Learning Objectives

Water is a unique substance.  It can exist in all three states at temperatures at the surface of the Earth.

Change of state:

Condensation: vapor to liquid
Evaporation: liquid to vapor
Freezing: liquid to solid
Melting: solid to liquid
Sublimation: solid to vapor directly
Deposition: vapor to solid directly
 

As the energy in the atmosphere (heat energy) changes water will change state, this is a dynamic process that occurs continually throughout the day.

Warm air has more energy, it can support more water as a vapor.
Cold air has less energy and cannot support as much water as vapor.

"Warm air holds more moisture than cold air."
This statement is often used to describe the relationship between water vapor in the atmosphere and temperature.

Humidity is a measure of the amount of water vapor in the atmosphere.

1) Absolute Humidity:    mass of water vapor in an air mass/volume of air in an air mass

This number is not as useful to meteorology because volume constantly changes with temperature.  It represents the density of water/air
 

2) Specific Humidity:  mass of water vapor in an air mass/mass of air in the air mass

This number relates the specific amount of water vapor present but does not give any useful meteorological information.

3) Relative Humidity: (actual water vapor content in an air mass/ capacity of the air mass) x 100

This is what is related in a weather report and determines the saturation of the air mass.
Warm air holds more moisture than cold air.  Warm air masses have a larger capacity for water vapor and cold air masses have less.  If relative humidity is the measure of how close a parcel is to saturation then as the parcel warms, capacity increases and humidity decreases (assuming no moisture is added).
 

Measuring Humidity:

Humidity is measured using a hygrometer, there are different types:

Hair Hygrometer: uses the change in the length of a hair for determining humidity.

Electrical Hygrometer: uses a change in electrical conductivity to determine humidity.

Infrared Hygrometer: uses the amount of energy absorbed to determine humidity.

Psychrometer: uses evaporative cooling to determine humidity.

Dew Point: the temperature to which a parcel of air must drop in order for condensation to occur.

Stability:

As a parcel of air rises due to temperature it encounters lower air pressures, this causes the air to expand.  As the air expands it cools.

Adiabatic Lapse Rate: air will cool 5.4 degrees F / 1000 ft as it rises and expands.  This assumes no heat added or removed.   10 degrees C/1000m

BUT...

As air rises it will encounter the dew point and condensation will occur.  Condensation releases heat energy (remember the "heat of vaporization").  This added heat slows the adiabatic cooling rate to an average of 3.3 degrees F/ 1000 ft.

Dry Adiabatic Lapse Rate: used for dry air = 5.4 degrees/1000 ft.

Moist Adiabatic Lapse Rate: used as an average for humid air = 3.3 degrees/1000 ft.  (this number will vary with the amount of moisture present but 3.3 gives a close approximation) 6 degrees C/1000m

An air parcel will continue to rise in the atmosphere as long as it is warmer than the surrounding air.  Once it reaches a temperature equal to the surrounding air it will stop rising.  If an air mass reaches the dew point cloud formation will occur.

The diagrams below depict unstable and stable air masses.
 

In this diagram the unstable air mass will continue to rise past 5000 ft until it reaches an altitude where the surrounding air temperature is equal.
 
 

In this diagram stability was reached at 5000 ft and the air parcel stopped rising.
 

Conditionally Stable: an air mass that is stable when dry but unstable when moist.  If a dry air mass is forced upward (by topography, frontal systems) it may become saturated becoming unstable.  The diagram below depicts conditional stability.