TOPIC 6:
INSOLATION AND THE SEASONS
Solar Radiation and Insolation
• Insolation – Incoming solar radiation
• Components of Insolation (by EM type)→Infrared – 48% →Visible Light – 44%→Ultraviolet – 7%
Effects of Earth’s Atmosphere on Insolation
• Absorption of EM→UV rays are absorbed by the ozone layer.
However, there is a hole in the ozone which allows UV to hit Earth and can cause cancer.
→Infrared radiation absorbed by carbon dioxide, water vapor and methane These are called greenhouse gasses
Effects of Earth’s Atmosphere on Insolation
• Reflection and Scattering→Conditions that cause HIGH REFLECTION
of insolation Clouds = lots of visible and IR reflected Aerosols – small suspended particles
in the atmosphere (dust, ice, water, ash) –Lots of aerosols cause lots of
scattering of insolation Low transparency of atmosphere
Balance of Energy from Insolation
→ Half of the insolation hitting Earth’s atmosphere will reach Earth’s surface (land or water) Transmitted through atmosphere
→ The Earth will re-radiate energy at longer wavelengths of EM usually in the form of heat (IR) Earth is cooler than the sun
Balance of Energy from Insolation
→The amount of heat absorbed, over time, will EQUAL amount of heat radiated so that Earth’s surface temperature will tend to balance Dynamic equilibrium
Factors Affecting Absorption and Reflection
of Insolation• Angle of Incidence – the angle at
which the sun’s energy hits the surface of the earth→Angle varies with latitude, time of
day and season→The higher the angle of incidence the
more energy is absorbed
Factors Affecting Absorption and Reflection
of Insolation• Characteristics of Earth’s Surface
→More energy is absorbed if Earth’s surface is dark and rough
Factors Affecting Absorption and Reflection
of Insolation• Change of State (of water) and
Transpiration→In general, when water is changing
phase solid liquid gas energy is absorbed
→If energy needs to be absorbed to change phase it isn’t available to raise temperature of Earth’s surface
Heating Land and Water• Land heats faster over time than
water because land has a lower specific heat
• Insolation can penetrate deeper into water.
• Heating water is done by convection so water layers mix.
Greenhouse Effect• Works on the idea that re-radiated
energy is at a longer wavelength than incoming insolation→Greenhouse gasses (CO2, H2O, CH4)
allow short wave EM in, but absorb longer wave EM that is re-radiated by Earth
• The Greenhouse Effect keeps Earth’s temperature warm enough to support life
Greenhouse Effect• Global Warming - an increase in
the Earth’s average temperature→Due to an increase in CO2 and methane
which traps re-radiated waves and raises Earth’s surface temperature
→Causes include burning of fossil fuels and deforestation
Variation of Insolation
• Insolation varies 2 ways→Intensity→Duration
Intensity of Insolation• Intensity of insolation -
amount of solar energy received by an area over a certain amount of time→Intensity of insolation received by an
area depends on the angle of incidence
Angle of Incidence→Intensity of insolation is the greatest
when the sun’s rays hit the surface of the earth at 90o
Angle of Incidence→At angles less than 90o the insolation
gets spread out over a larger area The same amount of energy is
spread out over a bigger area, so each part of that area receives less energy
Angle of Incidence
Angle of Incidence→As the angle of incidence increases,
the intensity of insolation increases
Angle of Incidence
Inte
nsity
of
Inso
latio
n
Intensity of Insolation→The sun’s rays are parallel to each
other as they approach Earth If the earth was flat, all insolation
would reach earth perpendicular to the surface (90o)
Since the earth is round the angle at which insolation hits the earth’s surface varies
Angle of Incidence→As the latitude increases, the angle of
incidence decreases
Latitude
Ang
le o
f In
cide
nce
Angle of Incidence→As the latitude increases, the
intensity of insolation decreases
Latitude
Inte
nsity
of
Inso
latio
n
Intensity of Insolation
Angle of Incidence• Only one location on earth receives
the sun’s energy at 90o at a given time→The location varies throughout the
yearDate Location
March 21June 21
September 23December 21
Intensity of Insolation• The angle of incidence varies for a
given location throughout the year→Angle is the highest on the day that
the sun is highest in the sky Most intense insolation = summer
solstice= June 21 for New York→Angle is lowest on the day that the
sun is the lowest in the sky Least intense insolation = winter
solstice = Dec. 21 for New York
Intensity of Insolation
June 21Summer Solstice in NYS
Dec 21Winter Solstice in NYS
Dec 21Winter Solstice in NYS
Intensity of Insolation• The angle of incidence varies for a
given location during the day→Angle is the highest a solar noon
Most intense insolation
Intensity of Insolation
Duration of Insolation• Duration of insolation - the length
of time between sunrise and sunset
• Duration of insolation varies for a given location throughout the year→Longest day = longest duration of
insolation June 21 in New York
→Shortest day = shortest duration of insolation December 21 in New York
Duration of InsolationSun’s path in summer in NYSJune 21
Sun’s path in winter in NYS
Dec 21
Duration of Insolation• Duration of insolation is usually
different at different locations on Earth→June 21 is the longest day in the
Northern Hemisphere, shortest day in the Southern Hemisphere
Duration of Insolation• Duration of insolation is 12 hours
all over the earth on the equinoxes March 21, September 23
Duration of Insolation
Insolation and Surface Temperatures
• The intensity of insolation and the duration of insolation together determine the surface temperatures for an area→The more intense the insolation, the
more energy an area receives, the higher the temperature
→The longer insolation is received at an area, the more energy absorbed, the higher the temperature
Insolation and Surface Temperatures
Insolation and Surface Temperatures
• The Earth heats up when it absorbs more energy than it radiates
• When the energy gained by an area is GREATER than the energy it radiates, its temperatures will rise
• When the energy gained by an area is LESS than the energy it radiates, its temperatures will fall
Minimums and Maximums of Temperature
• The highest temperatures of the year for an area tend to occur later than the time of maximum insolation
Minimums and Maximums of Temperature
Yearly• Highest surface temperature in
NYS: Late July – Early August→Day of maximum duration and
intensity of insolation occurs on June 21
→Why the delay? Earth is still receiving more energy than it is losing Heat surplus
Minimums and Maximums of Temperature
Yearly• Lowest surface temperature in
NYS: Late January – Early February→Day of minimum duration and
intensity of insolation occurs on Dec 21
→Why the delay? Earth is still receiving less energy than it is radiating Heat deficit
Minimums and Maximums of Temperature
Minimums and Maximums of Temperature
Daily• Solar noon is the time when the
sun is the highest in the sky for the day
• Mid-afternoon (~3pm) tends to be the time period for the highest temperatures for the day→Even though the sun is getting lower
in the sky, the earth is still receiving more energy than it is radiating (heat surplus)
Minimums and Maximums of Temperature
• When would you expect the lowest temperatures for the day to occur? Explain.
