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Chapter 12
Objectives
The ocean and the atmosphere are one interconnected system. The surface processes on
planet Earth are the result of H2O moving through this water cycle which is being driven byenergy from the Sun. The ocean's energy and water exchange with the atmosphere produce
atmospheric circulation cells, pressure belts, and wind belts. The idealized circulation
patterns are modified by the continents' own particular effects on heating, cooling, and
precipitation. The result is an intriguing mix of weather and climate, producing somewhat
predictable but widely variable atmospheric and oceanographic phenomena that are the topics
covered in this chapter.
After studying this chapter, you should be able to:
Describe the causes of uneven solar heating on Earth. Understand why Earth has seasons and how seasonal changes in solar energy affectatmospheric temperature, pressure, and density.
Explain the nature, origin, and consequences of the Coriolis effect in both theNorthern and Southern Hemisphere.
Discuss the locations and characteristics of Earth's major atmospheric circulationcells, pressure belts, wind belts, and boundaries.
Know the difference between weather (meteorology) and climate (climatology). Indicate the conditions required for the formation of tropical cyclones (hurricanes)and explain what types of destruction are caused by them.
Describe the cause of Earth's greenhouse effect and why it has increased in the recentpast.
Insolation
From Wikipedia, the free encyclopedia
Not to be confused withInsulation (disambiguation).
Insolation is a measure ofsolar radiationenergy received on a given surface area in a given
time. It is commonly expressed as averageirradiancein watts per square meter (W/m2) or
kilowatt-hours per square meter per day (kWh/(m2day)) (or hours/day). In the case
ofphotovoltaicsit is commonly measured as kWh/(kWpy) (kilowatt hours per year per
kilowatt peak rating).
The object or surface that solar radiation strikes may be a planet, a terrestrial object inside the
atmosphere of a planet, or any object exposed to solar rays outside of an atmosphere,
includingspacecraft. Some of the solar radiation will be absorbed, while the remainder will
be reflected. Usually the absorbed solar radiation is converted to thermal energy, causing an
increasing in the object's temperature. Some systems, however, may store or convert a
portion of the solar energy into another form of energy, as in the case of photovoltaics
http://en.wikipedia.org/wiki/Insulation_(disambiguation)http://en.wikipedia.org/wiki/Insulation_(disambiguation)http://en.wikipedia.org/wiki/Insulation_(disambiguation)http://en.wikipedia.org/wiki/Solar_radiationhttp://en.wikipedia.org/wiki/Solar_radiationhttp://en.wikipedia.org/wiki/Solar_radiationhttp://en.wikipedia.org/wiki/Irradiancehttp://en.wikipedia.org/wiki/Irradiancehttp://en.wikipedia.org/wiki/Irradiancehttp://en.wikipedia.org/wiki/Photovoltaicshttp://en.wikipedia.org/wiki/Photovoltaicshttp://en.wikipedia.org/wiki/Photovoltaicshttp://en.wikipedia.org/wiki/Spacecrafthttp://en.wikipedia.org/wiki/Spacecrafthttp://en.wikipedia.org/wiki/Spacecrafthttp://en.wikipedia.org/wiki/Spacecrafthttp://en.wikipedia.org/wiki/Photovoltaicshttp://en.wikipedia.org/wiki/Irradiancehttp://en.wikipedia.org/wiki/Solar_radiationhttp://en.wikipedia.org/wiki/Insulation_(disambiguation) -
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orplants. The proportion of radiation reflected or absorbed depends on the
object'sreflectivityoralbedo.
[edit]Projection effect
The insolation into a surface is largest when the surface directly faces the Sun. As the angle
increases between the direction at a right angle to the surface and the direction of the rays of
sunlight, the insolation is reduced in proportion to thecosineof the angle; seeeffect of sun
angle on climate.
Figure 2
One sunbeam one mile wide shines on the ground at a 90 angle, and another at a 30 angle.
The one at a shallower angle distributes the same amount of light energy over twice as much
area.
In this illustration, the angle shown is between the ground and the sunbeam rather than
between the vertical direction and the sunbeam; hence the sine rather than the cosine is
appropriate. A sunbeam one mile (1.6 km) wide falls on the ground from directly overhead,
and another hits the ground at a 30 angle to the horizontal.Trigonometrytells us that
thesineof a 30 angle is 1/2, whereas the sine of a 90 angle is 1. Therefore, the sunbeam
hitting the ground at a 30 angle spreads the same amount of light over twice as much area (if
we imagine the sun shining from the south atnoon, the north-south width doubles; the east-
west width does not). Consequently, the amount of light falling on each square mile is only
half as much.
This 'projection effect' is the main reason why thepolar regionsare much colder
thanequatorial regionson Earth. On an annual average the poles receive less insolation than
does the equator, because at the poles the Earth's surface are angled away from the Sun.
http://en.wikipedia.org/wiki/Plantshttp://en.wikipedia.org/wiki/Plantshttp://en.wikipedia.org/wiki/Plantshttp://en.wikipedia.org/wiki/Reflectivityhttp://en.wikipedia.org/wiki/Reflectivityhttp://en.wikipedia.org/wiki/Reflectivityhttp://en.wikipedia.org/wiki/Albedohttp://en.wikipedia.org/wiki/Albedohttp://en.wikipedia.org/wiki/Albedohttp://en.wikipedia.org/w/index.php?title=Insolation&action=edit§ion=1http://en.wikipedia.org/w/index.php?title=Insolation&action=edit§ion=1http://en.wikipedia.org/w/index.php?title=Insolation&action=edit§ion=1http://en.wikipedia.org/wiki/Cosinehttp://en.wikipedia.org/wiki/Cosinehttp://en.wikipedia.org/wiki/Cosinehttp://en.wikipedia.org/wiki/Effect_of_sun_angle_on_climatehttp://en.wikipedia.org/wiki/Effect_of_sun_angle_on_climatehttp://en.wikipedia.org/wiki/Effect_of_sun_angle_on_climatehttp://en.wikipedia.org/wiki/Effect_of_sun_angle_on_climatehttp://en.wikipedia.org/wiki/Trigonometryhttp://en.wikipedia.org/wiki/Trigonometryhttp://en.wikipedia.org/wiki/Trigonometryhttp://en.wikipedia.org/wiki/Sinehttp://en.wikipedia.org/wiki/Sinehttp://en.wikipedia.org/wiki/Sinehttp://en.wikipedia.org/wiki/Noonhttp://en.wikipedia.org/wiki/Noonhttp://en.wikipedia.org/wiki/Noonhttp://en.wikipedia.org/wiki/Polar_regionhttp://en.wikipedia.org/wiki/Polar_regionhttp://en.wikipedia.org/wiki/Polar_regionhttp://en.wikipedia.org/wiki/Equatorial_regionhttp://en.wikipedia.org/wiki/Equatorial_regionhttp://en.wikipedia.org/wiki/Equatorial_regionhttp://en.wikipedia.org/wiki/File:Seasons.too.pnghttp://en.wikipedia.org/wiki/Equatorial_regionhttp://en.wikipedia.org/wiki/Polar_regionhttp://en.wikipedia.org/wiki/Noonhttp://en.wikipedia.org/wiki/Sinehttp://en.wikipedia.org/wiki/Trigonometryhttp://en.wikipedia.org/wiki/Effect_of_sun_angle_on_climatehttp://en.wikipedia.org/wiki/Effect_of_sun_angle_on_climatehttp://en.wikipedia.org/wiki/Cosinehttp://en.wikipedia.org/w/index.php?title=Insolation&action=edit§ion=1http://en.wikipedia.org/wiki/Albedohttp://en.wikipedia.org/wiki/Reflectivityhttp://en.wikipedia.org/wiki/Plants -
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Solar Radiation Map of Africa and Middle East
Apyranometer, a component of a temporary remote meteorological station, measures
insolation onSkagit Bay,Washington.
[edit]Earth's insolation
Direct insolationis the solarirradiancemeasured at a given location on Earth with a surface
element perpendicular to the Sun's rays, excluding diffuse insolation (the solar radiation that
is scattered or reflected by atmospheric components in the sky). Direct insolation is equal to
thesolar constantminus the atmospheric losses due toabsorptionandscattering. While the
solar constant varies with theEarth-Sun distanceandsolar cycles, the losses depend on the
time of day (length of light's path through the atmosphere depending on theSolar elevation
angle),cloud cover,moisturecontent, and otherimpurities. Insolation is a fundamental abiotic
factor[1]
affecting the metabolism of plants and the behavior of animals.
