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Chapter 4
Moisture & Atmospheric Stability
Hydrologic cycle
1. Water is continually cycled between the
atmosphere, land & seas
2. Powered by the Sun
3. Plants absorb water through their roots, and
then release it into the atmosphere - called
transpiration
4. Water balance - precipitation is exactly
matched by evaporation & transpiration 380,000 cubic km per year. (would cover Earth
to depth of 1 m.)
H2O – A unique substance
• Readily converted from state to state
• Solid phase (ice) is less dense than the
liquid phase (water)
• Water expands as it cools through 4°C
• Water has an unusually high specific heat
• Unusual properties mostly due to water’s
ability to form hydrogen bonds
Water's Changes of State
1. Liquid, Solid, Gas
2. One calorie - the amount of heat required to
raise the temperature of one gram of water by
1 degree C
3. Latent Heat - heat energy that is transferred to
or from a body, thereby changing its state, but
not its temperature. (solid to liquid, liquid to
gas - ice to water, water to steam). Latent
means hidden.
Water's Changes of State - 2
1. Evaporation - the process of converting a
liquid to a gas (vapor). Latent heat of
vaporization must be supplied. Most energetic
molecules escape. Liquid cools.
2. Condensation - the process in which water
vapor changes to the liquid state. Latent heat
of condensation is released.
Water's Changes of State - 3
1. Melting - the process in which a solid
changes to a liquid. Requires heat.
2. Freezing - reverse of melting - releases
latent heat of fusion
Water's Changes of State - 4
1. Sublimation - conversion of a solid
directly to a gas
2. Deposition - reverse of sublimation.
Vapor converted directly to solid. (Frost)
Water in the Atmosphere
1. Humidity - describes the amount of
water vapor in the air
2. Absolute humidity - mass of water
vapor in a given volume of air:
mass of water vapor (gm) / volume of air
(cubic m) [mixed units]
Water in the Atmosphere - 2
1. Mixing ratio - mass of water vapor in a
unit of air, compared to the mass of the
dry air
mass of water vapor (gm) / mass of dry
air (kg)
Vapor Pressure & Saturation
1. Pressure - caused by change of
momentum of molecules as they hit a
surface
2. (Water) Vapor pressure - that part of
the total atmospheric pressure that is
due to its water vapor content
Vapor Pressure & Saturation - 2
1. Saturation -- number of molecules
evaporating = number condensing
2. Saturated vapor pressure - pressure
when the air is saturated (cannot hold
any more water)
Vapor Pressure & Saturation - 3
1. For every 10°C change in temperature,
amount of water needed for saturation
doubles
2. Rate of evaporation depends on
temperature and vapor pressure in the
air
Relative Humidity
1. Relative humidity is the ratio of the air's
actual water content to the amount of
water vapor required for saturation at
that temp & pressure
2. Same thing as RH = actual mixing ratio /
saturation mixing ratio
Relative Humidity - 2
1. Relative humidity can be changed by
adding/subtracting water or by changing
the temperature
2. If the water vapor content remains
constant, decreasing the temperature
increases the relative humidity
How to Change Relative Humidity
1. Add or subtract moisture
2. Increase or decrease the temperature
Dry Air at 100% Relative
Humidity?
• The RH is a measure of how much water
the air has in it, relative to the amount that
it could actually hold at that temperature.
• Cold air with 100% humidity (all the water
it can hold) can actually contain less water
than hotter air with a lower RH.
How Relative Humidity Changes
1. RH = (actual mixing ratio / saturated
mixing ratio) x 100%
2. Suppose mixing ratio at 20°C is 7g/kg
3. RH = (7g/kg / 14g/kg) x 100 = 50% (see
Table 4-1)
4. Suppose temperature rises to 20°C.
5. RH = (7g/kg / 26.5g/kg) x 100 = 26% decreases
How Relative Humidity Changes - 2
1. Suppose temperature falls to 1°C.
2. RH = (7 g/kg / 3.9g/kg) x 100 - increases,
but is > 100%!!!
3. What actually happens is that 7 - 3.9 =
3.1 gm condenses to form droplets
Humidifiers & Dehumidifiers
• In winter, the air is cold and cannot hold a lot of
water vapor. Water evaporates from the skin,
leading to dry skin. The solution is to add
moisture, with an humidifier.
• In summer, the air can be very humid (high RH),
preventing the body from cooling by sweating.
The solution is to remove moisture from the air
(dehumidifier).
