Isobars• They are lines that connect places with equal air pressure .• Example :On the map isobars marked: 1004 represents an area of high pressure 976 represents an area of low pressure

Map showing ISOBARS

Look at the map ,notice the series of circles in the east .This pattern is a low pressure system(L) ,or low .Isobar readings decrease toward the center of a low pressure system.The map shows another set of isobars in the west . This pattern is a high- pressure system (H),or high. The center of this system has high has higher air pressure than the surrounding area .

Importance of ISOBARS• Isobars give scientists an idea of wind speed .• Air moves fastest where pressure differences are greatest .

• Isobars spaced closely together show a large change in pressure over a small area .

They indicate that wind speeds will be high.• Widely spaced isobars indicate gentle wind .

Air pressure and weather •Different types of weather develop in highs than in lows.• In general , areas of high pressure have fair weather .•Cumulus clouds might be present , but generally little or no

rain falls .•A low-pressure area usually has clouds and precipitation.• Storms and rain often follow a drop in air pressure .•When the barometer reading drops suddenly ,it is likely that

precipitation will fall .

What are fronts ?

Air Masses• a body of air extending hundreds or thousands of miles

sideways and sometimes as high as the stratosphere and having nearly the same conditions of temperature and humidity.

Types of air massesHumid Air masses

Dry Air masses

Air Fronts• When air masses move across the surface of the earth they

may collide.• These collisions happen halfway between one of the poles and

the equator.• The 2 air masses don’t mix much when they collide.• Each keeps its own temperature ,humidity and pressure.• As a result ,a boundary forms, this boundary is called “Front”.

Types of Air FrontsScientists classify fronts by the

characteristics and movements of the two air masses.

1-Warm FrontWhen a warm air mass moves into an area of

cooler air ,the boundary between the air is called

“Warm front”The warmer ,less dense air slides up over the

cooler denser air in a wide ,gentle slope

•As the warmer air cools ,its relative humidity increases, and the water vapor condenses into clouds.•A warm front usually results in steady rain .

2)Cold Front•When a cold air mass moves into an area of warmer air , the boundary between air masses is called “Cold front”• The colder ,denser air slides under the warmer air is pushed up .

•As the warmer air cools ,the water vapor condenses and form clouds that brings precipitation

3)Occluded front

This produces a wedge of warm air between 2 masses of cold air .The formation of an occluded front often indicates that a storm will happen but will not last for long.