Over the course of a year the average solar radiation arriving at the top of the Earth's
atmosphere at any point in time is roughly 1,366wattsper square meter[2][3]
(seesolar
constant). The radiant power is distributed across the entireelectromagnetic spectrum,
although most of the power is in thevisible lightportion of the spectrum. The Sun's rays
areattenuatedas they pass through theatmosphere, thus reducing the insolation at the Earth's
http://en.wikipedia.org/wiki/File:SolarGIS-Solar-map-Africa-and-Middle-East-en.pnghttp://en.wikipedia.org/wiki/Pyranometerhttp://en.wikipedia.org/wiki/Pyranometerhttp://en.wikipedia.org/wiki/Pyranometerhttp://en.wikipedia.org/wiki/Skagit_Bayhttp://en.wikipedia.org/wiki/Skagit_Bayhttp://en.wikipedia.org/wiki/Washington_(U.S._state)http://en.wikipedia.org/wiki/Washington_(U.S._state)http://en.wikipedia.org/wiki/Washington_(U.S._state)http://en.wikipedia.org/w/index.php?title=Insolation&action=edit§ion=2http://en.wikipedia.org/w/index.php?title=Insolation&action=edit§ion=2http://en.wikipedia.org/w/index.php?title=Insolation&action=edit§ion=2http://en.wikipedia.org/wiki/Direct_insolationhttp://en.wikipedia.org/wiki/Direct_insolationhttp://en.wikipedia.org/wiki/Irradiancehttp://en.wikipedia.org/wiki/Irradiancehttp://en.wikipedia.org/wiki/Irradiancehttp://en.wikipedia.org/wiki/Solar_constanthttp://en.wikipedia.org/wiki/Solar_constanthttp://en.wikipedia.org/wiki/Solar_constanthttp://en.wikipedia.org/wiki/Absorption_(electromagnetic_radiation)http://en.wikipedia.org/wiki/Absorption_(electromagnetic_radiation)http://en.wikipedia.org/wiki/Absorption_(electromagnetic_radiation)http://en.wikipedia.org/wiki/Light_scatteringhttp://en.wikipedia.org/wiki/Light_scatteringhttp://en.wikipedia.org/wiki/Light_scatteringhttp://en.wikipedia.org/wiki/Earth%27s_orbithttp://en.wikipedia.org/wiki/Earth%27s_orbithttp://en.wikipedia.org/wiki/Earth%27s_orbithttp://en.wikipedia.org/wiki/Solar_cyclehttp://en.wikipedia.org/wiki/Solar_cyclehttp://en.wikipedia.org/wiki/Solar_cyclehttp://en.wikipedia.org/wiki/Solar_elevation_anglehttp://en.wikipedia.org/wiki/Solar_elevation_anglehttp://en.wikipedia.org/wiki/Solar_elevation_anglehttp://en.wikipedia.org/wiki/Solar_elevation_anglehttp://en.wikipedia.org/wiki/Cloud_coverhttp://en.wikipedia.org/wiki/Cloud_coverhttp://en.wikipedia.org/wiki/Cloud_coverhttp://en.wikipedia.org/wiki/Moisturehttp://en.wikipedia.org/wiki/Moisturehttp://en.wikipedia.org/wiki/Moisturehttp://en.wikipedia.org/wiki/Atmospheric_pollutionhttp://en.wikipedia.org/wiki/Atmospheric_pollutionhttp://en.wikipedia.org/wiki/Atmospheric_pollutionhttp://en.wikipedia.org/wiki/Insolation#cite_note-0http://en.wikipedia.org/wiki/Insolation#cite_note-0http://en.wikipedia.org/wiki/Insolation#cite_note-0http://en.wikipedia.org/wiki/Watthttp://en.wikipedia.org/wiki/Watthttp://en.wikipedia.org/wiki/Watthttp://en.wikipedia.org/wiki/Insolation#cite_note-1http://en.wikipedia.org/wiki/Insolation#cite_note-1http://en.wikipedia.org/wiki/Insolation#cite_note-1http://en.wikipedia.org/wiki/Solar_constanthttp://en.wikipedia.org/wiki/Solar_constanthttp://en.wikipedia.org/wiki/Solar_constanthttp://en.wikipedia.org/wiki/Solar_constanthttp://en.wikipedia.org/wiki/Electromagnetic_spectrumhttp://en.wikipedia.org/wiki/Electromagnetic_spectrumhttp://en.wikipedia.org/wiki/Electromagnetic_spectrumhttp://en.wikipedia.org/wiki/Visible_lighthttp://en.wikipedia.org/wiki/Visible_lighthttp://en.wikipedia.org/wiki/Visible_lighthttp://en.wikipedia.org/wiki/Attenuationhttp://en.wikipedia.org/wiki/Attenuationhttp://en.wikipedia.org/wiki/Attenuationhttp://en.wikipedia.org/wiki/Atmospherehttp://en.wikipedia.org/wiki/Atmospherehttp://en.wikipedia.org/wiki/Atmospherehttp://en.wikipedia.org/wiki/File:Pyranometer_2740.JPGhttp://en.wikipedia.org/wiki/Atmospherehttp://en.wikipedia.org/wiki/Attenuationhttp://en.wikipedia.org/wiki/Visible_lighthttp://en.wikipedia.org/wiki/Electromagnetic_spectrumhttp://en.wikipedia.org/wiki/Solar_constanthttp://en.wikipedia.org/wiki/Solar_constanthttp://en.wikipedia.org/wiki/Insolation#cite_note-1http://en.wikipedia.org/wiki/Insolation#cite_note-1http://en.wikipedia.org/wiki/Watthttp://en.wikipedia.org/wiki/Insolation#cite_note-0http://en.wikipedia.org/wiki/Atmospheric_pollutionhttp://en.wikipedia.org/wiki/Moisturehttp://en.wikipedia.org/wiki/Cloud_coverhttp://en.wikipedia.org/wiki/Solar_elevation_anglehttp://en.wikipedia.org/wiki/Solar_elevation_anglehttp://en.wikipedia.org/wiki/Solar_cyclehttp://en.wikipedia.org/wiki/Earth%27s_orbithttp://en.wikipedia.org/wiki/Light_scatteringhttp://en.wikipedia.org/wiki/Absorption_(electromagnetic_radiation)http://en.wikipedia.org/wiki/Solar_constanthttp://en.wikipedia.org/wiki/Irradiancehttp://en.wikipedia.org/wiki/Direct_insolationhttp://en.wikipedia.org/w/index.php?title=Insolation&action=edit§ion=2http://en.wikipedia.org/wiki/Washington_(U.S._state)http://en.wikipedia.org/wiki/Skagit_Bayhttp://en.wikipedia.org/wiki/Pyranometerhttp://en.wikipedia.org/wiki/File:Pyranometer_2740.JPGhttp://en.wikipedia.org/wiki/File:SolarGIS-Solar-map-Africa-and-Middle-East-en.png -
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surface to approximately 1,000 watts per square meter for a surface perpendicular to the Sun's
rays at sea level on a clear day.
The actual figure varies with the Sun angle at different times of year, according to the
distance thesunlighttravels through theair, and depending on the extent of atmospheric hazeand cloud cover. Ignoring clouds, the average insolation for the Earth is approximately 250
watts per square meter (6 (kWh/m2)/day), taking into account the lower radiation intensity in
early morning and evening, and its near-absence at night.
The insolation of the sun can also be expressed in Suns, where one Sun equals 1,000 W/m2
at
the point of arrival, with kWh/(m2day) displayed as hours/day.
