Environmental Science 102
Humidity, Clouds, Fog


Water vapor present in the air.
At most, 4-5% of a given volume of air.
Specific Humidity (mass/total mass)
Absolute Humidity (mass/volume)

Relative Humidity

Ratio of the amount of water vapor present to the amount of water vapor that the air can hold.
Measured with a Sling Psychrometer or hygrometer
RH Equation:

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

Simply stated: compression causes air to warm, expansion causes air to cool
Rising air masses:
Sinking air masses:

Adiabatic warming/cooling occurs at a given rate:
Dry Adiabatic Lapse Rate:
Wet Adiabatic Lapse Rate:

Cloud Formation


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


Produces clouds, fog or dew.
Two Necessary Conditions:
Cloud Condensation Nuclei:
Hygroscopic nuclei:
Hydrophobic nuclei:

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.
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.


Defined as a cloud with its base at or very near the ground

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.

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.