Isoline maps are made up of lines that join points of equal value. Contour lines

drawn on a topographic map are another example of an isoline map. Weather maps

use lines called isobars plus other symbols to join together lines of equal

air pressure. Today most meteorologists use the

hectopascal for measuring atmospheric pressure. Many weather reports to the

public use of millibar. One hectopascal equals one millibar. On these

American weather maps the isobars are drawn at intervals of four millibars.

1,013 millibars is the average sea level air pressure. Therefore areas with an air

pressure of over 1,013 millibars are considered to be areas of high pressure,

and areas with an air pressure below 1,013 millibars are considered to be

areas of low pressure. On a weather map areas of high pressure or anticyclones

are identified by a letter H. These areas are associated with clear skies and

little wind. Areas of low pressure or depressions are usually identified by

the letter L and are associated with rain, clouds and high winds. High pressure

systems have enclosed isobars which increase in pressure towards their

center. Low pressure systems on the other hand contain isobars which decrease in

pressure towards the center. When isobars make a sharp bend around a low

this Bend area is called a trough. One of the main symbols employed on a weather

map is called the front. A front represents the boundary between two air

masses and appears on the weather map as a line with triangles or semi circles

attached. At the front of two masses there is stormy weather.

There are four types of front most commonly seen on weather maps. A cold

front is the boundary where a cold air mass pushes underneath the warm air mass forcing

the warm air mass upwards which forms clouds and rain. A cold front is

represented by a blue line with the triangles pointing towards the direction

of movement. A warm front is the boundary where a moving warm air mass as you're placing

a cold air mass. A warm front is represented as a red line with semi circles pointing

towards the direction of movement. Warm fronts move more slowly than cold fronts

because it is harder for warm air to push against the denser cold air. As the

warm air rises above the cold air, water vapor condenses forming high clouds.

Light precipitation can fall on areas as the warm front passes. A stationary front

is the boundary between two air masses which are not moving. Neither mass is

strong enough to move the other, therefore a stationary front can be

pushing back and forth over a given area for days. A stationary front is

represented as an alternating warm and cold front symbols. An occluded front is

a composite of two frontal systems that merge as a result of occlusion. Cold

fronts generally move faster than warm fronts, therefore the cold front will

overtake a warm front and force it aloft. An occluded front is represented as a

purple line with triangles and semi circles. Weather maps also show wind

conditions. Wind is the movement of air. Wind moves from high pressure areas

towards low-pressure areas. Winds do not travel in a straight line due to the

Coriolis effect. This effect is created by her rotation of the earth. The

Coriolis effect deflects wind to the left in the southern hemisphere and to the

right in the northern hemisphere. Thus in the southern hemisphere wind flows

clockwise around low-pressure systems and anti-clockwise in high-pressure

systems. In the northern hemisphere when clothes anti-clockwise around low

pressure systems and clockwise around high pressure systems. Generally wind

flows towards the center of low pressure systems and away from high pressure

systems. When isobars are close to giver wind is strong. When isobars are far

apart wind is weak. Now it is time for you to go and read some weather maps and

see if you can interpret them.