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HOW DOES HAIL FORM?

Hail forms in cumulonimbus (storm) clouds that

contain powerful, vertical air currents. Water droplets

in the clouds freeze and are whirled up and down.

Each time a hailstone is tossed upwards to the frozen

cloud top, a new layer of ice forms around it. The ice

builds up, layer by layer, until the hailstone becomes

too heavy to remain airborne and falls to earth.

WHY ARE CLOUDS DIFFERENT SHAPES?

The way clouds form depends on their height above

the ground and on the movement of air. Wispy cirrus

clouds form high in the troposphere. They are made of

ice crystals drawn into strands by the wind. Rapidly

rising pockets of warm air cause fluffy, mid-level

cumulus clouds to form. Low-lying stratus clouds are

formed by air that rises slowly over a large area.

HOW DO CLOUDS FORM?

When the Sun shines on seas and lakes, some of the

water in them evaporates into the warm air. If air

currents blow the warm air over land and it rises over

mountains, or if cold air pushes beneath the warm air

and forces it upwards, then the warm air cools. Cold

air cannot hold as much moisture as warm air, so the

water vapour condenses to form clouds.

WHY DOES IT SNOW?

Snow forms in clouds high in the atmosphere, where

it is so cold, water droplets freeze into ice crystals. The

ice crystals collide and join together to make bigger

crystals. When the crystals get too heavy to float, they

fall to the ground as snowflakes. When snow falls, the

air is just cold enough to let the flakes drift to the

ground before they melt.

Clouds are visible masses of moisture, made up of tiny ice crystals

or water droplets, which are so light that they float. Clouds form in

the troposphere (the lowest layer of the Earth’s atmosphere) when

water vapour rising high into the sky cools and condenses.

Snow falls in cold weather when ice

crystals formed high in clouds join together

and drop to the ground as millions of tiny

snowflakes. Sleet is a mixture of rain and

snow, or partly melted snow.

Hailstones are ice pellets that grow from ice crystals formed in

freezing stormclouds and then plummet to the ground. Large

hailstones can shatter glass, dent car roofs, and ruin crops.

4 SNOWFLAKES

Every snowflake has a

different hexagonal (six-

sided) structure. Different-

shaped snowflakes form

at various temperatures

and heights in the

atmosphere.

2 CLOUDS

There are three main types of

cloud: cirrus (meaning curl or

wisp of hair), cumulus (meaning

heap), and stratus (meaning

layer). Other clouds are mixtures

of these three types. Clouds are

made of ice or water depending

on their height above the ground.

CLOUDS

HAIL

Some stellar snowflakes

can grow as large as

5–7 cm (2–3 in) wide

Stellar (star)

snowflake with

six sides

CIRRUS CUMULUS STRATUS STRATOCUMULUS

LAYERS OF ICE 3

Layers of ice are clearly

visible inside this grapefruit-

sized hailstone. Hailstones

this size are uncommon, but

many are as big as marbles.

SNOW

FIND OUT MORE. Atmosphere 49 • Climate 62 • Ice 58 • Rivers 56–57 • Weather 50 • Winds 51