[4]When calculating the
output of, for example, a photovoltaic panel, the angle of the sun relative to the panel needs to
be taken into account as well as the insolation. (The insolation, taking into account the
attenuation of the atmosphere, should be multiplied by the cosine of the angle between thenormal to the panel and the direction of the sun from it). One Sun is a unit ofpower flux, not
a standard value for actual insolation. Sometimes this unit is referred to as a Sol, not to be
confused with a sol, meaning one solar day on, for example, a different planet, such as
Mars.[citation needed]
[edit]Distribution of insolation at the top of the atmosphere
Spherical triangle for application of the spherical law of cosines for the calculation the solar
zenith angle for observer at latitude and longitude from knowledge of the ho ur angle h
and solar declination . ( is latitude of subsolar point, and h is relative longitude of subsolarpoint).
http://en.wikipedia.org/wiki/Sunlighthttp://en.wikipedia.org/wiki/Sunlighthttp://en.wikipedia.org/wiki/Sunlighthttp://en.wikipedia.org/wiki/Airhttp://en.wikipedia.org/wiki/Airhttp://en.wikipedia.org/wiki/Airhttp://en.wikipedia.org/wiki/Insolation#cite_note-3http://en.wikipedia.org/wiki/Insolation#cite_note-3http://en.wikipedia.org/wiki/Insolation#cite_note-3http://en.wikipedia.org/wiki/Fluxhttp://en.wikipedia.org/wiki/Fluxhttp://en.wikipedia.org/wiki/Fluxhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/w/index.php?title=Insolation&action=edit§ion=3http://en.wikipedia.org/w/index.php?title=Insolation&action=edit§ion=3http://en.wikipedia.org/w/index.php?title=Insolation&action=edit§ion=3http://en.wikipedia.org/wiki/File:SolarZenithAngleCalc.pnghttp://en.wikipedia.org/wiki/File:SolarZenithAngleCalc.pnghttp://en.wikipedia.org/w/index.php?title=Insolation&action=edit§ion=3http://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Fluxhttp://en.wikipedia.org/wiki/Insolation#cite_note-3http://en.wikipedia.org/wiki/Airhttp://en.wikipedia.org/wiki/Sunlight -
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, the theoretical daily-average insolation at the top of the atmosphere. The calculation
assumed conditions appropriate for 2000 A.D.: a solar constant ofS0 = 1367 W m2
, obliquity
of = 23.4398, longitude of perihelion of = 282.895, eccentricity e = 0.016704. Contour
labels (green) are in units of W m2.
The theory for the distribution of solar radiation at the top of the atmosphere concerns
how the solarirradiance(the power of solar radiation per unit area) at the top of the
atmosphere is determined by the sphericity and orbital parameters of Earth. The theory could
be applied to any monodirectional beam of radiation incident onto a rotating sphere, but is
most usually applied to sunlight, and in particular for application innumerical weather
prediction, and theory for theseasonsand theice ages. The last application is known
asMilankovitch cycles.
The derivation of distribution is based on a fundamental identity fromspherical trigonometry,
thespherical law of cosines:
where a, b and c are arc lengths, in radians, of the sides of a spherical triangle. Cis the angle
in the vertex opposite the side which has arc length c. Applied to the calculation of
solarzenith angle, we equate the following for use in the spherical law of cosines:
http://en.wikipedia.org/wiki/File:InsolationTopOfAtmosphere.pnghttp://en.wikipedia.org/wiki/Irradiancehttp://en.wikipedia.org/wiki/Irradiancehttp://en.wikipedia.org/wiki/Irradiancehttp://en.wikipedia.org/wiki/Numerical_weather_predictionhttp://en.wikipedia.org/wiki/Numerical_weather_predictionhttp://en.wikipedia.org/wiki/Numerical_weather_predictionhttp://en.wikipedia.org/wiki/Numerical_weather_predictionhttp://en.wikipedia.org/wiki/Seasonshttp://en.wikipedia.org/wiki/Seasonshttp://en.wikipedia.org/wiki/Seasonshttp://en.wikipedia.org/wiki/Ice_ageshttp://en.wikipedia.org/wiki/Ice_ageshttp://en.wikipedia.org/wiki/Ice_ageshttp://en.wikipedia.org/wiki/Milankovitch_cycleshttp://en.wikipedia.org/wiki/Milankovitch_cycleshttp://en.wikipedia.org/wiki/Milankovitch_cycleshttp://en.wikipedia.org/wiki/Spherical_trigonometryhttp://en.wikipedia.org/wiki/Spherical_trigonometryhttp://en.wikipedia.org/wiki/Spherical_trigonometryhttp://en.wikipedia.org/wiki/Spherical_law_of_cosineshttp://en.wikipedia.org/wiki/Spherical_law_of_cosineshttp://en.wikipedia.org/wiki/Spherical_law_of_cosineshttp://en.wikipedia.org/wiki/Zenith_anglehttp://en.wikipedia.org/wiki/Zenith_anglehttp://en.wikipedia.org/wiki/Zenith_anglehttp://en.wikipedia.org/wiki/File:InsolationTopOfAtmosphere.pnghttp://en.wikipedia.org/wiki/Zenith_anglehttp://en.wikipedia.org/wiki/Spherical_law_of_cosineshttp://en.wikipedia.org/wiki/Spherical_trigonometryhttp://en.wikipedia.org/wiki/Milankovitch_cycleshttp://en.wikipedia.org/wiki/Ice_ageshttp://en.wikipedia.org/wiki/Seasonshttp://en.wikipedia.org/wiki/Numerical_weather_predictionhttp://en.wikipedia.org/wiki/Numerical_weather_predictionhttp://en.wikipedia.org/wiki/Irradiancehttp://en.wikipedia.org/wiki/File:InsolationTopOfAtmosphere.png -
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The distance of Earth from the sun can be denoted RE, and the mean distance can be denoted
R0, which is very close to 1AU. The insolation onto a plane normal to the solar radiation, at a
distance 1 AU from the sun, is thesolar constant, denoted S0. The solar flux density
(insolation) onto a plane tangent to the sphere of the Earth, but above the bulk of the
atmosphere (elevation 100 km or greater) is:
and
The average ofQ over a day is the average ofQ over one rotation, or the hour angle
progressing from h = toh = :
Let h0 be the hour angle when Q becomes positive. This could occur at sunrise
when , or for h0 as a solution of
or
cos(ho) = tan()tan()
If tan()tan() > 1, then the sun does not set and the sun is already risen at h = , so ho = . If
tan()tan() < 1, the sun does not rise and .
is nearly constant over the course of a day, and can be taken outside the integral
http://en.wikipedia.org/wiki/Astronomical_unithttp://en.wikipedia.org/wiki/Astronomical_unithttp://en.wikipedia.org/wiki/Astronomical_unithttp://en.wikipedia.org/wiki/Solar_constanthttp://en.wikipedia.org/wiki/Solar_constanthttp://en.wikipedia.org/wiki/Solar_constanthttp://en.wikipedia.org/wiki/Solar_constanthttp://en.wikipedia.org/wiki/Astronomical_unit -
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Let be the conventional polar angle describing a planetaryorbit. For convenience, let = 0
at the vernalequinox. Thedeclination as a function of orbital position is
where is the obliquity. The conventionallongitude of perihelion is defined relative to thevernal equinox, so for the elliptical orbit:
or
1.
With knowledge of , ande from astrodynamical calculations
[5]
and So from aconsensus of observations or theory, can be calculated for any latitude and . Note
that because of the elliptical orbit, and as a simple consequence ofKepler's second
law,does not progress exactly uniformly with time. Nevertheless, = 0 is exactly the time
of the vernal equinox, = 90 is exactly the time of the summer solstice, = 180 is exactly
the time of the autumnal equinox and = 270 is exactly the time of the winter solstice
.
[edit]Application to Milankovitch cycles
Obtaining a time series for a for a particular time of year, and particular latitude, is a
useful application in the theory ofMilankovitch cycles. For example, at the summer solstice,
the declination is simply equal to the obliquity . The distance from the sun is
Past and future of daily average insolation at top of the atmosphere on the day of the summer
solstice, at 65 N latitude. The green curve is with eccentricity e hypothetically set to 0. The
red curve uses the actual (predicted) value ofe. Blue dot is current conditions, at 2 ky A.D.
