LESSON 1: ISOBARS
What's the difference between a High and a Low? 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.
kinds of weather are likely to be found in each?
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.
- This activity will assist you in creating and interpreting weather maps
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.
How would pressure measurements change as the High approaches and then passes to
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).
- Maps from The Weather
- Maps from Tiger Census Maps
Written by: Gene Rempel and Mike
Last Modified September 19, 1998