Environmental Science 102
Humidity, Clouds, Fog


Water vapor present in the air.
Percent Composition: % of all components; 0-5%
Specific Humidity (mass/total mass):
Absolute Humidity (mass/volume):
Vapor Pressure: portion of atmospheric pressure caused by water vapor content.
Saturation Vapor Pressure: pressure exerted by water vapor content when evaporation and condensation are in equilibrium.

Relative Humidity

Ratio of the amount of water vapor present to the amount of water vapor that the air can hold.
Indicates how near an air mass is to saturation − it is not a measure of actual quantity of water in air.

Two ways that RH can change:

Dew-Point Temperature

Temperature at which Relative Humidity will equal 100%, if the air is cooled.
Can the Relative Humidity ever be greater than 100%?

Adiabatic Process

"occurring without loss or gain of heat"
Compression causes air to warm
Expansion causes air to cool
Dry Adiabatic Lapse Rate:
Wet Adiabatic Lapse Rate:

Cloud Formation


Lifting Processes

Orographic Lifting:
Frontal Wedging Lifting:
Convergence Lifting:
Convective Lifting:


Visible aggregates of minute droplets of water or tiny ice crystals.
Formed when water condenses aloft due to adiabatic cooling.

Cloud Classification

Based on Form and Height
Form: Three basic types
Cirrus – high, thin clouds with a veil-like or wispy appearance.
Cumulus – globular, cauliflower-like masses, often with flat bases and as rising domes or towers.
Stratus – sheet-like or blanket-like clouds covering large areas.

Cloud Height

High Clouds: Form above 6,000 m
Middle Clouds: Form between 2000 to 6000 m
Low Clouds: Form below 2000 m
Vertical Development: Clouds with bases at low heights, and tops which extend to middle or high heights.

Vertical Development

Cumulonimbus: “Thunderstorms”
Tall, tower-like, dark, dense, billowy clouds with tops at high heights (up to 12 – 20 km)
Top spreads out into an anvil head
Produce heavy precipitation, lightning, thunder and occasionally hail and/or tornadoes.


Cloud with base at or near the ground

Evaporation Fogs

Fogs formed by the addition of water vapor to air causing saturation.
Steam Fog – forms when cool air moves over warm water.
Frontal Fog – during frontal wedging, warm air is forced to rise over cold air. Cooling of the rising warm air can result in precipitation. The precipitation then falls through the cold air below. Evaporation of the fallen precipitation can result in fog.

Fog Formed by Cooling

When the temperature of the air at ground level falls below the dew point, condensation produces fog
Radiation Fog – caused by radiation cooling of the ground and air.
Advection Fog – occurs when warm, moist air moves over a cold surface.
Upslope Fog – created when humid air moves up a slope.