Physical GeographyChapter 5

Clouds and Precipitation:The Transfer of Latent Heat

The Global Water Budget•Earth has a global water budget—if

water is lost in one place or in one form, it is moved to another place or another form

•The total amount of water (in whatever form) varies from place to place, but stays constant over the planet as a whole

Where is all of Earth’s water found?

•Oceans = 97.2%•Glaciers = 2.0%•Underground sources

(aquifers, underground pools & groundwater) = 0.5%•Lakes (half saline, half fresh) = 0.2%•Pore spaces in soil (“soil water”) = 0.04%•Atmospheric water, streams, living things = 0.01%

The Hydrologic Cycle•The hydrologic cycle is the planetary circulation

of water within and between the different sources of water on Earth—it is a closed system.

•The power source behind the hydrologic cycle is the radiant energy of the sun.

Residence Time•The amount of time a given amount

of water may remain in a particular segment of the hydrologic cycle is its residence time.

•Residence time can vary from hours (evaporation followed by a thundershower), to millions of years (trapped in deep aquifers)

Residence Time•As water changes its “residence,” it may

also change state.•When water changes state it moves

around latent heat. The evaporation and condensation phase changes are especially significant...

How about a diagram???

Latent Heat Transfer

evaporation—latent heat absorbed

condensation—latent heat released

melting—

latent heat absorbed

freezing—latent heat released

ice (solid)

water vapor (gas) water (liquid)

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Saturation The saturation point is the point at which a given parcel

of air is holding the maximum amount of water vapor that it can possibly hold at a given temperature and pressure.

– Temperature is the key! If the air is not saturated, evaporation can continue, as

long as there is moisture available to be evaporated

Three Factors Influencingthe Rate of Evaporation

1. Temperature of the water– The warmer the water, the faster the

molecules are moving and the more likely they will be able to escape the surface (evaporate)

Three Factors Influencingthe Rate of Evaporation

2. Temperature of the air– Warm air can hold more water vapor

suspended in it– Warm air transfers heat to the water and

speeds up water molecules to the point where they can evaporate

– Cold air can hold less water as a vapor and reaches its saturation point more quickly

Three Factors Influencingthe Rate of Evaporation

3. Degree of windiness– Saturation is reached quickly right above the

water– Wind blowing over a wet surface will reduce

saturation above that surface by moving water vapor molecules away from the surface. This leaves room for more molecules to evaporate

Vapor Pressure

Vapor pressure--the portion of total air pressure made up of water vapor molecules

Saturation vapor pressure--the pressure exerted by the maximum amount of water vapor a parcel of air can hold at a given temperature.

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Relative Humidity The amount of water vapor in the air at a given temperature,

compared with the maximum amount of water vapor which could be in the air if it were saturated

RH = actual/maximum x 100 = ___ %

RH = relative humidity Actual = the actual amount of water vapor in the air right now Maximum = the maximum amount of water vapor the air can hold at

the given temperature and pressure (in other words, saturation point)

RH Example:

If the room you’re sitting in has 5 grams of water vapor actually suspended in it, but the maximum amount of water vapor that the air could possibly hold is 10 grams, then:

RH = actual/maximum x 100 = ___ %

RH = 5g / 10g x 100 = 50%

Relative Humidity What happens when relative humidity

reaches 100%?– Saturation– Condensation

Clouds or fog (if cooling continues)

Two Ways to ChangeRelative Humidity

Change the temperature of the air– Temperature up, RH down– Temperature down, RH up

Add or subtract water vapor– In the atmosphere, water is added through evaporation,

or lost through precipitation (rain, snow, etc.)

The Dew Point

The dew point is the temperature at which saturation is reached.

The Adiabatic Process

What’s

happening,

here?

The Adiabatic Process

The process by which rising air cools (as it expands) and sinking air warms (as it is compressed) in the atmosphere

The physical principle involved:– When a gas expands, it cools– When a gas is compressed, it warms

The Adiabatic Process As an air mass rises through the atmosphere, it

moves into an area of lower density, allowing the molecules the freedom to expand.

As air expands, there are fewer collisions between molecules and the air begins to cool.

So rising air expands and cools down. If the air mass cools enough to reach the dew point temperature, condensation will occur and a cloud will form.

The Adiabatic Process On the other hand, a sinking air mass will move down

through the atmosphere into a region of increasingly more molecules of air.

The pressure of all of these molecules will compress the air mass, forcing the molecules closer to one another.

This increases the number of molecular collisions, speeding up the molecules, which translates into an increase in temperature.

So sinking air is compressed and warms up.

The DAR

The rate at which unsaturated air will cool as it rises is called the Dry Adiabatic lapse Rate, or DAR (the air is not actually “dry”, it’s just not saturated).

Although this rate can vary based on several atmospheric variables, a commonly-used average value is:

10ºC/1000m (5.5ºF/1000ft)

The LCL

The lifting condensation level (LCL) is the elevation at which condensation occurs.

As it rises, expands, and cools, the air’s relative humidity increases (getting closer to 100%) until eventually the air parcel reaches its dew point temperature.

At that point, saturation has been reached and a cloud begins to form.

The elevation where this happens is the LCL.

You can “see” the LCL:Look at the flat bottom of the cloud

A Quick Reminder!

The following five conditions all occur at the same time:

Saturation Condensation RH=100% Dew point temperature LCL (lifting condensation level)

The Latent Heat of Condensation

Once condensation occurs, the water molecules begin to give off the latent heat of condensation. This heat becomes sensible heat that can be measured.

This heat interferes with the adiabatic cooling that is going on, slowing down the cooling process. So the air continues to get colder as it rises, but it cools at a slower rate.

The SAR (or MAR) The rate at which a saturated parcel of air will

cool as it rises is called the Saturated Adiabatic lapse Rate, or SAR (also called the MAR, or moist adiabatic lapse rate)

Again, the rate varies, but we’ll use an average value of:

5ºC/1000m (3.3ºF/1000ft) As the air parcel continues to rise, it

continues to cool, though more slowly.

Buoyancy– The tendency of a substance to rise, especially in a fluid

An air-filled balloon (or you!) in water A helium balloon in air

– Density is the key Equilibrium level

– Where both the rising and the still air are the same density The opposite of buoyancy is stability

– The substance does NOT want to rise

Stability vs. Buoyancy

Stable air

Unstable air

Conditionally unstable air

Condensation nuclei and cloud droplets

Classifying CloudsAre you paying attention?

Extra Credit Section!!!

Cloud types…

Fog: A cloud on the ground

The Four Common Types of Fog

Dew: Condensation on Earth’s surface