Isobars and isotherms are lines on weather maps which represent patterns of pressure and temperature, respectively. They show how temperature and pressure are changing over space and so help describe the large-scale weather patterns across a region in the map.
Why do I care? Isobars and isotherms make it easier to read and analyze weather maps. By looking at patterns of temperature and pressure, you can determine weather conditions in the next few days ahead.
What is an isotherm?
Isotherms are lines of constant or equal temperature. They are often used on weather maps by meteorologists to give a large scale view of temperatures across the U.S. If you have ever looked at a weather map in a newspaper, the isotherms are used to divide the color-filled temperatures. For example, in the map below, temperatures in the 60's may be represented by a yellow color, while temperatures in the 70's may be represented by an orange color. The line that divides the yellow from the orange is the isotherm. All of the locations between the 60 degree isotherm and the 70 degree isotherm will have a temperature between 60 and 70 degrees.
What is an isobar?
Isobars are lines of constant or equal pressure on a weather map. They can be used to find areas of low or high pressure over a broad area (like the U.S.), and they can tell us how intense the system may be. On weather maps, you may have noticed areas that have a large “L” or “H” over a region with lines circling around them. The “L” stands for low pressure (where we would expect to find the lowest pressure) and “H” stands for high pressure (where we would expect to find the highest pressure). The lines circling them are isobars. Generally the lowest pressure is where precipitation is most likely to fall, and high pressures are usually associated with clear and sunny conditions. Where the isobars are close together, windy conditions may be expected. Elongated areas of low pressure on surface and upper air weather maps are called "troughs" and elongated areas of high pressure are called "ridges."
So why do we use isotherms and isobars?
Isotherms and isobars allow us to view large scale processes much more easily than looking at the raw data from individual weather stations itself. For example, Figure C is temperature data across the U.S. Figure D is of the same data, but this time, colors and isotherms have been added. Just by glancing, it is much easier to see where the warmest and coldest temperatures will be across the U.S., and it gives you an idea of what to expect in certain regions.
Figure C: Temperature Data on a Map