Natural Changes in Relative
Humidity
1. Daily changes in temperature
2. Temperature changes that result as air
moves horizontally from one location to
another
3. Temperature changes as air moves
vertically
Dew Point Temperature
1. Temperature to which a parcel of air
would need to be cooled to reach
saturation
2. Dew point temperatures are shown on
weather maps
Dew Point Temperature - 2
1. Example - Air at 23°C contains 5 g
vapor/kg of air
2. Cooling air to 5°C (Table 4-1) will cause
saturation & condensation
3. Thus dewpoint is 5°C.
Dewpoint Temps
Sept 15, 2005
Measuring Humidity
1. Instruments are called hygrometers
2. Psychrometer - wet & dry bulbs, observe
temperature, look up tables
3. Hair hygrometer - not very useful
4. Electric hygrometer - electrical resistance
changes as humidity changes
Adiabatic Temperature Changes
1. Adiabatic - no heat lost or gained
2. Adiabatic cooling is the basis of cloud
formation
3. A rising parcel of air expands because the
atmospheric pressure decreases, and the
parcel cools adiabatically (the expanding
parcel has to do work)
Adiabatic Temperature Changes- 2
1. Unsaturated air cools at 10°C per km of
ascent - this rate is called the dry
adiabatic rate.
2. If the parcel of air rises enough, it will
cool to its dewpoint, and condensation
will occur.
3. The altitude at which this occurs is the
lifting condensation level.
Adiabatic Temperature Changes- 2
1. Above lifting condensation level, latent
heat is liberated and slows the rate of
cooling.
Rate is called the wet adiabatic rate.
5°C per km (high moisture content),
9°C per km (low moisture content)
Lifting Processes
1. Orographic lifting - air is forced to rise over a
mountainous barrier
2. Frontal wedging - warmer, less dense, air is
forced over cooler, denser air.
3. Convergence - a pileup of horizontal air flow
results in upwards movement
4. Localized convective lifting - unequal
surface heating heats localized pockets of air
Orographic Lifting
1. Lifting causes adiabatic cooling, clouds, rain
on windward side of mountains
2. Often get rain shadow deserts on leeward
side of mountains. Water is extracted from the
air on the windward side. Air on the leeward
size is warmer than on the windward side, and
therefore has a lower RH.
3. See Box 4-6
Orographic Effects
•
•
•
Causes of rain shadows.
Water is extracted from the air on the
windward side
Air on the leeward size is warmer than
on the windward side and therefore has
a lower RH
Rain shadow desert
Frontal Wedging
•
Warmer, less dense, air is forced over cooler,
denser air.
Convergence
1. Collision of air masses
2. Obstacle slows or restricts horizontal air
flow (wind)
3. Air moving from a relatively smooth
surface to a rougher surface
Localized Convective Lifting
• On warm summer days, unequal heating
of the Earth’s surface may cause pockets
of air to be warmed more than the
surrounding air
• Warm parcels of air (thermals) rise above
the lifting condensation level, and clouds
form
Rain Shadows & Chinooks
• Orographic lifting by mountains leads to
rain shadows.
• Air coming down has lost a lot of its water
vapor (as rain), and is heated.
• RH therefore falls.
• Chinooks are winds that move down the
eastern slope of the Rockies - snow eater
Atmospheric Stability
1. A rising air parcel cooler than surrounding air stable air - parcel tends to sink
2. A rising air parcel warmer than surrounding air
- unstable air - parcel tends to keep rising
3. Absolute Stability - environmental lapse rate
is less than wet adiabatic rate
Atmospheric Stability
1. Absolute Instability - environmental lapse
rate is greater than dry adiabatic rate
2. Conditional Stability - moist air has an
environmental lapse rate between the dry and
wet adiabatic rates. Air is stable with
unsaturated parcel of air, but unstable with a
saturated parcel of air.
3. "Conditional" because air must be forced
upward before it reaches the level where it is
unstable and rises on its own
Stability & Daily Weather
• Instability is enhanced by:
1. Intense solar heating warming the lower-most
layer of the atmosphere
2. The heating of an air mass from below as it
passes over a hot surface
3. General upward movement of air caused by
processes such as orographic lifting, frontal
wedging, convergence
4. Radiation cooling from cloud tops
Stability & Daily Weather - 2
• Stability is enhanced by:
1.Radiation cooling of Earth's surface after
sunset
2.The cooling of an air mass from below as
it passes over a cool surface
3.General subsidence within an air column
End Chapter 4
Chapter 4 Homework: GIST: 3, 6, 7 Problems: 1, 4