GEOGRAPHY 204

PHYSICAL GEOGRAPHY

Spring, 1999

 

LECTURE NOTES

LECTURE 7 -- ATMOSPHERIC STABILITY

 

TERMS:

Define DEW POINT -- Temperature at which air becomes SATURATED, when condensation begins

Define Relative Humidity

Ratio of Actual Amount of Water Vapor in the Air

(at a particular temperature)

Relative to Amount of Water if Air was SATURATED

Expressed as a Percentage

ATMOSPHERIC STABILITY/INSTABILITY

Generally, a parcel of air begins to rise because it becomes warmer than the surrounding air -- e.g. Sun shining on a parking lot

The parcel can only continue to rise AS LONG AS IT IS WARMER THAN THE SURROUNDING AIR

As long as it is warmer than the surrounding air, it is less dense -- thus it is MORE BUOYANT

Same thing in the reverse: a parcel of air that is cooler than the surrounding air will sink -- more dense

(this happens on Mountain Slopes, where radiational cooling at night causes cooler air to flow down slope and pool on valley floors)

ENVIRONMENTAL LAPSE RATE -- this is the actual temperature of the atmosphere with elevation -- measured by the weather balloon -- radiosonde

The Environmental Lapse Rate can be very different from the Moist or Dry Lapse Rates -- influenced by air masses at different levels

THE TEMPERATURE PROFILE OF THE ATMOSPHERE determines whether the air is STABLE or UNSTABLE --

that is, whether rising parcels of air will continue to be warmer than the surrounding air, and be able to continue to rise (UNSTABLE)

Or whether the surrounding air will be warmer than the rising air parcel, causing it to SINK -- STABLE AIR

 

Temperature is average kinetic energy,

HEAT is Total kinetic energy

USUALLY Temperature decreases with elevation

But sometimes TEMPERATURE MAY NOT DROP WITH ELEVATION:

TEMPERATURE INVERSION

When layer of air above is warmer than surface air --

Causes STABILITY

OR Cold Air Drainage from Mountain Slopes at Night -- denser

May be caused by REGIONALLY SUBSIDING AIR, as in Tropics

STABILITY Can Cause

AIR POLLUTION -- E.G. LOS ANGELES, MEXICO CITY

DRY ADIAB L. R. -- Cools at rate of 10 degrees C/1000 m

If Mean Environmental Lapse Rate is 6.4 deg., this means that most air parcels that are DRY won't rise very far before they are COOLER than the surrounding air -- Become Stable

WET ADIAB. L. R. -- COOLS MORE SLOWLY DUE TO RELEASE OF LATENT HEAT

Very Warm Air -- 4 degr. C per 1000 m -- This means VERY UNSTABLE AIR -- Cools more slowly than Most Environmental Lapse Rates

Source of Energy for Hurricanes, Thunderstorms -- Very Buoyant Air

Very Cold Air -- cools almost as fast as Dry Adiab. L. R.

 

CONDITIONALLY STABLE:

When Environmental Lapse Rate is Not Very Steep

May be Stable for DRY ALR, when parcel cools rapidly, reaches same temp as surrounding air

But UNSTABLE FOR WET ALR -- Heat Released keeps Air Parcel Warmer than Surrounding Air

In general, there is some moisture in every air parcel

If the initial temperature of the air parcel is WARMER than the temperature at which condensation occurs, that is, when the Relative Humidity is below 100%

Then the air parcel will cool at the DRY ADIABATIC LAPSE RATE

When it cools to the temperature at which the Relative Humidity reaches 100%, then

Condensation will commence, and LATENT HEAT WILL BE RELEASED

When Latent Heat is released, the RATE OF COOLING IS SLOWED as the parcel continues to rise

THIS SLOWER RATE OF COOLING IS THE SATURATED OR WET ADIABATIC LAPSE RATE

The WET Adiabatic Lapse Rate varies with the Temperature of the Air Parcel, since

Temperature Controls the Rate of Condensation or Vaporization

The WET Adiabatic Lapse Rate Varies between about

4oC per 1000 m for VERY WARM Saturated Air, to

9oC per 1000 m for VERY COLD Saturated Air (almost dry, since very cold air holds much less moisture)

The Standard WET Adiabatic Lapse Rate is 6oC per 1000 m

 

 

TEMPERATURE DIFFERENCES FROM PLACE TO PLACE:

Temperature -- cycles

Daily variations

Seasonal variations (annual)

Variations with Latitude

Temperature "gradient"

 

HEAT BALANCE, Relative to Climate

The Temperature of a place is in part related to

How much energy goes into Heating the Land and Air above it
SENSIBLE HEAT

and How much goes into Evaporating Water -- LATENT HEAT

LATENT HEAT --

Energy Used to Evaporate Water (or MELT ice)

Heat must be applied to cause water to change state

from solid to liquid to gas

This energy is used to Break HYDROGEN BONDS

between Water Molecules

This Heat Energy is Stored -- you don't feel it --

it doesn't contribute to Temperature

So: PLACES THAT HAVE LOTS OF WATER AND SUNSHINE --

Aren't as HOT as Places with No Water

Greeks thought that people would Burn Up if they traveled to the TORRID

Zone -- Near the Equator

Because that's where there is the most Sunshine

Why Isn't this True?

Two Factors:

Lots of Sunshine = Lots of Evaporation (cools things down)
like sweating -- takes heat from your skin

Plus, Lots of Rainstorms (Water to evaporate)

and Clouds to absorb some sunshine

What about DESERTS?

Lots of Sunshine, but Little Water --
Almost all Energy goes to SENSIBLE HEATING -- HOT!

ENERGY BALANCE -- Page 80

Brazil -- Lots of Evaporation (LH), less Sensible Heat (SH)

Egypt -- No Water, Lots of SH, Virtually NO Latent Heat Stored

Paris -- Higher Latitude (48 degrees N)

Some Water, Limited Sunshine (in winter)

So, Heat LOSS in WINTER -- Longwave Going OUT

Heat Deficit

Part goes to Latent Heat, Part to Sensible Heating

Russia -- High Latitude -- Middle of Continent

Fairly Dry, but Cold in Winter

Strong Heat Loss in Winter -- Lots of Longwave Going OUT

Heat Deficit in Winter

Factors that Influence Temperature of a Place

Latitude -- How Much Solar Radiation received

Location Relative to Water --

Water Warms Slowly and Cools Slowly

Continentality -- if in Center of Land Mass -- Greater Daily and Seasonal Temperature change -- Earth heats up FASTER and Cools FASTER

Cloudiness

Surface Cover -- Albedo

Vegetation -- Uses Water and Sunshine in Photosynthesis --

Also Transpires -- releases water -- more humid, less HOT

Topography -- Aspect, shadows

 

NEXT TIME: ATMOSPHERIC CIRCULATION, WINDS

Go To Lecture 8
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