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Southern AER

A Quarterly Activity Bulletin of The South Carolina Department of Natural Resources-Southeast Regional Climate Center
Spring 2000
Volume 6, No. 1

Mapping the Weather

Since the weather effects everyone, it is important that we keep track of what the weather is like so we will know when to expect storms, or when to plan outdoor activities.   The most vital tool used by weather forecasters is a weather map.   Meteorologists use the information conveyed in a weather map to make predictions and advisories.  Weather maps are better than tables because maps help us visualize the changes in weather over space, as well as how fast storm systems are moving. 

Weather Maps

There are many weather elements recorded on a weather maps.  Symbols have been developed so that many weather elements can be reported in a small amount of space.


Weather is a phenomenon that occurs everywhere on earth. At any given moment at every location on earth, weather is taking place. Due to the fact that there are a limited number of stations that record weather observations, estimations are made for places without a recording station using observations from nearby stations.  Interpolation is the technique used to estimate various weather variables at locations where observations are not available by using the closest available observations. The interpolation process is more accurate when there are many observations available. Once the variables are interpolated, lines are drawn to show what locations have equal values. These lines are called Isopleths. Interpolation can be done on a variety of weather variables. We will first look at how temperatures are interpolated.


Isotherms are lines on a map that connect points of equal temperature. Since latitude plays a large part in controlling temperature variations, isotherms often run east to west. There are exceptions to this rule, of course, since other factors that control temperatures include land and water, ocean currents, and elevation. Temperatures vary between coastal locations and inland locations. Temperatures may also vary due to warm or cold ocean currents. Since temperature decreases with height, mountainous areas often have a lower temperature than areas with relatively flat terrain. See the figures below as well as the page of tips to keep in mind when drawing isopleths

Draw lines to connect points of equal temperature. All lines on one side of the isotherm should be less than that isotherm value, and all lines on the other side of the isotherm should be more than that isotherm value. If the given temperature values jump more than one degree, you can decide on the placement of the isotherm by dividing the distance between the observations so that there is an equal distance between degrees.

Your turn

Take the below map and draw your own isotherms. Start with 50 degrees and draw an isotherm for every five degrees (50, 55, 60, 65...). Remember, although isotherms generally run east to west, this is not always the case.

Map courtesy of the AMS's The Datastreme Project
Click here to get a copy of the map to print out. (don't forget to print it landscape by changing the orientation under properties!)


Isobars are lines on a map that connect points of equal pressure. These lines are drawn at intervals of 4 mb, with 1000 being the base value. Isobars are very useful when predicting the weather, as high pressure is associated with clear weather and low pressure is associated with stormy weather. Isobars are different than isotherms in that they are not as easy to draw because they do not follow as predictable patterns. In addition, low and high pressure areas will often have closed isobars around the points of lowest or highest pressure that create a bullseye on the map. See the below pictures for an example.


Now that you have had some practice interpolating temperatures, try interpolating the pressures on the map of the US.

Map courtesy of the AMS's The Datastreme Project
Click here to get a copy of the map to print out. (don't forget to print it landscape by changing the orientation under properties!)


1.   Why are weather maps better than tables or charts when it comes to making predictions and advisories?

2.   What is interpolation?

3.   Why is interpolation necessary?

4.   What are isobars?

5.   What are isotherms?

6.   Why do isotherms generally run east to west? Why is it that they do not always run east to west?

Permission is granted for the reproduction of materials contained in this bulletin.

Southern AER
Southeast Regional Climate Center
S.C. Department of Natural Resources
1201 Main Street, Suite 1100
Columbia, South Carolina 29201

The South Carolina Department of Natural Resources prohibits discrimination on the basis of race, color, sex, national origin, disability, religion, or age. Direct all inquiries to the Office of Human Resources, P.O. Box 167, Columbia, SC 29202.