What is an Isobar? - Definition & Examples

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  • 0:00 Definition of an Isobar
  • 1:38 What Are Isobars Used For?
  • 3:08 Lesson Summary
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Lesson Transcript
Instructor: Linda Fye
Learn the definition of an isobar, then explore the way in which they are applied on a map and what they show. After completing the lesson, you'll understand why isobars are useful tools for predicting weather.

Definition of an Isobar

Have you ever wondered how a meteorologist knows if your day is going to be windy or what direction the wind is going to blow from? Well, they have a mapping tool that helps them predict wind and other things about the weather. This tool is called an isobar. An isobar is a line on a map that shows a meteorologist what the pressure is at the surface of the earth. They are lines that connect equal points of pressure. Isobars can be used to map atmospheric or air pressure in a way that makes it easier to understand.

But first, what is atmospheric pressure? It's basically the force of the atmosphere on the surface of the earth, and it is caused by gravity. Although it's a little simplistic, you can think of it as the 'weight' of the air. But, atmospheric pressure varies from place to place on the surface for a number of reasons.

Pressure and temperature are often tied together. In general, warm air tends to rise. If the ground is warm in an area, it causes the air to rise, and this creates low pressure. The opposite is true with cold air. Cold air tends to sink, and this leads to high pressure. The most common way to measure air pressure is with a barometer, and it is usually recorded with the unit of pressure called a millibar. A millibar just reflects how much air pressure there is in a specific unit of area. For example, average pressure at sea level is 1013.25 millibars. Meteorologists look at the differences in pressure to learn about the weather. Just to give you an idea of what they're looking at, air pressure of 1032 millibars is considered high pressure, and 984 millibars is low pressure.

What Are Isobars Used For?

Once air pressure is measured, it can be plotted on a map. Points of equal pressure can be connected with a line, and these are the isobars. They are commonly shown on a map in increments of four millibars. Isobars drawn on a map this way make it visually easier for a meteorologist to interpret.

One way to think about an isobar map is like it's showing elevation. Areas of high pressure are like mountains, and areas of low pressure are like valleys. Air moves horizontally when there are different pressures in different areas. This mechanism that creates wind is called the pressure gradient force. The air movement, or wind, goes from high pressure to low pressure. So, if you think about it like elevation, the wind blows from the high pressure at the top of the 'mountain' to the low pressure in the 'valley.' This is how meteorologists determine wind direction. They can tell wind speed by how close the isobars are together. The closer they are, the faster the air moves from high to low pressure.

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