BISL 04-Weather and Climate

HAIL

Precipitation in the form of solid

lumps of ice. Hail is produced inside

storm clouds in which frozen

droplets grow in size as they rise

and fall within the cloud.

The drop attaches itself to a
nucleus or solid particle.

Then the surface of
the drop freezes.

Condensation

nucleus

Drop

Nucleus

Periphery

A

HOW CRYSTALS

ARE FORMED

A

Vertical air currents

cause the

microdroplets to

ascend and descend

within the cloud.

B

The droplets freeze, and

each time they are carried

upward in the cloud, they

acquire a new layer of ice.

This process, called

accretion, increases the

size of the hailstone.

A cloud with a greenish

tinge or rain with a

whitish color can

portend a

hailstorm.

C

When the hailstones

are too heavy to be

supported by the

ascending air

currents, they fall to

the ground.

If the drops

crystallize near

the freezing

level, they fall

in the form of

sleet.

C

SLEET

ASCENDING

WARM

CURRENT

WARM

ASCENDING

CURRENT

SNOWFALL

3 miles

(5 km)

-39° F

(-39° C)

ICE

CRYSTAL

2 miles

(3 km)

-9° F

(-23° C)

0.6 mile

(1 km)

19° F

(-7° C)

SNOWFLAKE

HOAR FROST

Similar to frost but

thicker. It usually

forms when there

is fog.

FROST

Frost forms when the

dew point of the air is

less than 32° F (0° C),

and the water vapor

transforms directly into

ice when it is deposited

on surfaces.

Most snowflakes disintegrate before

they reach the ground. They fall as

snowflakes only when the air near

the ground is very cold.

B

The ice

crystals

combine and

form

snowflakes.

The record of annual snowfall

Mount Rainier, Washington.

From February 19, 1971, to

February 18, 1972.

10 feet

(3.11 m)

CROSS SECTION OF A HAILSTONE

0.2 to 2 inches

(5 to 50 mm)

The typical range of

hailstone sizes

The flakes

measure

between 0.04

and 0.8 inch

(1 and 20 mm).

No two snowflakes are

identical to each other.

Most have

six points.

TYPES OF CRYSTALS

Plate

Column

Dendrite

Needle clusters

Layers

of ice

32° F (0° C)

DEW POINT

DEW

Water vapor that condenses

during the night into very small

drops. The condensation forms

on surfaces that radiate heat

during the night, such as plants,

animals, and buildings.

41° F (5° C)

27° F (-3° C)

Temperature of the air

Temperature of the ground

VARIED FORMS

Snow crystals can have a variety of shapes; most of

them have six points, although some have three or 12,

and they have hexagonal symmetry in a plane. They

can also be cubic crystals, but these form under

conditions of extremely low temperature in the

highest regions of the troposphere.

HYDROMETEORS

Drops of condensed or frozen water

in the atmosphere are called

hydrometeors. These include rain,

fog, hail, mist, snow, and frost.

Very small hail (0.2

inch [5 mm] or less

in diameter) is

called snow pellets.

6

7

that fell on April 14, 1986, in

Gopalganj, Bangladesh.

The heaviest

hailstones

2 pounds

(1 kg)

SNOW

Tiny ice crystals combine to

form a hexagonal star, or

snowflake. They form at

-4° F (-20° C).

42 METEOROLOGICAL PHENOMENA WEATHER AND CLIMATE 43