Stability and Cloud Development

To form a cloud we need to increase the relative humidity to approximately 100%. This is accomplished by lifting an air parcel.

Lifting Mechanisms

  1. Orographic Lifting: Windward side and Leeward side of mountains
  2. Frontal Lifting: A Front is a transition zone between two air masses of different density
  3. Convection: hot air rises
  4. Convergence and Divergence

Atmospheric Stability is measure by comparing the temperature of a rising (sinking) parcel to the surrounding air (or environmental) temperature. When a rising or sinking parcel exchanges no heat with its environment, this is known as an Adiabatic process.

How the temperature changes with altitude is referred to as the lapse rate. There are three important lapse rates to know:

Environmental Lapse Rate (ELR) - describes how the air temperature changes with altitude on any give day at any given time. It is continually changing.

Dry adiabatic lapse rate (DALR): If the specific humidity of the parcel does not change (in other words there is no condensation or evaporation) the temperature of a rising air parcel decreases approximately 10C per 1000m increase in altitude.

Moist adiabatic lapse rate (MALR): If the rising air is saturated then the parcel's temperature decreases because of expansion; however, condensation (deposition) releases heat and so the parcel's temperature does not cool as much as if condensation did not occur. The MALR is not constant but for our purposes we will assume it is a constant value of 6C per 1000m.

The stability of a layer in the atmosphere is characterized as either:

Absolutely Stable: ELR is less than the moist adiabatic lapse rate

Absolutely Unstable: ELR is greater than the dry adiabatic lapse rate

Conditionally Unstable: ELR between DALR and MALR.

What processes stabilize and destabilize a layer in the atmosphere. 

                STABILIZE                                 DESTABILIZE                 

nighttime radiation cooling                Solar heating                              

cold advection                             warm advection                             

air moving over a cold surface             air moving over a warm surface             

Sinking motion is called Subsidence        Mixing steepens ELR

Dew point temperature also changes 2C per kilometer, due to a rapid decrease in air pressure. Dew point temperature is related to the actual vapor presser of the rising air. A decrease in the total air pressure causes a decrease in the vapor pressure and hence a decrease in the dew point

OROGRAPHIC RAIN SHADOW EXAMPLE

Clouds and Stability