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Water and the Atmosphere
-water exists on Earth in all 3 states of matter
-over 98% of all water on Earth in liquid state
-mostly in the ocean
-overall atmosphere doesn’t contain much water vapor
-water vapor in the atmosphere
1. Contributes to the greenhouse effect making
the planet warmer
2. Serves as one principal in weathering and
erosion of the land
3. Maintaining life for all organisms
-cycling of water vapor in and out of the atmosphere makes
it all possible
-Evaporation and Condensation
-water can undergo liquid to gas or gas to liquid phase change
at any temperature in either direction
-Evaporation—more molecules are leaving than are
returning
-Condensation—more molecules are returning than
are leaving
-if the air were perfectly dry and still more molecules would
evaporate than would condense
-an equilibrium condition between evaporation and
condensation occurs in saturated air
-Saturated air—occurs when process of evaporation
and condensation are in balance
-air will remain saturated as long as
1. Temperature remains constant
2. Process of evaporation and condensation remain
balanced
-increase and decrease in temperature mean an
increase and decrease in the kinetic energy of
water molecules
-decrease in kinetic energy of water molecules means
its more likely for water molecules to come
together and form water droplets
-increase in kinetic energy of water molecules means
its less likely for water molecules to come
together and form water droplets
-if temperature increases in an equilibrium
condition more water vapor must be added to
maintain the saturated condition
-warm air can hold more water vapor than
cooler air
-warm air on a typical summer day can hold
5 times as much water vapor as cold air on
a cold winter day
-Humidity—amount of water vapor in the air
-damp, moist air more likely to have condensation
-dry air more likely to have evaporation
-Absolute Humidity--measurement of the amount of
humidity at a particular time
-15 g of water vapor per cubic meter of air
-the maximum absolute humidity is determined by the
temperature of the water vapor (the kinetic
energy of the water vapor)
-Relative Humidity--the relationship between the
actual absolute humidity at a particular temperature and the
maximum absolute humidity at that temp.
-ratio between
1. Amount of water vapor in the air
2. Amount of water vapor needed to saturate the air at
that temperature
-Relative humidity=absolute hum. At present tempx100
max.absolute humidity at present
temp.
-ex. 5 g/m3 of water vapor at 10C (50F)
-maximum possible at 10C (50F) is
10g/m3
-5/10 x 100=50%
-Condensation Process
-Condensation depends on 2 factors
1. Relative Humidity
2. Temperature of the air
-molecules of water join together to form liquid water
on the surface as dew or in the air as droplets of water
making up clouds or fog
-may also form solid water in the form of frost or
snow
-before condensation can occur air must be saturated
-relative humidity must be 100%
-parcel of air can become saturated by
1. Adding water to the air by evaporation
2. Cooling, which reduces the capacity of the air to
hold water vapor
3. Combo. Of 1 and 2
-Dew Point—temperature at which condensation begins
-if dew point is above 0C the water vapor will
condense as a liquid (dew)
-if dew point is below 0C the water vapor will
condense as a solid (frost)
-Dew and frost often seem to form on calm,
clear, and cool nights
-Dew and frost also seem to form
1. In open areas instead of under trees and
shelter
2. On objects such as grass rather than flat
bare ground
3. In low-lying areas before they form
on slopes or the sides of hills
-Condensation in air
-water vapor molecules need something to condense
upon
-Condensation nuclei—tiny particles present in the air
upon which water vapor condenses
-salt crystals especially effective
-after water molecules begin to condense on a condensation
nucleus other water molecules will join the liquid water already
formed and a tiny droplet begins to increase in volume
-water droplets that make up a cloud are about
1500 times larger than a condensation nucleus
-Fog and Clouds
-fog and clouds both form when air containing water vapor and
condensation nuclei has been cooled to the dew point
-Fogs form
-calm,clean, and cool nights with high
relative humidity
-may form in humid air over oceans and move
inland
-temporary fog over melting snow, over a body of
water in cold air, or over streets after a summer shower
-Sun does not “burn off” fog
-sun warms air increasing the air
capacity to hold water vapor
-relative humidity decreases and
fog evaporates back into air
-Clouds form:
-when a mass of air above the surface is cooled to
its dew point temperature
-in general, the mass of air is cooled because
something has given it an upward push
-3 major causes of upward air movement
1. Convection resulting from
differential heating
2. Mountain ranges that serve as barriers
to moving air masses
3. The meeting of moving air masses with
different densities
-cold dense air mass meeting a warm
