The atmosphere’s water - MSU Billings 2009/Suits/Geog... · Atmospheric Moisture and...
Transcript of The atmosphere’s water - MSU Billings 2009/Suits/Geog... · Atmospheric Moisture and...
The atmosphere’s water
Atmospheric Moisture and Precipitation
• Properties of Water
• The Hydrosphere and the Hydrologic Cycle
• Humidity
• The Adiabatic Process
• Clouds
• Precipitation
• Air Quality
Main points for today
• Water’s properties are due to its polar nature
•Water has a very high heat capacity and high heat of fusion
• Warm air can contain more water vapor than cold air
• Relative Humidity and Dew Point Temperature are the most
common ways to express how much water is in the air
•The Environmental Lapse rate is the observed change in air
temperature with change in elevation
• An Adiabatic Process is the expansion or contraction of an air
mass without heat loss or gain
Main points for today (cont)
• Clouds are classified by altitude and shape
• Precipitation can occur because of 1) convection, 2)
orographic lifting, and/or 3) frontal processes
• Air Quality
majority of water occurs as ocean saltwater
remaining water (only 2.8%) is accounted for by glaciers and ice sheets,
and groundwater
only 0.001% occurs in the atmosphere
The Hydrosphere and the Hydrologic Cycle
Figure 4.2, p. 121
the global water
balance
constantly
cycles between
these reservoirs
while the
atmosphere
contains
relatively little
water, it is
responsible for
the largest flow
The Hydrosphere and the Hydrologic Cycle
Figure 4.3, p. 122
Properties of Water• Physical States
– only natural substance that occurs naturally in three states on the earth’s surface
• Heat Capacity
– Highest of all common solids and liquids
• Surface Tension
– Highest of all common liquids
• Latent Heat of Fusion
– Highest of all common substances
• Compressibility
– Virtually incompressible as a liquid
• Density
– Density of seawater is controlled by temperature, salinity and pressure
– Liquid has maximum density at +4oC; solid phase has lower density!
Properties of Water (cont’)
• Radiative Properties
– transparent to visible wavelengths
– virtually opaque to many infrared wavelengths
– large range of albedo possible
• water 10 % (daily average)
• Ice 30 to 40%
• Snow 20 to 95%
• Cloud 30 to 90%
Molecular Structure of Water
Water's unique molecular structure and hydrogen bonds enable all 3 phases to exist in earth's atmosphere.
water moleculeice
Molecular Structure of Water
Water's unique molecular structure and hydrogen bonds enable all 3 phases to exist in earth's atmosphere.
ice
Water can exist
in three states -
solid (ice), liquid
(water), and gas
(water vapor)
Water exists in
the air in the form
of water vapor,
clouds, fog, and
precipitation
Three States of Water
Figure 4.1, p. 121
Energy associated with phase change
80 calories 100 calories 540 calories!
Why does it take so much energy to
evaporate water?
• In the liquid state, adjacent water molecules attract
one another
– “-” charge on O attracted to “+” charge on H
– we call this hydrogen bonding
• This same hydrogen bond accounts for surface
tension on a free water surface
– column of water “sticks together”
Water vapor pressure
• Molecules in an air parcel all contribute to pressure
• Each subset of molecules (e.g., N2, O2, H2O) exerts a partial
pressure
• The VAPOR PRESSURE, e, is the pressure exerted by water
vapor molecules in the air
– similar to atmospheric pressure, but due only to the water vapor
molecules
– often expressed in mbar (2-30 mbar common at surface)
Water vapor saturation• Water molecules move
between the liquid and gas phases
• When the rate of water molecules entering the liquid equals the rate leaving the liquid, we have equilibrium
– The air is said to be saturated with water vapor at this point
– Equilibrium does not mean no exchange occurs
How do we express the amount of water vapor in
an air parcel?