For this summer solstice calculation, the role of the elliptical orbit is entirely contained within
the important product , which is known as the precession index, the variation of
http://en.wikipedia.org/wiki/Orbithttp://en.wikipedia.org/wiki/Orbithttp://en.wikipedia.org/wiki/Orbithttp://en.wikipedia.org/wiki/Equinoxhttp://en.wikipedia.org/wiki/Equinoxhttp://en.wikipedia.org/wiki/Equinoxhttp://en.wikipedia.org/wiki/Declinationhttp://en.wikipedia.org/wiki/Declinationhttp://en.wikipedia.org/wiki/Declinationhttp://en.wikipedia.org/wiki/Longitude_of_periapsishttp://en.wikipedia.org/wiki/Longitude_of_periapsishttp://en.wikipedia.org/wiki/Longitude_of_periapsishttp://en.wikipedia.org/wiki/Insolation#cite_note-4http://en.wikipedia.org/wiki/Insolation#cite_note-4http://en.wikipedia.org/wiki/Insolation#cite_note-4http://en.wikipedia.org/wiki/Kepler%27s_laws_of_planetary_motion#Second_lawhttp://en.wikipedia.org/wiki/Kepler%27s_laws_of_planetary_motion#Second_lawhttp://en.wikipedia.org/wiki/Kepler%27s_laws_of_planetary_motion#Second_lawhttp://en.wikipedia.org/wiki/Kepler%27s_laws_of_planetary_motion#Second_lawhttp://en.wikipedia.org/w/index.php?title=Insolation&action=edit§ion=4http://en.wikipedia.org/w/index.php?title=Insolation&action=edit§ion=4http://en.wikipedia.org/w/index.php?title=Insolation&action=edit§ion=4http://en.wikipedia.org/wiki/Milankovitch_cycleshttp://en.wikipedia.org/wiki/Milankovitch_cycleshttp://en.wikipedia.org/wiki/Milankovitch_cycleshttp://en.wikipedia.org/wiki/File:InsolationSummerSolstice65N.pnghttp://en.wikipedia.org/wiki/File:InsolationSummerSolstice65N.pnghttp://en.wikipedia.org/wiki/File:InsolationSummerSolstice65N.pnghttp://en.wikipedia.org/wiki/File:InsolationSummerSolstice65N.pnghttp://en.wikipedia.org/wiki/File:InsolationSummerSolstice65N.pnghttp://en.wikipedia.org/wiki/File:InsolationSummerSolstice65N.pnghttp://en.wikipedia.org/wiki/File:InsolationSummerSolstice65N.pnghttp://en.wikipedia.org/wiki/File:InsolationSummerSolstice65N.pnghttp://en.wikipedia.org/wiki/File:InsolationSummerSolstice65N.pnghttp://en.wikipedia.org/wiki/File:InsolationSummerSolstice65N.pnghttp://en.wikipedia.org/wiki/File:InsolationSummerSolstice65N.pnghttp://en.wikipedia.org/wiki/File:InsolationSummerSolstice65N.pnghttp://en.wikipedia.org/wiki/File:InsolationSummerSolstice65N.pnghttp://en.wikipedia.org/wiki/File:InsolationSummerSolstice65N.pnghttp://en.wikipedia.org/wiki/File:InsolationSummerSolstice65N.pnghttp://en.wikipedia.org/wiki/File:InsolationSummerSolstice65N.pnghttp://en.wikipedia.org/wiki/File:InsolationSummerSolstice65N.pnghttp://en.wikipedia.org/wiki/File:InsolationSummerSolstice65N.pnghttp://en.wikipedia.org/wiki/Milankovitch_cycleshttp://en.wikipedia.org/w/index.php?title=Insolation&action=edit§ion=4http://en.wikipedia.org/wiki/Kepler%27s_laws_of_planetary_motion#Second_lawhttp://en.wikipedia.org/wiki/Kepler%27s_laws_of_planetary_motion#Second_lawhttp://en.wikipedia.org/wiki/Insolation#cite_note-4http://en.wikipedia.org/wiki/Longitude_of_periapsishttp://en.wikipedia.org/wiki/Declinationhttp://en.wikipedia.org/wiki/Equinoxhttp://en.wikipedia.org/wiki/Orbit -
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which dominates the variations in insolation at 65 N when eccentricity is large. For the next
100,000 years, with variations in eccentricity being relatively small, variations in obliquity
will be dominant.
[edit]Applications
Inspacecraftdesign andplanetology, it is the primary variable affectingequilibrium
temperature.
In construction, insolation is an important consideration when designing a building for a
particular climate. It is one of the most important climate variables for human comfort and
building energy efficiency.[6]
The projection effect can be used inarchitectureto design buildings that are cool in summer
and warm in winter, by providing large vertical windows on the equator-facing side of thebuilding (the south face in thenorthern hemisphere, or the north face in thesouthern
hemisphere): this maximizes insolation in the winter months when the Sun is low in the sky,
and minimizes it in the summer when the noonday Sun is high in the sky. (The Sun's
north/south paththrough the sky spans 47 degrees through the year).
Insolation figures are used as an input to worksheets to sizesolar power systemsfor the
location where they will be installed.[7]
This can be misleading since insolation figures
assume the panels are parallel with the ground, when in fact they are almost always mounted
at an angle
[8]
to face towards the sun. This gives inaccurately low estimates for winter.
[9]
Thefigures can be obtained from an insolation map or by city or region from insolation tables that
were generated with historical data over the last 3050 years. Photovoltaic panels are rated
under standard conditions to determine the Wp rating (watts peak),[10]
which can then be used
with the insolation of a region to determine the expected output, along with other factors such
as tilt, tracking and shading (which can be included to create the installed Wp
rating).[11]
Insolation values range from 800 to 950 kWh/(kWpy) inNorwayto up to 2,900
inAustralia.
In the fields ofcivil engineeringandhydrology, numerical models of snowmelt runoff useobservations of insolation. This permits estimation of the rate at which water is released from
a melting snowpack. Field measurement is accomplished using apyranometer.
Conversion factor (multiply top row by factor to obtain side column)
W/m2
kWh/(m2day)
sun
hours/daykWh/(m
2y) kWh/(kWpy)
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W/m2 1 41.66666 41.66666 0.1140796 0.1521061
kWh/(m2day) 0.024 1 1 0.0027379 0.0036505
sun hours/day 0.024 1 1 0.0027379 0.0036505
kWh/(m2y) 8.765813 365.2422 365.2422 1 1.333333
kWh/(kWpy) 6.574360 273.9316 273.9316 0.75 1
1. Insolation is the energy received on the earths surface from the sun. It is the most
important single source of atmospheric heat.
2. The earths surface does not absorb all the energy that it receives. The proportion of the
solar radiation reflected from the surface is called Albedo.
3. On an average, insolation is highest near the tropics, marginally lower at the equator and
lowest at the poles.
4. Although the earth receives energy continuously from the sun, its temperature remains
fairly constant, the only variations being the long-term climatic changes.
This is so because the atmosphere loses an amount of heat equal to the gain through
insolation. This mechanism of maintaining the same temperature by the atmosphere is called
the Heat Budget or Heat Balance.
5. Assuming that 100 units of energy reach the top of the atmosphere of the earth, 14 units are
absorbed directly by the atmosphere and 35 units are lost to space through reflection.
The remaining 51 units reach the earths surface and are absorbed by the earth due to which
the surface gets heated. The heated surface of the earth starts radiating energy in the form of
long waves and this process is called Terrestrial Radiation.
Out of the total 51 units given up by the surface in the form of terrestrial radiation, the
atmosphere (mainly carbon dioxide and water vapour) absorbs about 34 units and the
remaining 17 units escape to space.
In this manner, the atmosphere receives a total of 14 + 34 = 48 units and this amount is
radiated back to space by the atmosphere. The total loss of energy to space thus amounts to
100 units: 35 units reflected by the atmosphere, 17 units lost as terrestrial radiation and 48
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units from the atmosphere. In this manner, no net gain or loss of energy occurs in the earths
surface.
6. Although the earth and its atmosphere as a whole have a radiation balance, there are
latitudinal variations. The heat/energy is transferred from the lower latitudes to the higher
latitudes through winds and ocean currents.
Earth's radiation balance is the equation of the incoming and outgoing thermal radiation.
An instrument for measuring the net radiation balance and albedo. Model shown CNR 1.
