Water in the Atmosphere
I. Atmospheric Moisture
Water exists on Earth
in 3 forms:
 Liquid
 Solid (ice)
 Gas
 In
our atmosphere, water
exists mainly in its gaseous
form: water vapor
 What
is the principal source
of water vapor in Earth’s
atmosphere?
 The
oceans!
A. Humidity
 Humidity:
the amount of water
vapor in the atmosphere
 Saturated:
when the air is
holding all the water vapor it
can at a given temperature
 As
the air temperature
increases, what
happens to the
amount of water
vapor that volume
of air can hold?
Warmer air can hold more water
vapor than cold air
1. Relative Humidity: ratio of the
amount of water vapor in the air to
the amount it can hold when
saturated.

Psychrometer: instrument
used to measure relative
humidity.
Sling
Electronic
What does it mean to say
the air is saturated?
It cannot hold any more
water!
What happens when the
air becomes saturated?
Fill in the blank…
The higher the relative
humidity, the
_______________ the
chance that water vapor
will condense into rain or
snow.
When a certain volume of
air is saturated, what is
its relative humidity?
100%
As outside temperatures
increase during the day,
what happens to relative
humidity?
Hygrometer
RH decreases (with
increasing temperatures)
If outside temperatures
stay the same or
decrease, what happens
to relative humidity?
RH increases (greater
chance of precipitation)
with decreasing
temperatures
2. Specific Humidity
 The
actual amount of moisture
in the air.
High
Low
B. Dew Point
 The
temperature to which air
must be cooled to reach
saturation
 At
any temperature lower than
the dew point, water vapor
begins to condense
What
happens during
condensation?
 Water
vapor changes to
liquid water
Dew: air contacts a cool surface
and loses heat until it reaches
saturation
Frost: if dew point falls
below freezing, water vapor
changes directly to solid ice
crystals, or frost

II. Clouds

Clouds are visible masses of liquid
water droplets suspended in the
atmosphere
A. Cloud Formation
Clouds form when water
vapor condenses into liquid
water droplets in the air
 In order for condensation to
occur:

1.
2.
air must be saturated (cooled
to dew point)
must have a solid surface to
condense on (condensation
nuclei)
 Condensation
Nuclei: small
particles in the air created by:
– dust
– volcanoes
– factory
smoke
– forest
fires
– ocean
salt
Several
processes may
bring about the cooling
necessary for clouds to
form:
1. Convective Cooling
 Most
clouds form this way
 Air temperatures decrease
as air rises
and expands
Adiabatic Temperature
Changes:
temperature changes without the
addition or removal of heat
 temperature changes due to rising or
sinking air


Warm air rises, expands and cools

What happens to cool air?
 Cool
air sinks, compresses and
warms
2. Forceful Lifting
Air cools as it is forced over a topographical
feature (like a mountain range).
3. Temperature Changes
Cold
Air
Warm
Air
Two masses of moist air with different temperatures mix
4. Advective Cooling
 Wind
carries warm moist air
over cold oceans or cold land
 The cold water or land absorbs
heat from the air and the air
cools
B. Classifications of Clouds
1. Stratus Clouds
low level clouds
 sheet-like or layered
 cover a large area
 Nimbostratus = stratus cloud with

rain

Altostratus = stratus formation at
higher altitude

2. Cumulus Clouds
puffy, piled, popcorn, or heaped
 form when warm moist air rises and
cools
 flat base
 Cumulonimbus: cloud of great
vertical development
(“thunderhead”)
 middle altitude clouds

3. Cirrus Clouds
cirrus means “curly”
 wispy, stringy
 high altitude clouds
 made up of ice crystals due to the
low temperature and high altitude
 seen prior to a snowfall or rainfall

III. Precipitation
 Any
moisture that falls from
the air to Earth’s surface
 May be liquid or solid
 Four main types:
rain
 snow
 sleet
 hail

1. RAIN: forms when separate
drops of water fall to the Earth
from clouds
2. SNOW: forms when water
vapor condenses directly into
ice crystals
3. SLEET: a mixture of snow and
rain; forms when rain passes
through a cold layer of air and
freezes into ice pellets
4. HAIL: balls or irregular lumps
of ice (hailstones); usually
form in cumulonimbus clouds
Big Hail!
Bad Hail!
How does precipitation
form?
 Clouds
produce
precipitation when its
droplets or ice crystals
become large enough to fall
as rain or snow
Coalescence:
Droplets are carried by the
updrafts and downdrafts in a cloud
 They collide and coalesce to form
larger droplets.
 When the droplets
become too large to
be sustained on
the air currents…
they begin to fall as
rain or snow.