• Absolute humidity
– mass of water vapor/volume of air (g/m3)
– changes when air parcel volume changes
• Specific humidity
– mass of water vapor/mass of air (g/kg)
• Mixing ratio
– mass of water vapor/mass of dry air (g/kg)
• Specific humidity and mixing ratio remain constant as long as water vapor is not added/removed to/from air parcel
• Dew point temperature– the temperature at which the air parcel would be saturated
Expressing the water vapor pressure
• Relative Humidity (RH) is ratio of actual vapor pressure to saturation vapor pressure
– 100 * e/eS
– Range: 0-100% (+)
– Air with RH > 100% is supersaturated
• RH can be changed by
– Changes in water vapor content, e
– Changes in temperature, which alter eS
Humidity Facts
• Humidity is the amount of water vapor in the
atmosphere
• Warm air can hold much more than cold air
• Cold dry air can have close to 0%
• Warm tropical air may have 4-5%
• Two ways to describe humidity (specific humidity and
relative humidity)
Specific Humidity
the actual quantity
of water vapor in
the air
expressed as
grams of water
per kilogram of air
(g/kg)
used to describe
the water content
of large air
masses, and how
it varies by latitude
Figure 4.5, p. 124
Relative Humidity
• a measure of the amount of water vapor
present in air relative to the maximum amount
that the air can hold at a given temperature (%)
• e.g. if relative humidity is 50%, then it contains
1/2 the amount of water vapour it could hold at
a given temperature
• relative humidity decreases as temperature
increases
Relative Humidity and Temperature
if no water vapour is added or removed from the air mass,
then relative humidity decreases as temperature increases
Figure 4.7, p. 125
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Dew Point Temperature
• as air is cooled it eventually becomes saturated (100% relative humidity)
• the temperature of saturation is called the dew point temperature
• if cooling continues, condensation begins and dew forms
Relationship between eS and T
• The saturation vapor pressure of water increases with temperature
– At higher T, faster water molecules in liquid escape more frequently causing equilibrium water vapor concentration to rise
– We sometimes say “warmer air can hold more water”
• There is also a vapor pressure of water over an ice surface
– The saturation vapor pressure above solid ice is less than above liquid water
Water vapor saturation and atmospheric
pressure; at what Temperature does water boil
Dewpoint Temperatures
• Dewpoint temperature is a measure of the water vapor content of the air• It is not a measure of temperature!
The Adiabatic Process• when a gas expands, its volume increases and its
pressure and temperature decrease
• air temperature change solely as a result of air
expansion or contraction is a result of the adiabatic
process
• the adiabatic lapse rate is used to quantify how the
temperature of air decreases as it rises or increases
as it ascends
• lapse rates differ for dry (unsaturated) and wet
(saturated) air masses
Dry Adiabatic Lapse Rate
Dry Adiabatic
Lapse Rate
(DALR)
- decrease in
temperature
with altitude:
10˚C/1000m
Figure 4.10, p. 127
Wet (Saturated) Adiabatic Lapse Rate
• ranges from 4 to 9 degrees C per 1000
meters
• varies because it depends on temperature,
pressure and water vapour content
• less than the DALR because as water
condenses it releases latent heat, so the
temperature decrease is less
Wet Adiabatic Lapse Rate
as a parcel of air rises, it cools and becomes saturated at the dew point
dew point lapse rate (1.8 degrees per 1000 meters) means that the dew point of the air parcel decreases as the air rises
when it reaches its dew point, condensation occurs (lifting condensation level)
Figure 4.10, p. 127
Water vapor is distributed
throughout the atmosphere
• Generally largest amounts are found close to
the surface, decreasing aloft
– Closest to the source - evaporation from ground,
plants, lakes and ocean
– Warmer air can hold more water vapor than colder
air
Stability & Instability
A rock, like a parcel of air, that is in stable equilibrium will return to its original position when pushed.
If the rock instead accelerates in the direction of the push, it was in unstable equilibrium.
Stability in the atmosphere
Stable Unstable Neutral
If an air parcel is displaced from its original height it can:
Return to its original height - Stable
Accelerate upward because it is buoyant - Unstable
Stay at the place to which it was displaced - Neutral
An Initial
Perturbation
Vertical Motion and Temperature
Rising air
expands, using
energy to push
outward against its
environment,
adiabatically
cooling the air
A parcel of air
may be forced to
rise or sink, and
change
temperature
relative to
environmental air
“Environmental Lapse rate”
• The lapse rate is the change of temperature with height in the atmosphere
• There are two kinds of lapse rates:
– Environmental Lapse Rate (~ 4°/1000 m)
• What you would measure with a weather balloon
– Parcel Lapse Rate
• The change of temperature that an air parcel would experience when it is displaced vertically
• This is assumed to be an adiabatic process (i.e., no heat exchange occurs across parcel boundary)
The Adiabatic Process• when a gas expands, its volume increases and its
pressure and temperature decrease
• air temperature change solely as a result of air
expansion or contraction is a result of the adiabatic
process
• the adiabatic lapse rate is used to quantify how the
temperature of air decreases as it rises or increases
as it ascends
• lapse rates differ for dry (unsaturated) and wet
(saturated) air masses
Dry Adiabatic Lapse Rate
Dry Adiabatic
Lapse Rate
(DALR)
- decrease in
temperature
with altitude:
10˚C/1000m
Figure 4.10, p. 127
Wet (Saturated) Adiabatic Lapse Rate
• ranges from 4 to 9 degrees C per 1000
meters
• varies because it depends on temperature,
pressure and water vapour content
• less than the DALR because as water
condenses it releases latent heat, so the
temperature decrease is less
Wet Adiabatic Lapse Rate
as a parcel of air rises, it cools and becomes saturated at the dew point
dew point lapse rate (1.8 degrees per 1000 meters) means that the dew point of the air parcel decreases as the air rises
when it reaches its dew point, condensation occurs (lifting condensation level)
Figure 4.10, p. 127
Conditionally unstable air
• What if the environmental lapse rate falls between the moist and dry adiabatic lapse rates?