Courtesy of Kipp & Zonen
The incoming solar radiation is short wave, therefore the equation below is called the shortwave radiation balance Qs:
Qs = G - R = D + H - R or depending on thealbedo(back-reflection to space): = (D+H)(1 - a)
G = global radiation D = direct shortwave radiation H = diffuse shortwave radiation R = reflected portion of global radiation (ca. 4%) a = albedoThe Earth's surface and atmosphere emits heat radiation in the infrared spectrum, called long
wave radiation. There is little overlap between the long wave radiation spectrum and the solar
radiation spectrum. The equation below expresses the long wave radiation balance Ql:
Ql = AE = AO - AG
AE = effective radiation AO = radiation of the Earth's surface AG = trapped radiation (radiation forcing, also known as the so called greenhouseeffect)
http://en.wikipedia.org/wiki/Albedohttp://en.wikipedia.org/wiki/Albedohttp://en.wikipedia.org/wiki/Albedohttp://en.wikipedia.org/wiki/Greenhouse_effecthttp://en.wikipedia.org/wiki/Greenhouse_effecthttp://en.wikipedia.org/wiki/Greenhouse_effecthttp://en.wikipedia.org/wiki/Greenhouse_effecthttp://en.wikipedia.org/wiki/File:Kippandzonen-CNR1.jpghttp://en.wikipedia.org/wiki/File:Kippandzonen-CNR1.jpghttp://en.wikipedia.org/wiki/File:Kippandzonen-CNR1.jpghttp://en.wikipedia.org/wiki/File:Kippandzonen-CNR1.jpghttp://en.wikipedia.org/wiki/Greenhouse_effecthttp://en.wikipedia.org/wiki/Greenhouse_effecthttp://en.wikipedia.org/wiki/Albedo -
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The two equations on incoming and outgoing radiation can be combined to show the net total
amount of radiation energy, total radiation balance Qt:
Qt = Qs - Ql = G - R - AE
The difficulty is to precisely quantify the various internal and external factors influencing the
radiation balance. Internal factors include all mechanisms affecting atmospheric composition(volcanism, biological activity, land use change, human activities etc.). The main external
factor is solar radiation. Thesun's average luminositychanges little over time.
External and internal factors are also closely interconnected. Increased solar radiation for
example results in higher average temperatures and higher water vapour content of the
atmosphere. Water vapour, a heat trapping gas absorbing infrared radiation emitted by the
Earth's surface, can lead to either higher temperatures through radiation forces or lower
temperatures as a result of increased cloud formation and hence increased albedo.
Atmospheric circulation is the large-scale movement of air, and the means (together with
the smallerocean circulation) by whichthermal energyis distributed on the surface of
theEarth.
The large-scale structure of the atmospheric circulation varies from year to year, but the basic
climatological structure remains fairly constant. However, individual weather systems - mid-
latitude depressions, or tropical convective cells - occur "randomly"[citation needed], and it is
accepted that weather cannot be predicted beyond a fairly short limit: perhaps a month in
theory, or (currently) about ten days in practice (seeChaos theoryandButterfly effect).
Nonetheless, as the climate is the average of these systems and patterns - where and when
they tend to occur again and again -, it is stable over longer periods of time.
As a rule, the "cells" of Earth's atmosphere shift polewards in warmer climates
(e.g.interglacialscompared toglacials), but remain largely constant even due tocontinental
drift.Tectonic upliftcan significantly alter major elements of it, however - for example
thejet stream-, andplate tectonicsshiftocean currents. In the extremely hot climates of
theMesozoic, indications of a thirddesertbelt at theEquatorhas been found; it was perhaps
caused byconvection. But even then, the overalllatitudinalpattern of Earth's climate was not
much different from the one today.
The wind belts girdling the planet are organised into three cells: the Hadley cell, theFerrelcell, and thePolar cell. Contrary to the impression given in the simplified diagram, the vast
bulk of the vertical motion occurs in the Hadley cell; the explanations of the other two cells
are complex. Note that there is one discrete Hadley cell that may split, shift and merge in a
complicated process over time[citation needed]
. Low and high pressures on earth's surface are
balanced by opposite relative pressures in the upper troposphere.
[edit]Hadley cell
Main article:Hadley cell
http://en.wikipedia.org/wiki/Solar_luminosityhttp://en.wikipedia.org/wiki/Solar_luminosityhttp://en.wikipedia.org/wiki/Solar_luminosityhttp://en.wikipedia.org/wiki/Ocean_circulationhttp://en.wikipedia.org/wiki/Ocean_circulationhttp://en.wikipedia.org/wiki/Ocean_circulationhttp://en.wikipedia.org/wiki/Thermal_energyhttp://en.wikipedia.org/wiki/Thermal_energyhttp://en.wikipedia.org/wiki/Thermal_energyhttp://en.wikipedia.org/wiki/Earthhttp://en.wikipedia.org/wiki/Earthhttp://en.wikipedia.org/wiki/Earthhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Chaos_theoryhttp://en.wikipedia.org/wiki/Chaos_theoryhttp://en.wikipedia.org/wiki/Chaos_theoryhttp://en.wikipedia.org/wiki/Butterfly_effecthttp://en.wikipedia.org/wiki/Butterfly_effecthttp://en.wikipedia.org/wiki/Butterfly_effecthttp://en.wikipedia.org/wiki/Interglacialhttp://en.wikipedia.org/wiki/Interglacialhttp://en.wikipedia.org/wiki/Interglacialhttp://en.wikipedia.org/wiki/Glacialshttp://en.wikipedia.org/wiki/Glacialshttp://en.wikipedia.org/wiki/Glacialshttp://en.wikipedia.org/wiki/Continental_drifthttp://en.wikipedia.org/wiki/Continental_drifthttp://en.wikipedia.org/wiki/Continental_drifthttp://en.wikipedia.org/wiki/Continental_drifthttp://en.wikipedia.org/wiki/Tectonic_uplifthttp://en.wikipedia.org/wiki/Tectonic_uplifthttp://en.wikipedia.org/wiki/Tectonic_uplifthttp://en.wikipedia.org/wiki/Jet_streamhttp://en.wikipedia.org/wiki/Jet_streamhttp://en.wikipedia.org/wiki/Jet_streamhttp://en.wikipedia.org/wiki/Plate_tectonicshttp://en.wikipedia.org/wiki/Plate_tectonicshttp://en.wikipedia.org/wiki/Plate_tectonicshttp://en.wikipedia.org/wiki/Ocean_currenthttp://en.wikipedia.org/wiki/Ocean_currenthttp://en.wikipedia.org/wiki/Ocean_currenthttp://en.wikipedia.org/wiki/Mesozoichttp://en.wikipedia.org/wiki/Mesozoichttp://en.wikipedia.org/wiki/Mesozoichttp://en.wikipedia.org/wiki/Deserthttp://en.wikipedia.org/wiki/Deserthttp://en.wikipedia.org/wiki/Deserthttp://en.wikipedia.org/wiki/Equatorhttp://en.wikipedia.org/wiki/Equatorhttp://en.wikipedia.org/wiki/Equatorhttp://en.wikipedia.org/wiki/Convectionhttp://en.wikipedia.org/wiki/Convectionhttp://en.wikipedia.org/wiki/Convectionhttp://en.wikipedia.org/wiki/Latitudinalhttp://en.wikipedia.org/wiki/Latitudinalhttp://en.wikipedia.org/wiki/Hadley_cellhttp://en.wikipedia.org/wiki/Hadley_cellhttp://en.wikipedia.org/wiki/Hadley_cellhttp://en.wikipedia.org/wiki/Ferrel_cellhttp://en.wikipedia.org/wiki/Ferrel_cellhttp://en.wikipedia.org/wiki/Ferrel_cellhttp://en.wikipedia.org/wiki/Ferrel_cellhttp://en.wikipedia.org/wiki/Polar_cellshttp://en.wikipedia.org/wiki/Polar_cellshttp://en.wikipedia.org/wiki/Polar_cellshttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/w/index.php?title=Atmospheric_circulation&action=edit§ion=2http://en.wikipedia.org/w/index.php?title=Atmospheric_circulation&action=edit§ion=2http://en.wikipedia.org/w/index.php?title=Atmospheric_circulation&action=edit§ion=2http://en.wikipedia.org/wiki/Hadley_cellhttp://en.wikipedia.org/wiki/Hadley_cellhttp://en.wikipedia.org/wiki/Hadley_cellhttp://en.wikipedia.org/wiki/Hadley_cellhttp://en.wikipedia.org/w/index.php?title=Atmospheric_circulation&action=edit§ion=2http://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Polar_cellshttp://en.wikipedia.org/wiki/Ferrel_cellhttp://en.wikipedia.org/wiki/Ferrel_cellhttp://en.wikipedia.org/wiki/Hadley_cellhttp://en.wikipedia.org/wiki/Latitudinalhttp://en.wikipedia.org/wiki/Convectionhttp://en.wikipedia.org/wiki/Equatorhttp://en.wikipedia.org/wiki/Deserthttp://en.wikipedia.org/wiki/Mesozoichttp://en.wikipedia.org/wiki/Ocean_currenthttp://en.wikipedia.org/wiki/Plate_tectonicshttp://en.wikipedia.org/wiki/Jet_streamhttp://en.wikipedia.org/wiki/Tectonic_uplifthttp://en.wikipedia.org/wiki/Continental_drifthttp://en.wikipedia.org/wiki/Continental_drifthttp://en.wikipedia.org/wiki/Glacialshttp://en.wikipedia.org/wiki/Interglacialhttp://en.wikipedia.org/wiki/Butterfly_effecthttp://en.wikipedia.org/wiki/Chaos_theoryhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Earthhttp://en.wikipedia.org/wiki/Thermal_energyhttp://en.wikipedia.org/wiki/Ocean_circulationhttp://en.wikipedia.org/wiki/Solar_luminosity -
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TheITCZ's band of clouds over theEastern Pacificand theAmericasas seen from space
The Hadley cell mechanism is well understood. The atmospheric circulation pattern
thatGeorge Hadleydescribed to provide an explanation for thetrade windsmatches
observations very well. It is a closed circulation loop, which begins at the equator with warm,
moist air lifted aloft in equatoriallow pressure areas(theIntertropical Convergence Zone,
ITCZ) to thetropopauseand carried poleward. At about30N/S latitude, it descends in ahigh
pressure area. Some of the descending air travels equatorially along the surface, closing the
loop of the Hadley cell and creating theTrade Winds.
