BISL 04-Weather and Climate

FIELD CAPACITY

The amount of moisture in

the soil remaining after

water has run off the

surface. Field capacity

determines whether, even

with a meteorological

drought, the land can

continue to absorb

existing water between

soil particles.

I

n deserts, drought from lack of rain is customary,

but in arid, semiarid, and subhumid regions,

desertification occurs when for weeks, months, or

years the land is degraded because of climatic variations.

A high-pressure center that stays in a certain location longer than

usual can be the cause of this phenomenon. Soils are able to put up

with a certain dry period, but when the water table decreases

drastically, the drought can turn into a natural catastrophe.

Water Scarcity

50 METEOROLOGICAL PHENOMENA

Air

Hygroscopic

water

Solid

particles

Solid

particles

SATURATED SOIL

The water that falls as

precipitation may be

more than the soil can

absorb, and it descends

toward aquifers.

WILTING

This results when less

water is available in the

upper layers of the soil.

AGRICULTURAL DROUGHT

When soil moisture exists only at the hygroscopic

level (surface moisture on soil particles), there is

no water available for vegetation.

Remaining

water

Solid

particulates

Gravitational

water

Capillary

water

(osmosis)

Capillary

water

Solid

particles

WEATHER AND CLIMATE 51

CYCLONIC CURRENT

METEOROLOGICAL DROUGHT

The condition that results when

precipitation is much lower than normal

levels for that location. It is generally

determined based on comparison with

average rainfall.

THE PROPORTION

OF WATER IN THE

SOIL

Excess

water

(saturation)

Saturation

threshold

(field

capacity)

Level of

wilting

Hygroscopic

coefficient

(minimum of

water)

Solid

particles

Space

between

1933-37 the pores

The Dust Bowl

was created.

1962-66

Affected the states

of the Northeast

1977

Water is rationed in

California.

UNITED

STATES

1975-76

Less than 50% of

the average rainfall

ENGLAND

SAHEL

1965-67

1.5 million deaths

caused by

drought

1967-69

Numerous

forest fires AUSTRALIA

INDIA

THE DRIEST ZONES

coincide with deserts. For example, in the

Atacama Desert in northern Chile, not a single

drop of water fell between 1903 and 1917.

The region of the Sahel has
endured periods of devastating
droughts lasting this long.

100 years

1

2

3

4

5

Areas of insufficient rain

for normal vegetation

and harvests

KEY

HIGH-PRES

SU

RE

A

R

E

A

B HIGH PRESSURE

A high-pressure center,

or anticyclone, is more

stationary than usual and

creates an abnormal

situation in the region.

C

DROUGHT

The jet-stream

currents are thrown

off course by the high-

pressure center, which

impedes rainfall. A dry

period begins.

A

RAIN

Caused by cyclonic

(low pressure) air

currents.