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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.

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i atmospheric moisture
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?

a humidity
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?
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.



fill in the blank
Fill in the blank…

The higher the relative humidity, the _______________ the chance that water vapor will condense into rain or snow.

As outside temperatures increase during the day, what happens to relative humidity?


2 specific humidity
2. Specific Humidity
  • The actual amount of moisture in the air.



b dew point
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
frost if dew point falls below freezing water vapor changes directly to solid ice crystals or frost
Frost: if dew point falls below freezing, water vapor changes directly to solid ice crystals, or frost
ii clouds
II. Clouds
  • Clouds are visible masses of liquid water droplets suspended in the atmosphere
a cloud formation
A. Cloud Formation
  • Clouds form when water vapor condenses into liquid water droplets in the air
  • In order for condensation to occur:
      • 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:



factory smoke

forest fires

ocean salt

1 convective cooling
1. Convective Cooling
  • Most clouds form this way
  • Air temperatures decrease as air rises and expands
adiabatic temperature changes
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?

2 forceful lifting
2. Forceful Lifting

Air cools as it is forced over a topographical feature (like a mountain range).

3 temperature changes
3. Temperature Changes

Cold Air

Warm Air

Two masses of moist air with different temperatures mix

4 advective cooling
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
1 stratus 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
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
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:





III. Precipitation
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
how does precipitation form
How does precipitation form?

Clouds produce precipitation when its droplets or ice crystals become large enough to fall as rain or snow

  • 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.