Physical Science 101
Atmospheric Moisture

Atmospheric Moisture

Water: H2O
2 hydrogen, 1 oxygen
Unique Properties

Phase Changes of Water

Phase changes from one state to another involve the exchange of latent heat and a temperature change.
Solid to Liquid:
Liquid to Solid:
Liquid to Gas:
Gas to Liquid:
Gas to Solid:
Solid to Gas:

Latent Heat

Changes in the amount of energy, but no change in the temperature.
Due to the breaking or forming of hydrogen bonds
Bonds between the "+" and "-" charge on the water molecule

Water - Ice Density

Water ice is less dense than liquid water.

Hydrologic Cycle

Describes where water is found (reservoirs) and the global and local flow of water among its reservoirs.


Defined as water vapor present in the air.
At most, the total amount of water in the atmosphere is 4-5% of a given volume of air.

Absolute Humidity
Defined as the mass of water vapor present per volume of air.

Mixing Ratio
Defined as the mass of water vapor present per mass of dry air.

Specific Humidity
Defined as the mass of water vapor present per mass of air.

Vapor Pressure
Defined as that part of the total atmospheric pressure attributable to the water vapor content

Saturation Vapor Pressure
The pressure exerted by the water vapor content when evaporation and condensation are in equilibrium
Imbalances in this equilibrium result in either net evaporation or net condensation
Controlled by temperature or vapor pressure.

Relative Humidity
Defined as the ratio of the amount of water vapor present to the amount of water vapor required for saturation.
The result is reported as a percentage.
Indicates how near an air mass is to saturation - it is not a measure of the actual quantity of water in the air.
Two ways that RH can change:
Measured with a Sling Psychrometer or hygrometer

Dew Point Temperature

The temperature to which the air needs to be cooled to reach saturation
The temperature at which the Relative Humidity will equal 100% if the air is cooled.

Adiabatic Process

Simply stated - compression causes air to warm- expansion causes air to cool
As an air mass rises in altitude it will expand. As it expands it cools.
If the same air mass sinks to a lower altitude it will be compressed. As it is compressed it warms.
Adiabatic warming and cooling occurs at a given rate - Dry Adiabatic Lapse Rate:
The Dry Adiabatic Lapse Rate is followed as long as no condensation occurs
If condensation occurs, the Wet Adiabatic Lapse Rate is followed:
What happens during condensation?

Cloud Formation: Example

1) Air starts at 0 meters, 20°C
2) As air rises it cools: 10°C/1000m Dry Adiabatic Lapse Rate
3) At 1000 m, temperature has dropped to 10°C.
4) 1000 m condensation starts
5) Condensation results in formation of water droplets - clouds form


Lifting Processes

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

Atmospheric Stability

Determined by comparing an air mass temperature to that of the surrounding air.
Stable Air - cooler air masses that resist vertical movement - tend to stay in place.
Unstable Air - warmer air that rises - tend to move to new position.

Types of Stability

Absolute Stability: the environmental lapse rate is less than the wet adiabatic lapse rate.
What this means:
Absolute Instability: the environmental lapse rate is greater than the dry adiabatic lapse rate.
What this means:
Conditional Instability: occurs when moist air has an environmental lapse rate is between the dry and wet adiabatic lapse rates.
What this means:

Stable or Unstable?

Stable air that is forced to rise will produce thin, horizontal clouds with only light to moderate precipitation, if any.
Unstable air that is forced to rise will produce columns of towering clouds, usually associated with heavy precipitation.

Changes in Stability

Warming of air near the surface (when compared to air aloft) causes instability.
What causes surface air to warm?

Cooling of air near the surface (when compared to air aloft) causes stability.