Ch-5, Part-7a:  Atmospheric Lifting Mechanisms;
Adiabatic Cooling

[ Ch-5 Main ]   [ Exam-2 Topics ]   [ Geog-100 Main ]

• All forms of precipitation (in nature) depend on lifting
  • Air rises
  • Air cools, adiabatically (by expansion)
  • Continued cooling raises relative humidity to saturation = 100%
  • Further rising of the air cools it further, beyond saturation -- "something has to give"
  • The air "gives up" water vapor via condensation and/or deposition/sublimation
    • Condensation and/or deposition/sublimation produces clouds of water droplets and/or ice crystals
    • Further rising and cooling plus condensation and/or deposition/sublimation yield precipitation:  rain, snow and variants
• The purpose here is to learn about the quite different mechanisms in nature that produce lifting
  • This can explain why some geographic areas are dry and others moist
  • This can explain why some precipitation is very localized while other precip covers broad regions
  • This can enable you to understand what happens and to make your own precipitation forecasts and/or explanations to others

Adiabatic heating and cooling

• Parcels of air that rise above the surface expand because of lower air pressure aloft
• Expanding air cools -- a process in nature
  • If the cooling is enough, the moisture saturation point (100% humidity) can be reached and clouds and precipitation can form
• Conversely, sinking air experiences increasing air pressure and warms
  • Warming air always has its humidity reduced below the saturation point (below 100%)
  • Hence, sinking air warms and produces clear weather
• The above is called adiabatic heating and cooling (by compression and expansion)

Dry Adiabatic lapse rate

• If air rises and cools without formation of clouds or precipitation, we have a Dry Adiabatic Lapse Rate of about 5.5 degrees F. per 1000 feet of elevation change (first Figure, below)
• Conversely, sinking air warms at the Dry Adiabatic Lapse Rate (second Figure, below)

Adiabatic Lapse Rate vs the Environmental Lapse Rate

• Remember, the Environmental Lapse Rate is what would be observed by pulling a thermometer up through the atmosphere --- without air rising or sinking
• The Adiabatic Lapse Rate is rate at which air cools by rising or warms by sinking
• The Dry Adiabatic Lapse Rate is the temperature changes that occur with falling air, and with rising air without clouds or precipitation forming
• The Adiabatic rate can cause rising air to cool slower or faster than the condition of the environmental lapse rate
• The above may sound complicated, but they are the critical factors behind clear vs. cloudy and precipitating weather, and between gentle vs. violent storms

Unstable air

• The moist adiabatic cooling rate is greater than the environmental lapse rate
• Condensation from cooling, rising air, makes the air rise even more
• Cooling from rising produces clouds and possible precipitation
• In other words, THIS IS HOW RAIN AND SNOW ARE MADE!!!!!

Stable air (below):  

• The moist adiabatic cooling rate is less than the environmental lapse rate
• Rising air wants to sink back down
• This causes stagnant air and clear skies

LIFTING IN GENERAL

• In nature, the way to get clouds and precipitation is by having air rise and expand, thus cooling adiabaticaly
• Therefore, an understanding of where and how clouds and rain can occur depends upon the processes which provide lifting of air in nature!!!!!

CONVERGENT LIFTING

• Think Intertropical Convergence Zone (ITCZ)
• NE and SE Tradewinds converge (meet) at/near the Equator
• The colliding air has to go somewhere; assisted by heating, it rises in giant Cumulonimbus thunderheads, often more than one storm per day

Convergent Lifting:  Think Intertropical Convergence Zone
(Assisted by solar heating at/near the Equator)

CONVECTIONAL LIFTING

• Surfaces receiving insolation (Sun's rays) heat up

• Air at/near these warmed surfaces also heat up

• Heated air expands and has fewer molecules per unit of volume (cubic meters, etc)

• Hence heated air has less density (weight)

• Thus heated air is lighter and rises

• This produces updrafts that sometimes rise into the stratosphere in giant cumulonimbus clouds

• The flat bottoms of cumulus and cumulonimbus clouds represent the LCL, Lifting Condensation Level, at which the air has been cooled to the saturation/100% relative humidity point

Convectional Heating Over Dark Surfaces
(Think Cumulus and Cumulonimbus Clouds!)

Condensation in Cumulus and Cumulonimbus Clouds
Releases Latent Heat of Vaporization and
Enhances Additional Rising of Air

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