– The atmosphere is unstable for saturated air parcels but stable for unsaturated air parcels
– This situation is termed conditionally unstable
• This is the typical situation in the atmosphere
Clouds
• Made up of water droplets and/or ice particles
• form when air is saturated AND contains
particles (condensation nuclei) e.g. dust, salts
• water can remain in liquid state below freezing
(supercooled) to as low as -12°C (10°F)
high (eg. cirrus)
middle (eg. altocumulus)
low (eg stratus, cumulus)
Clouds types - classified by altitude
thunder cloud (cumulonimbus)
extends from low to high
Figure 4.11, p. 128
Fog
• cloud layer at or close to the Earth’s surface
• radiation fog forms at night when air near the ground falls below the dew point temperature
• advection fog forms when warm moist air moves over a cool surface
• sea fog forms when cool marine air comes in contact with cold ocean currents
Precipitation
Precipitation formation requires:
• growth of droplets in clouds
• ice crystal process - ice particles act as freezing nuclei
• coalescence process - large droplets collide with smaller ones and coalesce
• produced in clouds well below the dew point temperature
• usually near cloud tops
• all precipitation begins as frozen water
• if it reaches the ground in liquid form -rain, drizzle (small drops)
Precipitation
Types of Precipitation:
• freezing rain (ice crystals freeze onto a frozen surface)
• snow (ice crystals have not melted)
• sleet (ice crystals melt as they fall)
• hail (melting and refreezing crystals that form in thunder clouds)
Precipitation: three mechanisms
Convectional
Orographic
Frontal (cyclonic)
1.) Convectional
warm air rises
cools to dew point
temperature - clouds
form
latent heat release
adds energy and
increases updraft
may produce
thunderstorms
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2.) Orographic lifting
air rising over a highland
intercepting slope =
windward slope (wetter)
change in temperature
alters the humidity
leeward
slope (drier)
(rain shadow)
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where air masses with different temperatures come together
warm air lifted by cold dense air along a weather front
leads to frontal precipitation
3.) Frontal (cyclonic) precipitation
Cold air Warm air
Thunderstorms are
intense convectional
storms associated
with massive
cumulonimbus clouds.
They may produce
heavy rains, hail,
thunder, lightening,
and intense
downdrafts
(microbursts) which
may create hazards
for humans
Figure 4.21, p. 137
Air Quality
Air pollutants are undesirable gases, aerosols, and particulates injected into the atmosphere by human and natural causes
Figure E4.1, p. 143
Smog over China
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Cloud type summary
High Clouds• High clouds
– White in day;
red/orange/yellow at sunrise
and sunset
– Made of ice crystals
– Cirrus
• Thin and wispy
• Move west to east
• Indicate fair weather
– Cirrocumulus
• Less common than cirrus
• Small, rounded white puffs
individually or in long rows
(fish scales; mackerel sky)
– Cirrostratus
• Thin and sheetlike
• Sun and moon clearly
visible through them
• Halo common
• Often precede
precipitation
Cirrus
Cirrus
Cirrus Display at Dawn
Cirrocumulus
Cirrocumulus
Cirrocumulus at Sunset
Cirrostratus
Cirrostratus with Halo
Contrails
Middle Clouds
• Altocumulus
– <1 km thick
– mostly water drops
– Gray, puffy
– Differences from cirrocumulus
• Larger puffs
• More dark/light contrast
• Altostratus
– Gray, blue-gray
– Often covers entire sky
– Sun or moon may show through dimly
• Usually no shadows
Altostratus
Alto Stratus Castellanus
Altocumulus
Altocumulus
Alto Cumulus Radiatus
Alto Cumulus
Alto Cumulus Undulatus
Low Clouds
• Stratus
– Uniform, gray
– Resembles fog that does not reach the ground
– Usually no precipitation, but light mist/drizzle possible
• Stratocumulus
– Low lumpy clouds
– Breaks (usually) between cloud elements
– Lower base and larger elements than altostratus
• Nimbostratus
– Dark gray
– Continuous light to moderate rain or snow
– Evaporating rain below can form stratus fractus
Stratus fractus
Looking down on an eastern
Atlantic stratus deck
Stratiform cloud layers
Stratocumulus cloud streets
Stratus undulatus
Stratus
A Layer of Stratocumulus
Cloud viewed from above
Vertically
“developed”
clouds• Cumulus
– Puffy “cotton”
– Flat base, rounded top
– More space between cloud elements than stratocumulus
• Cumulonimbus
– Thunderstorm cloud
– Very tall, often reaching tropopause
– Individual or grouped
– Large energy release from water vapor condensation
Cumulonimbus with Pileaus caps
Cumulonimbus Clouds Spawn Tornadoes
The Earth’s Hydrologic Cycle