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How to Read Isobars

Surface weather maps use isobars (lines of equal atmospheric pressure) to illustrate air pressure changes (measured in millibars) in much the same way as contour lines on a topographic map display differences in elevation. Isobars link areas of like pressure in long, sloping lines or, in some cases, circling high pressure centers (topographic mountain tops) or low pressure centers (topographic valleys). These isobars help meteorologists predict wind direction and wind speed, which becomes important in marine settings (where wind creates waves) and firefighting (where wind can spread fires).

Things You'll Need

Online weather map showing air pressure readings
Printout of weather map with air pressure readings
Three pencils (one #2, one blue, one red)

Instructions

- 1
  Access a weather map online that shows air pressure readings. If possible, print out the weather map for drawing and writing notes. Examine the air pressure readings on the weather map. Look for areas of like pressure, observing any patterns created.
- 2
  Notice that areas of like pressure either pack closely together, become spaced apart, or encircle an area of high (H) or low (L) pressure. Using the #2 pencil, create isobars by connecting areas of like pressure readings, taking note of the sloping lines they create and the distance between isobars. Note that isobars packed close together indicate steep pressure gradients and isobars spaced far apart signify shallow pressure gradients
- 3
  Look for isobars circling an area of pressure, noticing whether the enclosed pressure area is higher or lower than the isobar pressures. Write a red "H" in the circle for pressure areas higher than the isobar or a blue "L" for pressure areas lower than the isobar. Use the #2 pencil to specify wind direction around the central "H" or "L" pressure areas. Draw clockwise arrows around "H" areas in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. Pencil in counterclockwise arrows around "L" areas in the Northern Hemisphere and clockwise in the Southern Hemisphere.
- 4
  Indicate wind direction by drawing arrows with the #2 pencil, from areas of high pressure moving toward areas of low pressure. Observe the distance between isobars, noting that closely spaced isobars indicate stronger winds and wide spacing indicates gentler winds. Show this on the map with the size of the arrows indicating wind direction. Draw short arrows to illustrate strong winds due to a great contrast in pressure gradients and longer arrows for calmer winds and milder pressure gradients.

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