Chapter 15: Moisture, Clouds, and Precipitation
I. Changes of state of water
A. Heat energy
1. Measured in calories – one calorie is the heat necessary to raise the temperature
of one gram of water one degree Celsius
2. Latent heat
a. Stored or hidden heat
b. Not derived from temperature change
c. Important in atmospheric processes
B. Three states of matter
1. Solid
2. Liquid
3. Gas
C. To change state, heat must be
1. Absorbed, or
2. Released
D. Processes
1. Evaporation
a. Liquid is changed to gas
b. 600 calories per gram of water are added – called latent heat of vaporization
2. Condensation
a. Water vapor (gas) is changed to a liquid
b. Heat energy is released – called latent heat of condensation
3. Melting
a. Solid is changed to a liquid
b. 80 calories per gram of water are added – called latent heat of melting
4. Freezing
a. Liquid is changed to a solid
b. Heat is released – called latent heat of fusion
5. Sublimation
a. Solid is changed directly to a gas (e.g., ice cubes shrinking in a freezer)
b. 680 calories per gram of water are added
6. Deposition
a. Water vapor (gas) changed to a solid (e.g., frost in a freezer compartment)
b. Heat is released
II. Humidity
A. Amount of water vapor in the air
1. Saturated air is air that is filled with water vapor to capacity
2. Capacity is temperature dependent – warm air has a much greater capacity
3. Water vapor adds pressure (called vapor pressure) to the air
B. Measuring humidity
1. Specific humidity
a. Quantity of water vapor in a given mass of air
b. Often measured in grams per kilogram
2. Relative humidity
a. Ratio of the air's actual water vapor content compared with the amount of
water vapor required for saturation at that temperature (and pressure).
b. Expressed as a percent
c. Saturated air
1. Content equals capacity
2. Has a 100% relative humidity
d. Relative humidity can be changed in two ways
1. Add or subtract moisture to the air
a. Adding moisture raises the relative humidity
b. Removing moisture lowers the relative humidity
2. Changing the air temperature
a. Lowering the temperature raises the relative humidity
b. Raising the temperature lowers the relative humidity
e. Dew point temperature
1. Temperature to which a parcel of air would need to be cooled to reach saturation (relative humidity is 100% )
2. Cooling the air below the dew point causes condensation
a. e.g., dew, fog, or cloud formation
b. Water vapor requires a surface to condense on
f. Two types of hygrometers are used to measure humidity
1. Psychrometer
a. Compares temperatures of
1. Wet-bulb thermometer, and
2. Dry-bulb thermometer
b. If the air is saturated (100% relative humidity) then both thermometers read the
same temperature
c. The greater the difference between the thermometer readings, the lower the
relative humidity
2. Hair hygrometer – reads the humidity directly
III. Adiabatic heating/cooling
A. Adiabatic temperature changes occur when
1. Air is compressed
a. Motion of air molecules increases
b. Air will warm
c. Descending air is compressed due to increasing air pressure
2. Air expands
a. Air parcel does work on the surrounding air
b. Air will cool
c. Rising air will expand due to decreasing air pressure
B. Adiabatic rates
1. Dry adiabatic rate
a. Unsaturated air
b. Rising air expands and cools at 1°C per 100 meters (5.5 °F per 1000 feet)
c. Descending air is compressed and warms at 1°C per 100 meters
2. Wet adiabatic rate
a. Commences at condensation level
b. Air has reached the dew point
c. Condensation is occurring and latent heat is being liberated
d. Heat released by the condensing water reduces the rate of cooling
e. Rate varies from 0.5°C to 0.9°C per 100 meters
IV. Stability of air
A. Types of stability
1. Stable air
a. Resists vertical displacement
1. Cooler than surrounding air
2. Denser than surrounding air
3. Wants to sink
b. No adiabatic cooling
c. Absolute stability occurs when the environmental lapse rate is less than the wet
adiabatic rate
d. Often results in widespread clouds with little vertical thickness
e. Precipitation, if any, is light to moderate
2. Absolute instability
a. Acts like a hot air balloon
b. Rising air
1. Warmer than surrounding air
2. Less dense than surrounding air
3. Continues to rise until it reaches an altitude with the same temperature
c. Adiabatic cooling
d. Environmental lapse rate is greater than the dry adiabatic rate
e. Clouds are often towering
f. Conditional instability occurs when the atmosphere is stable for an unsaturated
parcel of air but unstable for a saturated parcel
B. Determines to a large degree
1. Type of clouds that develop
2. Intensity of the precipitation
V. Processes that lift air
A. Orographic lifting
1. Elevated terrains act as barriers
2. Result can be a rain shadow desert
B. Frontal wedging
1. Cool air acts as a barrier to warm air
2. Fronts are part of the storm systems called middle-latitude cyclones
C. Convergence where the air is flowing together and rising
D. Localized convective lifting where unequal surface heating causes localized pockets of
air to rise because of their buoyancy
VI. Condensation and cloud formation
A. Condensation
1. Water vapor in the air changes to a liquid and forms dew, fog, or clouds
2. Water vapor requires a surface to condense on
a. Possible condensation surfaces on the ground can be the grass, a car window, etc.
