# LESSON 1: ISOBARS

## What's the difference between a High and a Low? What kinds of weather are likely to be found in each?

Certain weather maps allow us to view High and Low pressure systems that control the weather. An isobar is a line connecting locations of equal barometric pressure. Isobar maps show where pressures are relatively high and low, and show us where pressure changes are gradual or dramatic over a distance.

## Exercise 1:

• This activity will assist you in creating and interpreting weather maps with isobars.
The image above shows an example of a weather map containing isobars. The thick black lines are called isobars. They are typically placed at intervals of 4 millibars.

Select Image 1 and print out the map with pressure readings.

1. Determine which 4 millibar intervals will be necessary to use for this map. (Keep in mind that the isobars are found every 4 millibars such as 996, 1000, 1004, etc.)
2. Draw in the isobars on the map you have printed out. Start with the 1000 mb line, then adding 996 mb, 1004 mb, etc. Label each isobar. Note: isobars form sets of curves that do not cross each other.

Isobars can be used to identify "Highs" and "Lows". The pressure in a high is greater than the surrounding air. The pressure in a low is lower than the surrounding air.

3. Label with an "H" the center of a high pressure area.
4. Label with an "L" the center of a low pressure area.

High pressure regions are usually associated with dry weather because as the air sinks it warms and the moisture evaporates. Low pressure regions usually bring precipitation because when the air rises it cools and the water vapor condenses.

5. Over which state would you expect to see rain or snow?
6. Over which state would you expect to see clear skies?

In the northern hemisphere the wind blows clockwise around centers of high pressure. The wind blows counterclockwise around lows. It is the opposite in the sourthern hemisphere.

7. Draw arrows around the "H" on your map to indicate the wind direction.
8. Draw arrows around the "L" on your map to indicate the wind direction.

Imagine that you live in El Paso, TX.

9. If a High were approaching you from the west, describe the shift in wind direction as the High approaches you and then passes to the east.
10. How would pressure measurements change as the High approaches and then passes to the east.

Imagine that you live in New York City.

11. If a Low were approaching you from the west, describe the shift in wind direction as the Low approaches you and then passes to the east.

The strength of the wind is determined by changes in the pressure, the pressure gradient. A large change in pressure over a short distance would indicate strong winds. On a weather map, isobars that are close together indicate a a strong pressure gradient.

12. Indicate on your map where you expect the strongest winds (the steepest pressure gradient).

## Other Information

### Image Credits:

Maps from The Weather Channel
Maps from Tiger Census Maps

Written by: Gene Rempel and Mike Hanson