Though the Hadley cell is described as lying on the equator, it is more accurate to describe it
as following the sunszenithpoint, or what is termed the "thermal equator," which undergoes
a semiannual north-south migration.
[edit]Polar cell
Main article:Polar vortex
The Polar cell is likewise a simple system. Though cool and dry relative to equatorial air, air
masses at the60th parallelare still sufficiently warm and moist to undergoconvectionand
drive athermal loop. Air circulates within thetroposphere, limited vertically by thetropopause at about 8 km. Warm air rises at lower latitudes and moves poleward through the
upper troposphere at both the north and south poles. When the air reaches the polar areas, it
has cooled considerably, and descends as a cold, dry high pressure area, moving away from
the pole along the surface but twisting westward as a result of theCoriolis effectto produce
thePolar easterlies.
The outflow from the cell createsharmonicwaves in the atmosphere known asRossby
waves. These ultra-long waves play an important role in determining the path of the jet
stream, which travels within the transitional zone between thetropopauseand theFerrel cell.
By acting as aheat sink, the Polar cell also balances the Hadley cell in the Earths energy
equation.It can be argued that the Polar cell is the primary weathermaker for regions above the middle
northern latitudes. While Canadians and Europeans may have to deal with occasional heavy
summer storms, there is nothing like a winter visit from aSiberianhigh to give one a true
appreciation of real cold. In fact, it is the polar high which is responsible for generating the
coldest temperature recorded on Earth: -89.2C atVostok Stationin 1983 inAntarctica.
The Hadley cell and the Polar cell are similar in that they are thermally direct; in other words,
they exist as a direct consequence of surface temperatures; their thermal characteristics
override the effects of weather in their domain. The sheer volume of energy the Hadley cell
transports, and the depth of theheat sinkthat is the Polar cell, ensures that the effects of
transient weather phenomena are not only not felt by the system as a whole, but exceptunder unusual circumstancesare not even permitted to form. The endless chain of passing
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highs and lows which is part of everyday life for mid-latitude dwellers is unknown above the
60th and below the 30th parallels. There are some notable exceptions to this rule. In Europe,
unstable weather extends to at least70 north.
These atmospheric features are also stable, so even though they may strengthen or weaken
regionally or over time, they do not vanish entirely.[edit]Ferrel cell
The Ferrel cell, theorized byWilliam Ferrel(1817-1891), is a secondary circulation feature,
dependent for its existence upon the Hadley cell and the Polar cell. It behaves much as an
atmospheric ball bearing between the Hadley cell and the Polar cell, and comes about as a
result of theeddycirculations (the high and low pressure areas) of the mid-latitudes. For this
reason it is sometimes known as the "zone of mixing." At its southern extent (in the
Northern hemisphere), it overrides the Hadley cell, and at its northern extent, it overrides the
Polar cell. Just as the Trade Winds can be found below the Hadley cell, theWesterliescan be
found beneath the Ferrel cell. Thus, strong high pressure areas which divert the prevailing
westerlies, such as aSiberian high(which could be considered an extension of the Arctichigh), could be said to override the Ferrel cell, making it discontinuous.
While the Hadley and Polar cells are truly closed loops, the Ferrel cell is not, and the telling
point is in theWesterlies, which are more formally known as "the Prevailing Westerlies."
While the Trade Winds and the Polar Easterlies have nothing over which to prevail, their
parent circulation cells having taken care of any competition they might have to face, the
Westerlies are at the mercy of passing weather systems. While upper-level winds are
essentially westerly, surface winds can vary sharply and abruptly in direction. A low moving
polewards or a high moving equator wards maintains or even accelerates a westerly flow; the
local passage of a cold front may change that in a matter of minutes, and frequently does. A
strong high moving polewards may bring easterly winds for days.
The base of the Ferrel cell is characterized by the movement of air masses, and the location of
these air masses is influenced in part by the location of the jet stream, which acts as a
collector for the air carried aloft by surface lows (a look at a weather map will show that
surface lows follow thejet stream). The overall movement of surface air is from the 30th
latitude to the 60th. However, the upper flow of the Ferrel cell is not well defined. This is in
part because it is intermediary between the Hadley and Polar cells, with neither a strong heat
source nor a strong cold sink to drive convection and, in part, because of the effects on the
upper atmosphere of surface eddies, which act as destabilizing influences.
[edit]Longitudinal circulation features
While the Hadley, Ferrel, and Polar cells are major factors in global heat transport, they do
not act alone. Disparities in temperature also drive a set of longitudinal circulation cells, and
the overall atmospheric motion is known as the zonal overturning circulation.
Latitudinal circulation is the consequence of the fact that incident solar radiation per unit area
is highest at the heat equator, and decreases as the latitude increases, reaching its minimum at
the poles. Longitudinal circulation, on the other hand, comes about because water has a
higher specific heat capacity than land and thereby absorbs and releases more heat, but the
temperature changes less than land. Even atmesoscales(a horizontal range of 5 to several
hundred kilometres), this effect is noticeable; it is what brings the sea breeze, air cooled by
the water, ashore in the day, and carries the land breeze, air cooled by contact with theground, out to sea during the night.
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Diurnal wind change in coastal area.
On a larger scale, this effect ceases to be diurnal (daily), and instead is seasonal or
evendecadalin its effects. Warm air rises over theequatorial,continental, and western PacificOcean regions, flows eastward or westward, depending on its location, when it reaches the
tropopause, andsubsidesin theAtlanticandIndian Oceans, and in the easternPacific.
The Pacific Ocean cell plays a particularly important role in Earth's weather. This entirely
ocean-based cell comes about as the result of a marked difference in the surface temperatures
of the western and eastern Pacific. Under ordinary circumstances, the western Pacific waters
are warm and the eastern waters are cool. The process begins when strong convective activity
over equatorial East Asia and subsiding cool air off South America's west coast creates a
wind pattern which pushes Pacific water westward and piles it up in the western Pacific.
(Water levels in the western Pacific are about 60 cm higher than in the eastern Pacific, a
difference due entirely to the force of moving air.)
[1][2][3]
[edit]Walker circulation
Main article:Walker circulation
The Pacific cell is of such importance that it has been named the Walker circulation afterSir
Gilbert Walker, an early-20th-century director of British observatories inIndia, who sought a
means of predicting when themonsoonwinds would fail. While he was never successful in
doing so, his work led him to the discovery of an indisputable link between periodic pressure
variations in the Indian Ocean and the Pacific, which he termed the "Southern Oscillation".
The movement of air in the Walker circulation affects the loops on either side. Under
"normal" circumstances, the weather behaves as expected. But every few years, the wintersbecome unusually warm or unusually cold, or the frequency of hurricanes increases or
decreases, and the pattern sets in for an indeterminate period.
The behavior of the Walker cell is the key to the riddle, and leads to an understanding of
theEl Nio(more accurately, ENSO or El Nio - Southern Oscillation) phenomenon.
If convective activity slows in the Western Pacific for some reason (this reason is not
currently known), theclimatedominoes next to it begin to topple. First, the upper-level
westerly winds fail. This cuts off the source of cool subsiding air, and therefore the surface
Easterlies cease.
