Childrens Illustrated Encyclopedia

559

Weather

Cloud formation

there are three main kinds of

clouds, which form at different

heights in the air. feathery, cirrus

clouds float highest of all. midway

to low are fluffy, cumulus clouds.

Sheets of stratus clouds often lie

low in the sky; gray stratus bears

rain. Cumulonimbus cloud, a type

of cumulus cloud, towers in the sky

and often brings thunderstorms.

CloudS

low-lying clouds at the top of a

hill cause the air to become cold,

foggy, and damp. this is because the

clouds contain many tiny droplets of

water. Clouds form in air that is rising.

the air contains invisible water vapor.

as the air ascends, it becomes cooler.

Colder air cannot hold so much vapor,

and some vapor turns into tiny droplets

or freezes to ice crystals, forming a

cloud. Slow-rising air produces

sheets of cloud. air that is ascending

quickly forms clumps of cloud.

Anvil of

cumulonimbus

clouds

Altostratus

Cumulonimbus

Cumulus

Stratus

Stratocumulus

Altocumulus

air maSSeS and frontS

Huge bodies of air, called air

masses, form over land and

sea. air masses containing

warm, cold, moist, or dry air

bring different kinds of

weather as they are carried by

the wind. a front is where two

air masses meet. the weather

changes when a front arrives.

Cold front

Cold air moves in

under warm air,

bringing heavy

rain followed

by showers.

Warm,

moist air

mass

Cold, moist

air mass

Warm air

mass

oCCluded front

a cold front overtakes

a warm front, lifting

warm air above it.

rain also falls along

an occluded front.

Cold air

mass

Cold air

mass

WeatHer foreCaSting

the weather centers in different countries receive

measurements of weather conditions from satellites and

observers around the world. they use this data to forecast

the weather that lies ahead. Supercomputers do the many

difficult calculations involved and draw charts of the weather

to come. forecasters use the charts to predict the weather

for the next few days, producing weather reports for

television, newspapers, shipping, and aircraft.

WeatHer CHart

a weather forecaster predicts

the day’s weather using a chart

showing air pressure and fronts

over a large region. lines called

isobars connect regions with the

same atmospheric pressure. tight

loops of isobars of decreasing

pressure show a low, where it is

windy and possibly rainy. isobars

of rising pressure indicate a high,

which gives settled weather.

HigHS and loWS

the pressure of the air varies from

time to time and from place to

place. regions of low pressure are

called cyclones, or lows. the air

rises and cools, bringing clouds

and rain. an anticyclone, or high,

is a region of high pressure. the

air descends and warms, bringing

clear, dry weather. Winds circle

around highs and lows, as can

be seen in this satellite picture

of a cyclonic storm.

HigH

Blue triangles indicate

the advancing edge

of a cold front.

Lines called

isobars give the

air pressure,

which is measured

in millibars.

Red semicircles

indicate the

advancing edge

of a warm front.

HigH indicates

regions of highest

air pressure.

LoW indicates

regions of lowest

air pressure.

LoW

ground

level

2 miles

(3.2 km)

6 miles

(9.7 km)

8 miles

(13 km)

10 miles

(16 km)

4 miles

(6.4 km)

CumuluS CloudS

Separate masses of cloud are called

cumulus clouds. altocumulus is

medium-high patchy cloud, and

low stratocumulus contains low,

dense clumps of cloud.

CirruS CloudS

Cirrus clouds form high in the sky

so they contain only ice crystals.

Cirrocumulus (above) and cirro-

stratus also form at high altitudes.

Warm front

long spells of

rain occur as

warm air rises

above cold air,

before the

front arrives

on the

ground.

Cold air

mass

Cirrocumulus

Cirrus clouds

Find out more

atmosphere

Climates

earth

rain and snow

Storms

Wind

Warm air mass

HigH

US_559_Weather_2.indd 559 27/01/16 2:40 pm