Transcript No Slide Title

GEOG. 1710 EARTH SCIENCE
LECTURE 7. ATMOSPHERIC MOISTURE AND
PRECIPITATION
Moisture in the atmosphere can be in a solid, liquid or
gaseous state.
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Water enters the atmosphere as WATER VAPOR (gas), via the
HYDROLOGICAL CYCLE. Water is turned into water vapor
by EVAPORATION (from oceans, moist soil etc.) and
TRANSPIRATION (from plants); together these processes are
termed EVAPOTRANSPIRATION. The change from liquid to
gas requires the absorption of heat energy - LATENT HEAT
OF EVAPORATION. This is usually absorbed from insolation,
surrounding soil etc. It follows that as temperature increases,
evaporation also increases;
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There is a limit to how much water vapor the air can hold
(absolute humidity), which depends on its temperature. If air
contains less water vapor than the maximum possible it is
UNSATURATED; if it contains the maximum possible for its
temperature, it is SATURATED.
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E.g. air containing 15 grams per kg at 300 C is unsaturated
air containing 15 grams per kg at 200 C is saturated
The air at 300 C can become saturated in two ways:
1. if more water vapor evaporates into it, until its water vapor
content becomes 27 g per kg;
2. if its water vapor content remains at 15 g per kg and its
temperature falls to 200 C.
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This brings us to a common measure of the air's humidity RELATIVE HUMIDITY = ratio of actual water vapor content
to the maximum possible amount that could exist if the air were
saturated.
E.g. the air containing 15 grams per kg at 300 C has a relative
humidity of 15/27 x 100% = 56%.
the air containing 15 grams per kg at 200 C has a relative
humidity of 15/15 x 100% = 100%.
In other words, for saturated air the relative humidity is 100%;
for unsaturated air the relative humidity is < 100%.
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Because the maximum amount of water vapor air can held
decreases with falling temperature, relative humidity increases
as temperature falls and decreases as temperature rises. E.g. as
the air containing 15 grams per kg at 300 C cools, its relative
humidity increases until finally it reaches 100% at 200 C. The
temperature at which air becomes saturated as it cools is called
the DEW POINT TEMPERATURE.
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What happens if air is cooled to or just below its dew point?
-> Condensation; the air can not hold any more water vapor so
some changes back into liquid water. This process is aided by
condensation nuclei - particles (dust, smoke) in the atmosphere
which act as collection centers for condensing water vapor. The
small water droplets typically form cloud droplets (i.e. a cloud is
a mass of liquid water droplets). When the water vapor changes
back
into
liquid
water,
the
LATENT
HEAT
OF
EVAPORATION is released back into the surrounding air.
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Examples of Condensation Processes
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1. Dew and frost: at night, ground cools by longwave radiation > air in contact with ground cooled -> saturation ->
condensation on exposed surfaces; if temperature > 0 -> dew; if
< 0 -> frost.
2. Fog: a. Radiation fog - on a still night, a whole layer of air
above the ground is chilled -> fog.
b. Advection fog - warm moist air moves over a colder surface > air cools, fog forms e.g. San Francisco.
c. Upslope fog - warm moist air flows up over a mountain ->
adiabatic cooling -> fog.
d. Evaporation fog - moisture evaporates into air that becomes
saturated and fog forms – common in swampy areas.
e. Valley fog – cold air sinks into valleys/low spots -> fog.
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Advection fog
Evaporation fog
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3. Cloud: as air rises through the atmosphere it undergoes adiabatic cooling > saturation -> cloud. Why does air rise? ->
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a. convective uplift - warm sunny day, unequal heating.
b. orographic uplift - note rain shadow effect; descending air
does not produce clouds.
c. frontal uplift - less dense warm air is forced to ride up over
more dense cool air when they meet
d. convergent uplift - air forced up by convergence from
surrounding area.
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Adiabatic processes
Air warmer than surrounding air will rise (less dense). How far does air have to rise
before it reaches its dew point and a cloud forms? This depends on:
(i) the water vapor content of the air and
(ii) the rate of adiabatic cooling. The rate of adiabatic cooling for unsaturated air is
fairly constant at about 10o C per km - this is the DRY ADIABATIC RATE (DAR).
The height at which the rising air reaches its dew point temperature and cloud
begins to form is the LIFTING CONDENSATION LEVEL. Once the air becomes
saturated it will continue to rise if it is still warmer than the surrounding air;
however, it will now cool at a slower rate due to the release of the LATENT HEAT
OF EVAPORATION during condensation - this rate, about 6o C per km, is the
MOIST ADIABATIC RATE (MAR). The air will keep rising until it attains the
same temperature as the surrounding air - the temperature of the surrounding air is
called the ENVIRONMENTAL LAPSE RATE (this refers to the fact that air is
normally colder with height – a typical value is about 6.4o C per 1,000 m, but this
rate can vary by a few degrees from place to place and over time).
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Atmospheric Stability
As
mentioned
previously,
air
warmer
than
its
surroundings will rise because it is less dense or buoyant. Such
air is said to be UNSTABLE, because it rises on its own accord.
Unstable air occurs where the air's adiabatic lapse rate is less
than the environmental lapse rate (i.e. the air remains warmer
than the surrounding air as it rises). If the air's adiabatic lapse
rate is more than the environmental lapse rate, it will be
STABLE, because it will become colder than the surrounding
air and stop rising.
CONDITIONAL INSTABILITY occurs if the average lapse
rate is between the dry and moist adiabatic lapse rates - the air
will be stable if unsaturated and unstable if saturated.
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unstable
stable
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Instability is often apparent by the presence of clouds of
pronounced vertical extent (indicating vigorous vertical air
movement) e.g. cumulus type clouds. Stable air can also produce
clouds if it is forced to rise (e.g. over a mountain) - in this case
thin layer clouds are produced (stratiform clouds).
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Precipitation Processes
Why don't all clouds produce precipitation? - cloud droplets are
so small that they are kept aloft by turbulence; the droplet must
grow a lot bigger to fall to Earth. How does this occur?
1. Ice Crystal Growth: the tops of many clouds are below
freezing. Supercooled cloud droplets and ice crystals can both
be present. Since water vapor is more strongly attracted to
ice than water, it is possible for water vapor to evaporate
from the cloud droplets and condense onto the ice crystals at
the same time.
2. Collision-Coalescence: As slightly larger droplets fall faster,
or as droplets are swept around by turbulence, collisions
occur and droplets stick together. Eventually, the drop grows
large enough to fall.
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Cloud Classification: Based on shape and height. Shapes:
1. Cirriform - thin, wispy clouds composed of ice crystals; found only
at high elevations.
2. Stratiform - thin sheets or layers, usually cover whole sky.
3. Cumuliform - puffy masses of cloud, non-continuous.
Heights:
A. High clouds - > 6000 m e.g. cirrus, cirrocumulus, cirrostratus
B. Middle clouds – 2000 – 6000 m e.g. altocumulus, altostratus
C. Low clouds - < 2000 m e.g. stratus, cumulus
D. Towering clouds – reach from low to high – usually cumulonimbus
The root "nimb" indicates that precipitation is falling from the
cloud e.g. nimbostratus, cumulonimbus.
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