less dense air mass
-upward movement of air may or may not result in
cloud formation—depends on atmospheric cond
-For dry air, the atmosphere is in a state of stability when a
Lifted parcel of air is cooler than the surrounding air
-being cooler it will be more dense than the
surrounding air
-if moved to a higher level and released in a stable environment
it will move back to its original level when the
atmosphere is stable
-stratus clouds form here
-Atmosphere is in a state of instability when a lifted parcel
Of air is warmer than the surrounding air
-being warmer, the parcel will be less dense than the
surrounding air
-if moved to a higher level it will continue moving after the
uplifting force is removed
-cumulus clouds usually form in an unstable
atmosphere
-as air moves upward eventually the dew point is
reached and the air becomes saturated
-as water vapor condenses forming droplets the
rising parcel of air cools at a slower rate
-condensation causes release of latent heat of
vaporization
-warms air parcel and accelerates
the ascent
-leads to further condensation
and the formation of towering cumulus
clouds often leading to rain
Precipitation
-water that returns to the surface of the Earth
-dew and frost form directly on the surface
-not precipitation
-Forms in clouds by 1 of 2 processes
1. The coalescence of cloud droplets
2. The growth of ice crystals
-coalescence of droplets thought to form in warm cumulus
clouds that form near the ocean in tropics
-ice crystals form in clouds that extend high enough in the
atmosphere to be above the freezing point of water
-ice crystals grow large enough to eventually fall to the
surface
-during the summer they melt as they fall
through warmer air
-during the winter they stay frozen and
fall as snow
-Cumulus clouds usually produce showers or
thunderstorms that last a brief time
-”piled up”
-Stratus clouds usually produce longer periods of
drizzle,rain, or snow
-”spread out”
-Cirrus clouds don’t produce precipitation
-may have meaning about coming weather
-”curly”
Weather Producers
-The shift of weather is related to 3 related weather producers
1. Movement of large bodies of air called air masses, that
have acquired the temperature and moisture
conditions where they have been located
2. The leading fronts of air masses when they move
3. The local high and low pressure patterns that are
associated with air masses and fronts
Air Masses
-large, more or less uniform body of air with nearly the
same temperature and moisture conditions
-forms when a large body of air stays over an area of land or
water for an extended period of time
-tend to retain the temp. and moisture characteristics of where they
were located and can move long distances
-classified according to temperature and moisture
conditions where they originate
-2 extreme possibilities for temp.
1. Polar air mass
2. Tropical air mass
-2 extreme possibilities for moisture
1. Maritime air mass—moist and formed over
ocean
2. Continental air mass—dry and formed over
land
-therefore there are 4 types of air masses
1. Continental Polar
2. Maritime Polar
3. Continental Tropical
4. Maritime Tropical
-when an area is under the influence of an air mass the location
is experiencing a period of air mass weather
-weather conditions will generally remain the same
from day to day with gradual changes
-will remain the same until a new air mass moves
in or until the air mass acquires the
conditions of the new location
-may take days or several weeks
-Weather Fronts
-boundary between air masses of different temperatures
-thin transition zone between 2 air masses that range
from 5-30 km wide
-air masses don’t mix except in this narrow zone
-Cold Front—formed when a cold air mass moves into
a warm air mass, displacing it in the process
-generally steep and forces the warm air mass
to rise quickly
-if warm air is moist, the dew point temp. is
quickly reached resulting in large towering cumulus
clouds and thunderclouds along the front
-thunderstorms often form along the front
-can be intense, but are usually over quickly
and are soon followed by rapid drop in
temp.
-passage of a cold front also marked by rapid shift in
wind direction and rapid increase in
barometric pressure
-Warm front
-forms when a warm air mass advances over a
mass of cooler air
-since its less dense than the cooler air mass
it’s displacing it generally overrides the cooler air
forming a long gently sloping front
-may produce high cirrus clouds a day or more
in advance of the front
-followed by thicker and lower stratus
clouds
-usually result in broad band of drizzle,
fog, and continuous rain
associated with stratus clouds
-light rain (and snow in winter) may last
for days as warm front passes
-Stationary front
-sometimes forces influencing the movement of cold or
warm air mass lessen or become balanced and
the front stops advancing
-stream of cold air moves along the north side of
the front and a stream of warm moves along the
south side in the opposite direction
-represents an unstable situation that can
result in a major atmospheric storm