Download Saturation Vapor Pressure

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
Liquid to Vapor
540-590 cal
absorbed
80 cal absorbed
Solid to liquid
Solid to vapor
Vapor to liquid
Liquid to solid
Vapor to solid
Type of Heat
Latent heat of
vaporization
Latent heat of
fusion
680 cal absorbed
Latent heat of
sublimation
540-590 cal released Latent heat of
condensation
80 cal released
Latent heat of
fusion
680 cal released
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
-
Adiabatic lapse rates
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 encouters 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
Cirrus Clouds - High Clouds
• "Feathers" or "Wisps" or small "Puffs"
• almost always High clouds
• Usually 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
• occur at all elevations
• can be thick and rain/snow producing
- Stratus
clouds look
like a gray
sky
- these are
Nimbostratus
clouds which
produce rain
- these cumulus cloud examples
developed due to heating of
• puffy clouds with some
land which caused the air to
vertical development
heat and rise into cooler air
Cumulus
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
Orographic lifting creating a lenticular cloud
Precipitation
• includes rain,drizzle, freezing rain, snow,
hail, ice pellets etc.
• occurs when the weight of the water which
has condensed in the air overcomes the
influences of the winds keeping it aloft
- falls under the influence of Earth's Gravity
- velocity doesn't continue to increase
> air resistance slows it down as it falls
- Max Velocity called Terminal Velocity
> varies with size of the water droplet
• most precip forms by a processes known as
Bergeron process
Precipitation (2:2)
• Rain = drops > 0.5 mm
• Drizzle = droplets ~ 0.2 & 0.5 mm
• Freezing rain = liquid water
(sometimes supercooled) which falls
onto freezing (or near freezing)
surfaces
• Snow = ice crystal in dendrites,
plates, columns, or needles form
• Pellets = aka, sleet; water droplets
freeze
• Hail = big ice pellets that have been
"recycled" through a cloud several
times
Dew/Frost
• saturated air near the ground gets cooled
to the saturation point, and water collects
on any surface available
- Frost is frozen condensation
- Dew is liquid condensation
• Dewpoint is the temperature that the air must
be cooled to (under constant pressure) to reach
saturation and cause condensation
Fog
• Essentially low level clouds
• created when air is heated or
cooled in contrast to land or water
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