Minimums and Maximums of Temperature
Heat Budget and Climate Change
Heat budget• Heat budget - the result of the
balance between heat absorbed and heat lost→The temperature of an object
• For Earth - the balance between the energy gained from the sun and lost through radiation from Earth to space→Earth’s average temperature
Climate Change• If there is a change in the heat
budget - or average temperatures of the earth - global climate change can occur
• In the past the climate of the earth has changed dramatically due to changes in the Earth heat budget
Examples of Climate Change
• Ice Ages/Long Warm Periods→Ice Ages – Large glaciers covered large
parts of Earth’s surface during cold periods Most recent ice age - 10,500 years ago
→During cold periods the average sea-level is lower and coastlines are further out than their present location Much of the water is in the glaciers
→During warm periods coastlines flood and sea levels rise
Examples of Climate Change
• El Niño/La Niña→El Niño occurs when a shift in wind
patterns across the Pacific push warm waters toward the west coast of South America Causes major changes in weather
patterns all over the world–Can cause droughts, flooding,
tornadoes El Niño events usually occur every 5-10
years
Examples of Climate Change
• El Niño/La Niña→La Niña occurs when very cold waters
occur off the coast of South America Also cause changes in global weather
patterns
Examples of Climate Change
• Global Warming→The average temperature of the earth is
rising
Causes of Heat Budget ShiftsSolar Energy Changes• Sunspots are dark areas on the sun’s
surface• When there are a lot of sunspots, the
sun emits more electromagnetic energy = more insolation
Causes of Heat Budget ShiftsChanges in Shape of Earth’s Orbit and Tilt• There are changes that occur in the
eccentricity of Earth’s orbit and the tilt of Earth’s axis
May cause cooler summers and warmer winters
Possible cause of ice ages
Causes of Heat Budget Shifts• Changes in Earth’s Axis
→Earth rotates on an axis tilted at 23 1/2 degrees
→Since earth is not a perfect sphere, it “bobbles” like a top North pole not always pointed at the
same place Called precession
Causes of Heat Budget Shifts• Changes in Earth’s Axis
Causes of Heat Budget Shifts• Changes in Earth’s Axis
Causes of Heat Budget Shifts• Changes in Earth’s Tilt
→Earth’s tilt on its axis changes through time Varies between 22o and 24o
Currently at 23½o
→When the tilt is greater summers are warmer, winters are colder
Causes of Heat Budget Shifts• Volcanic Eruptions
→Volcanic eruptions cause temperature of Earth to cool
→When volcanoes erupt they throw ash that can stay in the atmosphere for months or years Called aerosols = low transparency
→Aerosols reflect insolation back into space and prevent it from reaching the earth’s surface Less energy = cooler temperatures
Causes of Heat Budget ShiftsHuman Causes• Desertification
→Over use of land in areas with deserts causes grasslands to turn into desert areas Deserts heat faster than grasslands =
higher temperatures
Causes of Heat Budget ShiftsHuman Causes• Deforestation – Trees removed from
areas →More insolation reaching Earth’s surface→Less transpiration
Less water in atmosphere = less precipitation
Causes of Heat Budget ShiftsHuman Causes• Urbanization – Areas changed from
rural areas to cities→More paved surfaces = more absorption
of insolation→Increase in greenhouse gases
Carbon dioxide
Seasons• The mid-latitudes experience
seasons→Winter, Spring, Summer, Fall
• Seasons are distinguished by changes in temperature, moisture, weather conditions and vegetation
• Regions near the equator have little seasonal changes
SeasonsDirect Causes of the Seasons• Cyclic variations in:
→Intensity of insolation Angle of Incidence
→Duration of insolation
• Seasons follow the North-South path of the direct rays of the sun
SeasonsIndirect Causes of the Seasons• Tilt of Earth’s Axis• Revolution of Earth Around Sun
Seasons
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