Atmospheric Stability and Air Masses

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

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

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

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)

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

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

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)

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

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

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)

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)