Chapter 2: Weather Factors
Section 4: Water in the Atmosphere
Humidity: amount of moisture in the air
Relative Humidity: amount of moisture (water vapor) in the air compared to the amount of
water vapor it can hold
Psychrometer: instrument used to measure relative humidity
Condensation: process by which molecules of water vapor in the air become liquid water
Dew Point: temperature at which condensation begins
Cumulus: clouds that form less than 2km above the ground; look like fluffy, rounded piles of
cotton
Stratus: clouds that form in flat layers
Cirrus: clouds that form at high levels above 6km; wispy, feathery clouds made mostly of ice
crystals
Important Facts:
 Cold air can hold less moisture than warm air
 In the water cycle, water moves from lakes and oceans to the atmosphere and falls back
to Earth in a continuous pattern
 Clouds of all kinds form when water vapor in the air becomes liquid water or ice
crystals (cools-condenses)
 Clouds are formed whenever air is cooled to its dew point and particles are present
 Meteorologists classify clouds into 3 main types
1. Cumulus
 Clouds that form less than 2km above the ground
 Fluffy, rounded piles of cotton
 Indicate fair weather
 Cumulonimbus clouds produce thunderstorms
 Cirrocumulus clouds look like rows of cotton balls and indicate
that storms are on the way
2. Stratus
 Form in flat layers
 “Strato” means spread out
 Usually cover all or most of the sky
 When thickened they may produce drizzle, rain, or snow and are
then called nimbostratus clouds
3. Cirrus
 Wispy, feathery clouds with feathery hooked ends
 Form at high levels (6km or higher) and at low temperatures
 Made of ice crystals
o Clouds with “alto” as a prefix mean that they are high (usually between 2 and
6km)
o Clouds with “nimb” in the word mean that they are “rain clouds” or are related
to precipitation
o Fog is a type of cloud that forms at or near the ground when the ground is cooler
than the air
The Water Cycle (p.61 text)
1. Evaporation from oceans, lakes and streams (as well as from living things and plants) puts
water vapor into the air.
2. Water vapor cools and condenses to form clouds.
3. Rain and other forms of precipitation fall to the earth.
4. Water then runs into the ground or off the surface into oceans lakes and streams.
5. Process repeats.
Why are the tops of most mountains almost always covered by clouds?
When wind strikes the side of a mountain, the air is forced upward.
It is being pushed toward the dew point. (The air gets colder as it moves up in the troposphere)
Particles in the air collect water vapor, creating water droplets.
The air rises, is cooled and condensed and clouds are formed.
Cold air holds less moisture and so the cloud must release the moisture.
Rain or snow falls on the wayward side of the mountain (side facing wind).
By the time the air reaches the other side of the mountain it has lost most of its water vapor.
The air is cool and dry. This is the leeward side of the mountain.
Very little rain falls here and the air gets warmer which causes dessert conditions.
Example of this occurs at the Sierra-Nevada Mountains
Condensation forms on the particle at dew point and builds up- creating
water droplets and forming a cloud. Cold air holds less moisture and so it
cannot hold the waterParticle in the air
* *
. . * .
Dew Point . * . *
Precipitation falls in form of rain or snow on the windward side.
This leaves air moving to leeward side dry, causing
desert conditions on the leeward side of the
mountain.
due to wind
moist air moves upward
(water vapor)
The air gets colder as it rises
WINDWARD
Side of mountain the wind hits
LEEWARD