7/14/19

1

The Definition of “Desert”

Relative vs. absolute humidity

§ Absolute – tells you how much water vapor is in the atmosphere– Vapor pressure– Dew point temperature– Mixing ratio, specific humidity

§ Relative – tells you how close the atmosphere is to being saturated– Relative humidity

§ Why would these two ways of talking about humidity differ?

Vapor Pressure

§ Atmospheric Pressure – force per unit area exerted by the molecules that make up the atmosphere

§ Vapor pressure – pressure exerted by just the water vapor molecules in the air

Saturation

§ What do we mean when we say the atmosphere is saturated?

7/14/19

2

SaturationVapor

Pressure§ Why does

saturation vapor pressure increase as T increases?

Water Phase Change

§ Need to add energy to water to go from:– Solid to liquid– Liquid to gas– Solid to gas

§ Water releases energy when going from:– Gas to liquid– Liquid to solid– Gas to solid

Relative Humidity

§ Relative humidity tells us how close the atmosphere is to being saturated.

§ What can happen to the atmosphere to cause air to become saturated (RH=100%)?

Dew Point Temperature

§ Dew point temperature: The temperature to which air must be cooled, at constant pressure, in order to just become saturated

§ Dew point temperature is an absolute measure of humidity

7/14/19

3

Lifting Air: Adiabatic Ascent

§ How will absolute and relative humidity change as air is lifted?

§ If air descends?

Lifting often leads to cloud formation

Global Atmospheric Circulation

7/14/19

4

Defining Deserts

A Place May Be Defined As Desert Based on -

§ Climate – amount of precipitation only; or combination of meteorological variables that affect the water balance

§ Vegetation type or amount§ Type of surface – soil or substrate§ Water drainage

(similar areas regardless of definition)

Consider Climate

What aspects of climate control the dryness of the soil, and therefore the

water available for vegetation?§ Annual amount of precipitation

Is this all?

7/14/19

5

Many definitions of desert are based only on precipitation

amount.

Simple Precipitation Thresholds for

Defining a Place Desert

§ 25 cm (10 inches) annually– common

geographer’s definition

§ 5 cm annually

§ 25 cm annually in cold climates and 50 cm in

warm climates (why the distinction?)

§ Have experienced 12 consecutive months

without precipitation

§ Number of precipitation days (> 2.5 mm) per

month, regardless of amount

But only considering precipitation is an oversimplification

7/14/19

6

Consider Two Locations With About the Same Annual Precipitation (15-20”)

Sonoran Desert

Taiga

Sonoran Desert

Scandinavian TaigaApparently annual-total

precipitation alone is not a very satisfactory basis for defining a place as desert

7/14/19

7

To develop a better definition, answer the following.

Why does it seem to be a lot less desert-like at the high latitude

location?

Consider the amount of water loss at a location as well as

water gain

A Better Definition of Desert

water loss > water gainat surface at surfaceevaporation precipitationtranspiration dew and fog

(rapid soaking in) (ground water)

Evaporation & Precipitation

Subtropics

Arid Arid

7/14/19

8

So, just comparing precipitation (water gain) and evaporation

(water loss) identifies most of the major desert areas of the world.

Take a closer look at water loss.

§ Evaporation rate depends on- wind speed- solar radiation received at surface (depends on

cloudiness)- temperature- humidity

§ Transpiration rate (vegetation) depends on- amount of vegetation- type of vegetation- amount of soil water

7/14/19

9

Does this explain pattern of evaporation seen here?

Subtropics

Arid Arid

Thus, dry climates (surfaces) are related to:

§ Low annual precipitation, dew and fog deposition

§ High wind speed§ High temperature§ High fraction of possible sunshine§ Low humidity§ One more thing - Precipitation concentrated

during the warm season

A simple definition of desert that relies on the surface water

balance concept

Deserts - the “land of the empty bucket”

What does this mean?

§ If precipitation (gain) exceeds evaporation (loss), the bucket will generally be full.

§ If evaporation (loss) exceeds precipitation (gain), the bucket will generally be empty.

§ Thus, an empty bucket means water loss exceeds water gain, and the place is called a desert.

§ Loophole – what if the bucket is full during some seasons and not others?

§ Try this yourself with a bucket and see what happens.

7/14/19

10

But, measuring precipitation is a lot easier than measuring water loss

§ So, formulae have been developed that account for other variables such as temperature that are related to water loss

Desert if R(in)<T(F)/5

Using Soils/Substrate to Decide Whether a Place is a Desert

§ Salt content§ Extremely desiccated through a

deep layer

Using Vegetation to Decide Whether a Place is a Desert

§ Vegetation is either water conserving, or has deep roots to reach water table

7/14/19

11

Phreatophyte Xerophyte

Using Drainage to Decide Whether a Place is a Desert

Classifying Degrees of Aridity: Subjective Terms are Plentiful§ Hyper-arid§ Desert§ Sub-desert§ Semi-desert§ Absolute desert§ True desert

§ Super-arid§ Arid§ Semi-arid§ Extremely arid§ Demi-arid§ Hemi-arid

Sometimes Desert vs Non-Desert is Not Enough

Quantitative Indices of the Degree of Aridity

7/14/19

12

“P over PET”P = Water supply (precipitation)

ET = Evapotranspiration, the sum of evaporation and transpiration

PET = Potential evapotraspiration, the ET when the water supply is unlimited (water loss)

P/PET is the ratio of the amount of water gain to the maximum loss under the existing meteorological conditions (temperature, wind speed, etc.)

So low P/PET result from low precipitation amounts and high rates of loss

Based on UN Definition

§ Hyper-arid: P/PET < 0.03

§ Arid: 0.03 < P/PET < 0.20

§ Semi-arid: 0.20 < P/PET < 0.50

§ Subhumid: 0.50 < P/PET < 0.75

PET is calculated from observed solar radiation, wind speed, humidity, etc.

Does not apply to “cold deserts”

“Budyko Index”§ A radiational index of dryness§ Depth of water that could be evaporated

by total annual net radiation at the ground, divided by annual precipitation total

Annual Evap (radiation)Annual Precip

§ Local atmospheric conditions such as temperature, wind speed, etc., are not accounted for as with P/PET

Annual Precipitation

7/14/19

13

Net Shortwave Radiation Net Longwave Radiation

A Comparison

for the Arabian

Peninsula(P/PET)