Chapter 4
Moisture in the Atmosphere
Water on the Earth has three common
states
solid, liquid, and vapor
Each with its own unique properties
Earth is the only planet we know of that
has all three forms
Phase Change
Heat Transfer
Type of Heat
Liquid to Vapor
540-590 cal
absorbed
Solid to liquid
80 cal absorbed
Solid to vapor
680 cal absorbed
Vapor to liquid
Liquid to solid
540-590 cal
released
80 cal released
Vapor to solid
680 cal released
Latent heat of
evaporation
(vaporization)
Latent heat of
melting
Latent heat of
sublimation
Latent heat of
condensation
Latent heat of
fusion
Latent heat of
sublimation
Rain, ice, snow
The water cycle
Distribution of
water within the
hydrological
cycle
Atmospheric moisture
･ Water in the atmosphere
･ Requires
- vapor pressure- the amount of pressure
contributed by any volatile substance
> e.g., water
- air capable of "holding" vapor
> dependent on temperature
･ Evaporation- more water is becoming vapor than is
condensing (becoming liquid)
･ Condensation
- opposite effect
Relative humidity (RH)
• Water in atmosphere is dependent on temp.
• Saturation Vapor Pressure=
- the maximum amount of water vapor that air
can "hold"
- temperature dependent
- warm air "holds" more than cold air
RH =
Vapor Pressure
Saturation Vapor Pressure
• measured by a number of devices
Saturation
vapor pressure
vs temperature
Condensation phenomena
• As RH goes to 100% water vapor condenses
- i.e., it changes from vapor to liquid or solid
• forms clouds, rain, snow/ice, fog, dew
Releases latent heat stored during vaporization
Condensation Factors
• must get air mass to reach saturation (approx)
- accomplished by lifting & cooling, cooling, or
increasing amount of water being vaporized
• usually have to have something for the water
to condense onto...such as:
- aerosols -dust particles and large molecules
Environmental lapse rate
6.5° C per 1000 m =
Avg. lapse rate
dependent upon the local environmental conditions
i.e., empirical = derived by measuring the avg temp of
the air mass at the surface (TS) and at the top of the
troposphere (TT) and the elevation difference between
surface and troposphere (HST)
(( TS - TT ) / (HST))
Different from another more important lapse rate called
Adiabatic lapse rate
Cloud formation (1:2)
• lifting of an air mass cooling due to
adiabatic process
- ADIABATIC - no energy lost or gained by
exchanging with air that has different
characteristics
- explain this process...
Adiabatic lapse rates
(2:2)
Dry lapse rate (not at saturation)
> -10° C per 1000 meters (-5.5 F / 1000 ft)
- Wet lapse rate (at saturation point)
> Heat (latent) gained as water condenses
> air does not cool as fast
> -6° C per 1000 meters (-3.3 F / 1000 ft)
Means of lifting
• Heating (aka convectional lifting)
- warmed air rises (can also
have a lot of water vapor)
• Orographic Lifting (mountains)
- air encounters a barrier and
goes over the top of it
• Frontal Lifting (air masses
with different densities
- cool air is more dense than
warm air
> slides underneath warm
air, lifting it
Frontal lifting
• Warm Front
• Cold Front
Cloud terminology is descriptive
Based on cloud form or shape
• Cirrus = feathered or wispy
• Stratus= layered
• Cumulus=puffy
also linked to elevation
• low, middle, high, and vertically developed
- Alto = middle
also linked to precipitation
• nimbo (-us) = rain
sometimes linked to temperature
• warm vs. cold clouds
Divided into 10 cloud Genera with corresponding
species within each Genus
Cirrus Clouds - “hair-like” High Clouds
• "Feathers" or "Wisps" or small "Puffs"
• always High clouds- may be used a prefix
• Composed of Ice crystals
- a "mackerel"
sky with small,
puffed, cirrus
clouds known as
cirrocumulus
- and almost
layered
cirrostratus
clouds
Perspective is everything
• from below, the low clouds look like stratus
• from above they look like cumulus
• in reality they are some combination of both!
- three layers
are visible
here
- cirrus
> v. high
- altocirrus
> middle
- stratocumulus
> lower
Stratus Clouds
•
•
•
Layered clouds
Can occur at all altitudes
Can produce precipitation
• Stratus
clouds
produce dull
gray looking
skies
• These clouds
are
nimbostratus
- layered
and producing
rain
Cumulus
Puffy clouds with vertical development
Clouds on the right -lifting due to a cold front
Those on the left lifting due to heating and orographic effects
Cumulonimbus Clouds
• continued vertical development will eventually lead to
Cumulonimbus clouds
- these produce heavy local rains, strong winds, and
thunderstorms
Characterized by a tall, often flat-topped, puffy
•
cloud form
This slide is a movie and is not available online
Orographic lifting
creating a lenticular
cloud
Turbulance creating a
lenticular cloud
Rainbow- rain acts as a
prism, splitting white
sunlight into its spectral
colors
Virga- rain that evaporates before
reaching the ground
Mammatus clouds
Wall Cloud with lightning
Altocumulus
Castelanus- aka
“jellyfish” clouds
Noctilucent Clouds
“night glow” clouds
Precipitation
Includes rain, drizzle, freezing rain, snow sleet
hail, ice pellets, etc.
Occurs when the weight of the condensed water
becomes too great to keep it aloft
Falls under the influence of Earth’s gravity
• Velocity doesn’t continue to increase as it
falls
• Air resistance (friction) slows it down
• Achieves a stable speed called the terminal
velocity
• Varies according to the size of the
droplet/crystal
Most precip forms by a process called the
Bergeron Process
Dew/Frost
Saturated air near the ground gets cooled to
the saturation point
Water collects (condenses) onto any surface
available
• Frost means the temp is below freezing
point (32°F
• Dew means temp is above freezing
Dew point is the temp to which the air must
be cooled in order to force condensation to
occur
• happens under CONSTANT PRESSURE