Lesson 31 What is an isobar?

A weather map shows us weather conditions from many places A weather map shows us weather conditions from many places. How is air pressure shown?

Air pressure is measured in millibars. All of the weather stations call in their measurements to the National Weather Service. (remember that from unit 30?)

The meteorologists at the NWS put all of the millibar numbers on their master map. They then connect all of the places with the same air pressure, with a line.

See those black lines? They are called isobars. Every place on one of those lines has the same air pressure.

As you saw, isobars follow a curved path As you saw, isobars follow a curved path. The ends meet to form a closed figure.

Connect the millibar readings with isobars: 1025 1030 1025 1025 1035 1035 1030 1035 1030 1025

Did the numbers get higher, or lower as you go toward the center? They got higher, so the isobars show a high pressure area.

Dry air weighs more than moist air Dry air weighs more than moist air. The more the air weighs, the higher the pressure is. Weather in a high pressure area is generally dry and fair. A high-pressure area is shown with a H in the center.

Try this one: 980 980 950 950 925 950 980 925 925 950 980

What do you notice about the numbers? Higher or lower toward the center? Because they went lower, it is a low pressure area. How do you think you mark a Low pressure area?

Moist air weighs less than dry air. The less the air weighs, the lower the pressure is. The weather in a low pressure area is generally cloudy or rainy. A low pressure area is shown with an L in the center

You may see a map such as this on The Weather Channel.