Stability of Air
Two types of stability
1. Stable air
Resists vertical movement
Cooler than surrounding air
Denser than surrounding air
Wants to sink
Results in wide spread thin clouds
Precipitation is light if any
2. Unstable air
Air acts like a hot air balloon
Rising air
Warmer than surrounding air
Less dense than surrounding air
Continues to rise until it reaches an altitude with
the same temperature
Cools as it rises
Because it is cooler higher up
“Environmental Lapse Rate”
Because it cools as it expands
“Adiabatic cooling”
Clouds are often towering
Results in Heavy Precipitation
Condensation and Cloud Formation
Condensation
Water vapor changes to a liquid and forms dew, fog or clouds
Water vapor requires a surface to condense on
“Condendation Nucei”
Dust
Smoke
Ocean spray  Salt
Clouds
Made of millions and millions of
Droplets of water
Tiny crystals of ice
Types of Clouds based on two characteristics
1. Form
a. Cirrus -- High, white and thin
b. Cumulus
Globular cloud masses “cotton ball”
Fair weather clouds
c. Stratus
Sheets of clouds
Cover most of the sky
2. Height
a. High upper clouds
Above 18,000 feet
Cirrus
Cirrostratus
Cirrocumulus
b. Middle clouds
Between 6,000 – 18,000 feet
Altocumulus
Altostratus
c. Low level clouds
Below 6,000 feet
Stratus
Stratocumulus
Nimbostratus (nimbus means “rainy”)
d. Cumulonimbus
Reach multiple levels
Produce rain showers, thunderstorms and
tornados
Fog
Cloud with base near the ground
Types
Advection fog
Radiation fog
-
Upslope fog
-
Warm, moist air moves over a cool surface
Earth cools rapidly
Example clear calm cool nights
Air moves up a slope (hill or mountain) and
cools as it expands “adiabatic cooling”
Steam fog
-
Cool air moves over warm water and
moisture is added to the air