b. Possible condensation surfaces in the atmosphere are tiny bits of particulate matter
1. Called condensation nuclei
2. Dust, smoke, etc.
B. Clouds
1. Made of millions and millions of
a. Minute water droplets, or
b. Tiny crystals of ice
2. Classification based on
a. Form (three basic forms)
1. Cirrus – high, white, thin
2. Cumulus
a. Globular cloud masses
b. Often associated with fair weather
3. Stratus
a. Sheets or layers
b. Cover much or all of the sky
b. Height
1. High clouds
a. Above 6000 meters
b. Types
1. Cirrus
2. Cirrostratus
3. Cirrocumulus
2. Middle clouds
a. 2000 to 6000 meters
b. Types (alto as part of the name)
1. Altocumulus
2. Altostratus
3. Low clouds
a. Below 2000 meters
b. Types
1. Stratus
2. Stratocumulus
3. Nimbostratus (nimbus means "rainy")
4. Clouds of vertical development
a. From low to high altitudes
b. Called cumulonimbus
c. Often produce
1. Rain showers
2. Thunderstorms
VII. Fog
A. Considered an atmospheric hazard
B. Cloud with its base at or near the ground
C. Most fogs form because of
1. Radiation cooling, or
2. Movement of air over a cold surface
D. Types of fog
1. Fogs caused by cooling
a. Advection fog – warm, moist air moves over a cool surface
b. Radiation fog
1. Earth's surface cools rapidly
2. Forms during cool, clear, calm nights
c. Upslope fog: Humid air moves up a slope
2. Evaporation fogs
a. Steam fog
1. Cool air moves over warm water and moisture is added to the air
2. Water has a steaming appearance
b. Frontal fog, or precipitation fog
1. Forms during frontal wedging when warm air lifted over colder air
2. Rain evaporates to form fog
VIII. Precipitation
A. Cloud droplets
1. Less than 20 micrometers (0.02 millimeter) in diameter
2. Fall incredibly slow
B. Formation of precipitation
1. Bergeron process-supercooling and supersaturating
a. Temperature in the cloud is below freezing
b. Ice crystals collect water vapor
c. Large snowflakes form and
1. Fall to the ground as snow, or
2. Melt on their descent and form rain
d. Dominant in the middle latitudes
2. Collision-coalescence process
a. Warm clouds
b. Large droplets form
c. Droplets collide with other droplets during their descent
d. Common in the tropics
C. Forms of precipitation
1. Rain and drizzle
a. Rain – droplets have at least a 0.5 mm diameter
b. Drizzle – droplets have less than a 0.5 mm diameter
2. Snow – ice crystals, or aggregates of ice crystals
3. Sleet and glaze
a. Sleet
1. Wintertime phenomenon
2. Small particles of ice
3. Occurs when
a. Warmer air overlies colder air
b. Rain freezes as it falls
b. Glaze, or freezing rain – impact with a solid causes freezing
c. Hail
1. Hard rounded pellets
a. Concentric shells
b. Most diameters range from 1 to 5 cm
2. Formation
a. Occurs in large cumulonimbus clouds with violent up- and down drafts
b. Layers of freezing rain are caught in up- and down drafts in the cloud
c. Pellets fall to the ground when they become too heavy
d. Rime
1. Forms on cold surfaces
2. Freezing of
a. Supercooled fog, or
b. Cloud droplets
Notes from Tarbuck and Lutgens website