The consequence of this is twofold. In the eastern Pacific, warm water surges in from thewest since there is no longer a surface wind to constrain it. This and the corresponding effects
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of the Southern Oscillation result in long-term unseasonable temperatures and precipitation
patterns in North and South America, Australia, and Southeast Africa, and disruption of
ocean currents.
Meanwhile in the Atlantic, high-level, fast-blowing Westerlies which would ordinarily be
blocked by the Walker circulation and unable to reach such intensities, form. These windstear apart the tops of nascenthurricanesand greatly diminish the number which are able to
reach full strength.
[edit]El Nio - Southern Oscillation
Main article:El Nio-Southern Oscillation
El Nio andLa Nia are two opposite surface temperature anomalies in the Southern Pacific,
which heavily influence the weather on a large scale. In the case of El Nio warm water
approaches the coasts of South America which results in blocking the upwelling of nutrient-
rich deep water. This has serious impacts on the fish populations.
In the La Nia case, the convective cell over the western Pacific strengthens inordinately,resulting in colder than normal winters in North America, and a more robust cyclone season
in South-East Asia and Eastern Australia. There is increased upwelling of deep cold ocean
waters and more intense uprise of surface air near South America, resulting in increasing
numbers of drought occurrence, although it is often argued that fishermen reap benefits from
the more nutrient-filled eastern Pacific waters.
The neutral part of the cycle - the "normal" component - has been referred to humorously by
some as "La Nada", which means "the nothing" in Spanish.
[edit]See also
Chapter 6 - Atmospheric and Oceanic Circulation
Air Pressure and Wind Basics
Air pressure depends on the density and temperature of the air in addition to the altitude.
Air pressure was first measured by Torricelli (1600s), Galileo's pupil.
Barometer (Gk: baros = weight)
- mercury barometer: the weight of the air pushes mercury up an evacuated glass tube
- anneroid barometer (anneroid = without fluid): weight of air pushes in on partiallyevacuated chamber
Atmospheric pressure is commonly measured in inches or milimeters of mercury, or as
milibars or kilopascals of pressure.
Winds are produced bypressure differences between two locations. Wind speedis measured
by ananemometer. Wind speed is recorded in miles per hour (mph), kilometers per hour
(kmph), or knots (1 knot = 1.15 mph). Wind direction is measured by a wind vane. Wind
direction is the direction that the wind is coming from (not the direction it is moving toward).
TheBeaufort Wind Scale was developed in 1806 to estimate wind speed from visual cues atsea. It was modified for using visual cues on land by Simpson in 1926.
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Driving Forces of the Wind
Gravitational Force - compresses the atmosphere
Pressure Gradient Force - air moves from high pressure (more compressed) toward low
pressure (less compressed)
Coriolis Force - air in motion appears to be deflected as Earth spins west to east
Friction Force - drag against the Earth's surface slows the wind; slows surface winds the
most, high level winds the least
- Pressure Gradient Air pressure differences are the result of uneven heating of the
atmosphere. In some areas, stronger heating leads to expansion of air, making it less dense
(fewer molecules and less weight per cubic foot or cubic meter of air). Hot, rising air is
associated with low pressure. Cold, dense, sinking air is associated with high pressure.
Pressure gradient refers to the pressure difference (inches or mm of mercury or milibars)divided by the distance over which the pressure drop occurs. Since wind (moving air) is
caused by pressure differences, the greater the pressure gradient the stronger the wind.
Air pressure maps plot lines of equal pressure called isobars. Isobars are drawn at equal
increments of air pressure. The more closely spaced the isobars are, the greater the pressure
gradient and the stronger the winds.
The pressure gradient force, if it acted by itself (which it doesn't), would produce winds thatmoved at very high speeds at right angles to the isobars, the shortest path from high to low
pressure. But...(see the next two forces)
- Coriolis Force As winds move north or south they are deflected due to the rotation of the
Earth. As the Earth spins on its axis, a person standing on the equator moves from west to
east at around 1000 mile per hour. At the poles, on the other hand, that person would not
move at all, just spin around in place. So, the equator and anything on it moves west to east
faster than any other place on Earth. The west to east motion decreases from the equator to
the pole.
As winds move away from the equator, their west to east momentum carries them to the east
of a true poleward trajectory. In the northern hemisphere they are deflected to the right. In the
southern hemisphere they are deflected to the left. For the opposite case, as air masses movetoward the equator, their west to east momentum lags behind the west to east motion of the
Earth at lower latitude and they curve to the west.In the northern hemisphere moving air
(wind) is deflected to the right. In the southern hemisphere winds are deflected to the left.
The strength of the coriolis force is zero at the equator, half its maximum strength at 30
latitude, and maximum at the poles. Fast winds and winds covering the greatest distances are
deflected the most. In the absence of friction (approximated in the upper atmosphere), the
coriolis force would cause the winds to blow parallel to isobars, in circles, clockwise around
high pressure and counterclockwise around lows in the northern hemisphere. In the southern
hemisphere the effect is the opposite, counterclockwise around highs and clockwise around
lows. These isobar parallel circular winds,geostrophic winds, only occur in the upperatmosphere, away from friction with the Earth's surface.
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- Friction Force Friction with the Earth's surface only affects wind speed up to altitudes
around 500 m (1640 ft). Friction prevents geostrophic winds at low altitude. Rather, low
level winds move at an angle across isobars.
The net effect of these forces is that near surface winds spiral outward away from high
pressure centers and inward toward low pressure centers.
- In the northern hemisphere, where winds are deflected to the right,
winds spiralclockwise around high pressure andcounterclockwise around low pressure
- In the southern hemisphere, where winds are deflected to the left, winds
spiralcounterclockwise around high pressure andclockwise around low pressure
Low pressure centers are zones of convergence, with winds spiralling inward. These are
calledcyclones.
Tornadoes and hurricanes are strong cyclonic storms.
High pressure centers are zones of diveregence, with winds spiralling outward. These are
calledanticyclones.
Primary Gobal Pressure Systems, Atmospheric Circultion, and Climate Belts
Equatorial Low-Pressure - Intertropical Convergence Zone (ITCZ) The equatorial region
is the most strongly heated area on the Earth. It is there that we find the most vigorous
upward convection. Low pressure is found all along the equator. Winds converge on the
intertropical convergence zone from the northeast (northeast tradewinds) and the
southeast (southeast tradewinds). The trades are fairly strong and consistent. Right at the
ITCZ winds are weak and variable. Sailors in the days of sailing ships called thisthedoldrums.
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Subtropical Highs At latitudes around 25 to 30 north and south of the equator there are
several more-or-less continuous and stationary centers of high pressure. For example
theBermuda High (or Azores High) in the north Atlantic Ocean, thePacific High (or
Hawaiian High) in the north Pacific, and highs over the south Atlantic, south Pacific, and
south Indian oceans. Winds diverging from these highs toward the equator form the
tradewinds. Winds diverging from the highs towards the poles are deflected to the east innorthern and southern hemispheres forming the prevailing westerlies in midlatitudes.
Subpolar Lows A series of low pressure centers encircle Antarctica summer and winter. In
the northern hemisphere, theIcelandic Low in the north Atlantic andAleutian Low in the
north Pacific spawn cyclonic storms in winter but weaken or die out in summer as the
subtropical highs strengthen in the north Atlantic and north Pacific.
Polar Highs High pressure dominates in the polar regions because the air is very cold and
dense. Antarctica is the coldest place on Earth because it lies over the south pole, because it
is continental, and because the ice sheet is very thick and so the surface elevation is also
high. High pressure dominates Antarctica year-round. The north pole, however, lies in theArctic Ocean. The ocean has a moderating effect on the arctic. High pressure is less well
developed in the north polar summer, but devlops over land as the Canadian and Siberian
Highs in winter.
Climate Belts Simplified
It is commonly known that when materials (such as atmospheric gases) are heated they
expand thereby becoming less dense or "lighter." In a room with a radiator or other such
heater, as the air around the radiator becomes heated it expands and rises to the ceiling and is
replaced by cooler denser air. This is an example ofconvection. A similar process occurs
near the coast in summer. Land heats up faster during the day than the water does. Air over
the land is heated, expands, and rises. It is replaced by cooler, denser air from over the water
forming a coolsea breeze. Convection also occurs on a global scale driven by the uneven
heating of the Earth.
equatorial: hot & wet
The equatorial regions are the most strongly heated areas on the Earth's surface. It is there
that we find the most vigorous upward convection. Hot air is capable of holding much water
vapor. Hot, humid air rises over the equator. As it rises to high altitude it expands because the
air pressure decreases (there is less mass of air above it). As air expands due to thisdecreasing pressure, it also cools. Since cool air is able to hold less water vapor than warm
air, condensation occurs. This is why the equatorial regions normally have very high rainfall.
