A. Air Parcel

• an "air parcel" is a balloon-like blob of air - that expands and cools as it rises and compresses and warms as it sinks!

• if it does this without any interchange of heat with its outside surroundings, it is considered to be an "adiabatic process"

• from observation, as a whole, the atmosphere in the vertical is neither moving away nor closing in.

• hence, over large horizontal distances (~10 km or greater) the vertical pressure gradient force (Pv) and the gravitational force (G) must balance:

P v + G = 0             or, expressed another way:       Pv = - G
• the balance of G and Pv is known as "hydrostatic equilibrium"

                
    Hydrostatic Equilibrium
 

C. Behavior of Rising and Sinking Air

• departures from hydrostatic balance (vertical motions) produce weather (clouds and precipitation)

• to understand how vertical motions produce weather, we must ask two important questions:

2) how does the parcel behave, relative to the environment(the surrounding air) when displaced vertically?

Answer to Question #1:

The parcel undergoes adiabatic expansion & cooling as it ascends; or adiabatic compression & warming as it descends !!

Adiabatic Process:

• no external heat addition or removal occurs
• temperature change is accomplished only expansion/compression of the parcel

• the external addition/subtraction of heat; examples:
 
• radiative heating/cooling

• sensible heating/cooling

• latent heating (condensation)/cooling (evap.)

Answer to Question #2 (how does the parcel behave, relative to the environment(the surrounding air) when displaced vertically):

(1) "Stable Equilibrium" - after (forcibly) being lifted, or lowered, the air will tend to return to its original position - i.o.w., the air resists vertical motions

(2) "Unstable Equilibrium" - when given a "push" it will move farther from its original position - it promotes vertical motions
 

Lapse Rates --  the rate at which temperature decreases with height

"Dry Adiabatic Lapse Rate"

• during ascent the parcel cools at 10 degrees Celsius per every 1 kilometer......10^oC/km

• during descent the parcel warms at 10^oC/km

                    
         Dry Adiabatic Lifting
 

"Moist (Wet) Adiabatic Lapse Rate"

• as the parcel ascends RH increases, until the parcel is saturated (i.o.w., RH=100%, or Temp = dew point)

• any further lifting will result in the parcel cooling at a slower rate because of the heat added during condensation - cooling still takes place - just at a slower rate!!
• the moist adiabatic lapse rate varies with air temperature, because warm air can "hold" more water vapor than cold air; the average = 6^oC/km

                
Dry and Moist Adiabatic Lifting
 

"Environmental Lapse Rate" - the actual lapse rate in the troposphere - it varies from place to place & with time (the average = 6.5 ^oC/km)

(1) Stable - the parcel cools off more quickly than the environment

(2) Unstable - the parcel cools off more slowly than the environment

(3) Conditional Instability - the environmental lapse rate lies between the dry lapse rate & the moist lapse rate

                                                 
Adiabatic and Environmental Lapse Rates and Stability in the Lower Atmosphere