1. Atmospheric Stability and Air Masses

2. Adiabatic Temperature Changes
Adiabatic temperature changes occur when air is compressed or expands
When it expands, air cools
When it is compressed, air warms
In the atmosphere, rising air changes temperature at known rates

3. Adiabatic Temperature Changes
The dry adiabatic lapse rate is 1°C/100m and applies until air reaches its dew point
The wet adiabatic lapse rate is more variable (0.5 to 0.9°C/100m) and depends on the moisture content of the air
These are different from the environmental lapse rate which is the change in temperature of each layer of the atmosphere

4. Atmospheric Stability
Stability = tendency of an air parcel to move vertically (up or down)
Atmospheric stability is dependent on temperature and pressure
Temperature differences between an air parcel and the surrounding air lead to density differences and movement upwards or downwards
More movement = less stable

5. Atmospheric Stability
Air warmer than its surroundings will tend to rise (because of its lower density)
Air cooler than its surroundings will tend to sink (because of its greater density)
Air at the same temperature as its surroundings will tend to remain at the same height (because there is no density difference)

6. Atmospheric Stability
In stable conditions, vertical movement is discouraged
In unstable conditions the air parcel tends to move upward or downward and to continue that movement.
Unstable air patterns can cause convection currents in the atmosphere
These convection currents create clouds and over time they can build up into the clouds that create thunderstorms

7. Atmospheric Stability
The atmosphere is neutral when air only moves based on adiabatic heating or cooling
When conditions are extremely stable, cooler air near the surface is trapped by a layer of warmer air above it = inversion - no vertical air motion

8. Air Masses
Weather patterns result from the movement of large bodies of air = air masses
Air masses are classified based on their temperature and moisture properties acquired from a source region (Fig 14.2)

9. Air Masses
Arctic (A) air masses originate in the Arctic and are very cold
Polar (P) air masses originate at high latitudes and are cold
Tropical (T) air masses originate at low latitudes and are warm
Continental (c) air masses form over land and are dry

10. Air Masses Maritime (m) air masses form over water and are moist
The source regions can be combined to describe the different temperature and moisture combinations

11. Air Masses Air masses are divided by boundaries called fronts
Warm fronts are produced as warm air moves up over a cold air mass
The clouds formed create moderate precipitation
When this occurs, they form a wedge shape (Fig 14.3)

12. Air Masses
Cold fronts are produced when cold air advances into a region of warm air
The boundary between them tends to be a steeper wall of air and causes heavy precipitation (Fig 14.5)