BISL 04-Weather and Climate

FRANCE

GERMANY

BELARUS

POLAND

Bonn UKRAINE

Prague

Kraków

Kiev

Collision

14 CLIMATOLOGY WEATHER AND CLIMATE 15

Cool air

Cool air

Warm air

Cold air Warm air

Cold air

A long Rossby wave develops

in the jet stream of the high

troposphere.

1

The Coriolis effect

accentuates the wave action

in the polar air current.

2

The formation of a meander of warm

and cold air can provide the conditions

needed to generate cyclones.

Rossby Waves 3

Large horizontal atmospheric waves that are
associated with the polar-front jet stream.
They may appear as large undulations in the
path of the jet stream. The dynamics of the
climatic system are affected by these waves
because they promote the exchange of
energy between the low and high latitudes
and can even cause cyclones to form.

OCCLUDED FRONTS

When the cold air replaces the cool air

at the surface, with a warm air mass

above, a cold occlusion is formed. A

warm occlusion occurs when the cool air

rises above the cold air. These fronts are

associated with rain or snow, cumulus

clouds, slight temperature fluctuations,

and light winds.

STATIONARY FRONTS

These fronts occur when there is no

forward motion of warm or cold air—that

is, both masses of air are stationary. This

type of condition can last many days and

produces only altocumulus clouds. The

temperature also remains stable, and there

is no wind except for some flow of air

parallel to the line of the front. There

could be some light precipitation.

Entire Continents

Fronts stretch over large geographic areas.

In this case, a cold front causes storm

perturbations in western Europe. But to the

east, a warm front, extending over a wide

area of Poland, brings light rain. These fronts

can gain or lose force as they move over the

Earth's surface depending on the global

pressure system.

Severe imbalance

in the cold front

Very dense clouds

that rise to a

considerable altitude

Thick rain

clouds

A barely noticeable

imbalance of a warm front

Rain below

the front

Warm Fronts

These are formed by the action of winds. A

mass of warm air occupies a place formerly

occupied by a mass of cold air. The speed of the cold

air mass, which is heavier, decreases at ground level

by friction, through contact with the ground. The

warm front ascends and slides above the cold mass.

This typically causes precipitation at ground level.

Light rain, snow, or sleet are typically produced, with

relatively light winds. The first indications of warm

fronts are cirrus clouds, some 600 miles (1,000 km) in

front of the advancing low pressure center. Next,

layers of stratified clouds, such as the cirrostratus,

altostratus, and nimbostratus, are formed while the

pressure is decreasing.

Behind the cold front,

the sky clears and the

temperature drops.

The cold front forces the warm

air upward, causing storms.

There could be

precipitation in the area

with warm weather.

Cold front

A warm front can be 125 miles (200 km)

long. A cold front usually covers about

60 miles (100 km). In both cases, the

altitude is roughly 0.6 mile (1 km).

125 miles

(200 km)

As the clouds extend

over a region, they

produce light rain

or snow.

The mass of cold air takes the form

of a retreating wedge, which has

the effect of lifting the warm air as

it moves over the mass of cold air.

If the

warm front

moves faster than

the retreating wedge of

cold air, the height of the

advancing warm front

continues to increase.

Surface warm front

KEY

Surface cold front

Cool air

Cold front Warm front

Cold air Warm air

W

hen two air masses with different temperatures and moisture content collide, they

cause atmospheric disturbances. When the warm air rises, its cooling causes water

vapor to condense and the formation of clouds and precipitation. A mass of warm

and light air is always forced upward, while the colder and heavier air acts like a wedge. This

cold-air wedge undercuts the warmer air mass and forces it to rise more rapidly.

This effect can cause variable, sometimes stormy, weather.

Cold Fronts

These fronts occur when cold air is moved by the
wind and collides with warmer air. Warm air is
driven upward. The water vapor contained in the air forms
cumulus clouds, which are rising, dense white clouds. Cold
fronts can cause the temperature to drop by 10° to 30° F
(about 5°-15° C) and are characterized by violent and
irregular winds. Their collision with the mass of ascending
water vapor will generate rain, snow flurries, and snow. If
the condensation is rapid, heavy downpours, snowstorms
(during the cold months), and hail may result. In weather
maps, the symbol for a cold front is a blue line of
triangles indicating the direction of motion.