It is here that we find tropical rainforests such as those in the Amazon, Congo, and Indonesia.
Areas of upward convection are dominated by low atmospheric pressure.
Atmospheric Convection and Rainfall
desert belts: hot & dry
The rising air at the equator is replaced by low-level air from higher latitudes north and south
of the equator. To balance the air moving toward the equator at low altitude, the convecting
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air moves away from the equator, toward the north and south, at high altitude. It is now cool
because of expansion and dry because it has dropped off excess moisture. To complete the
convection loop, in the regions around 25 degrees north and 25 degrees south of the equator,
this cool and dry air descends back to the surface (subtropical highs). As it descends, the
pressure increases (because there is now more air overhead). The increased pressure increases
the temperature of the air and therefore increases the capacity of the air to hold water vapor.Now the air is very dry and has the capacity to soak up much evaporation. Consequently,
these latitudes are very dry with high evaporation and low rainfall. These are the desert belts,
including the Sahara, Mojave and Sonoran deserts of the U.S. southwest and Mexico, the
Kalahari and Namib in southern Africa, the Australian desert, and the Atacama Desert on the
west coast of South America. Areas of descending air are dominated by high atmospheric
pressure.
midlatitude: temperate - cool and moist
The midlatitudes are a battleground between very cold, dense polar air and warm air moving
poleward from the subtropical highs. The boundary between them is called thepolarfront. The polar front is an undulating boundary. The undulations are calledRossby Waves.
In the midlatitudes, these undulations are sites where cold air pushes equatorward and warm
air pushes poleward. Cyclonic circulation (convergence, counterclockwise in northern
hemisphere) develops around the southward bulges of the polar front. As these undulations
of the polar front sweep southward and eastward (northern hemisphere) cold dense air pushes
under warmer, less dense air. As the warmer air rises, it expands, cools, and condenses some
of its water vapor. That is why cold fronts bring clouds and rain. During the summer the
polar front lies farther north and we seldom see summer cold fronts on Long Island.
polar: cold & dry
The polar regions are the coldest on Earth. The air is very cold, dense, and dry. High
pressure dominates. Much of Antarctica is essentially a desert because so little precipitation
falls (though what does fall remains frozen).
Ocean Circulation
surface currents Wind drives both waves and surface currents in the oceans. Warm surface
waters at the equator are driven westward by the easterly trade winds. When the equatorial
currents reach the western edge of the ocean basin (east coast of some continent) they are
diverted to the north and south along the continents. In the central Atlantic this northwardflowing branch is called the Gulf Stream. It carries warm water from the equator, Caribbean
and Gulf of Mexico, northward along the east coast of North America, across the North
Atlantic, and to arctic Ocean off northwestern Europe. The westerlies help to drive the Gulf
Stream eastward across the Atlantic toward Europe. The warmth of the Gulf Stream
moderates the climate of northwestern Europe. The waters in the North Atlantic cool. The
cooled waters then flow southward along the coast of Europe and Africa. This southern
current is called the Canary Current. It brings cool waters down to northwest Africa and
keep the coast here relatively cool. Eventually this current approaches the equator where the
waters warm again and the easterly winds drive them westward across the Atlantic again to
start another clockwise loop. Such large surface current loops, calledsubtropical gyres, are
found in all the open ocean basins. They generally flow clockwise in the northern hemisphereand counterclockwise in the southern hemisphere.
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deep ocean currentsAs water chills in the North Atlantic it becomes more dense. Also, when
the cold water begins to freeze to form sea ice the ice that forms is from pure water; the salt is
left behind in the remaining sea water. The sea water gets saltier. The saltier the water the
denser it becomes. These cold, salty, dense surface waters sink down to the bottom of the
Atlantic. The sinking waters are replaced by less dense surface waters from the south. The
sinking waters flow southward along the bottom of the ocean as surface waters flownorthward to replace them. The North Atlantic Deep Water continues south until they meet a
northward flowing bottom current of even denser waters that formed off the coast of
Antarctica. The North Atlantic Deep Water then rides up above the Antarctic Bottom Water
and continues southward at intermediate depths until they eventually rise to the surface near
Antarctica. From there they follow other currents that carry them throughout the oceans.
ocean conveyor belt There is an ocean conveyor belt that mixes waters through all the ocean
basins from the sea bottom to the sea surface, connecting the surface, intermediate, and
bottom water currents. This mixing moves heat, dissolved gases, and nutrients through the
oceans in one grand cycle. Breakdown of this conveyor belt may have been responsible for
sudden changes in the Earth's climate in the past.
global warming and sudden cooling in Europe? The Earth is warming, largely due to the
release of greenhouse gases from industrial and agricultural activity. As a result of the
warming, the rate of melting and release of icebergs from Greenland into the far north
Atlantic and Arctic Ocean is increasing. The increased influx of fresh water into the sea willmake these surface waters less dense which could slow or stop the formation of deep water
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(sinking of surface water). This should slow the northward movement of the Gulf Stream.
Because heat carried into the far north Atlantic helps to moderate the climate of densely
populated northwestern Europe, a weakening or cessation of the Gulf Stream would cause
major social, agricultural, and economic problems. Geologic evidence has shown that this has
happened very rapidly in the past yielding a sudden cooling of northwestern Europe within
about two decades time.
Climate change refers to a statistically significant variation
in either the mean state of the climate or in its variability,
persisting for an extended period (typically decades or
longer). Climate change may be due to natural internal
processes or external forcings, or to persistent anthropogenic
changes in the composition of the atmosphere or in land use.
The Earth is the only planet in our solar system that supports life. The complex process ofevolution occurred on Earth only because of some unique environmental conditions that were
present: water, an oxygen-rich atmosphere, and a suitable surface temperature.
Mercury and Venus, the two planets that lie between Earth and the sun, do not support life.
This is because Mercury has no atmosphere and therefore becomes very hot during the day,
while temperatures at night may reach -140C. Venus, has a thick atmosphere which traps
more heat than it allows to escape, making it too hot (between 150 and 450 C) to sustain
life.
Only the Earth has an atmosphere of the proper depth and chemical composition. About 30%
of incoming energy from the sun is reflected back to space while the rest reaches the earth,
warming the air, oceans, and land, and maintaining an average surface temperature of about
15C.
The chemical composition of the atmosphere is also responsible for nurturing life on our
planet. Most of it is nitrogen (78%); about 21% is oxygen, which all animals need to survive;
and only a small percentage (0.036%) is made up of carbon dioxide which plants require for
photosynthesis.
The atmosphere carries out the critical function of maintaining life-sustaining conditions on
Earth, in the following way: each day, energy from the sun (largely in the visible part of the
spectrum, but also some in the ultraviolet, and infra red portions) is absorbed by the land,
seas, mountains, etc. If all this energy were to be absorbed completely, the earth would
gradually become hotter and hotter. But actually, the earth both absorbs and, simultaneously
releases it in the form of infra red waves (which cannot be seen by our eyes but can be felt as
heat, for example the heat that you can feel with your hands over a heated car engine). All
this rising heat is not lost to space, but is partly absorbed by some gases present in very small
(or trace) quantities in the atmosphere, called GHGs (greenhouse gases).
Greenhouse gases (for example, carbon dioxide, methane, nitrous oxide, water vapour,ozone), re-emit some of this heat to the earth's surface. If they did not perform this useful
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function, most of the heat energy would escape, leaving the earth cold (about -18C) and
unfit to support life.
However, ever since the Industrial Revolution began about 150 years ago, man-made
activities have added significant quantities of GHGs to the atmosphere. The atmospheric
concentrations of carbon dioxide, methane, and nitrous oxide have grown by about 31%,
151% and 17%, respectively, between 1750 and 2000 (IPCC 2001).
Variations of the Earth's surface temperature for the past 140
years
The Earths surface temperature is shown year by year (red bars)
and approximately decade by decade (black line, a filtered annualcurve suppressing fluctuations below near decadal time-scales).
There are uncertainties in the annual data (thin black whisker bars
represent the 95% confidence range) due to data gaps, random
instrumental errors and uncer