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 THE DIFFERENCE BETWEEN ISOBARS AND ISOHYPSES

METEOROLOGIST JEFF HABY

Although they look similar when plotted on a weather chart, there is a distinct difference between isobars and isohypses. An isobar connects points of equal barometric pressure normalized to sea level while an isohypse connects points of equal geopotential height. Isohypses are also called height contours. Isobars are plotted only on surface charts while isohypses are plotted on upper air charts such as the 850, 700, 500, and 300 mb analysis and forecast charts.

What is an isohypse and how is it found? An isohypse, or height contour, represents the distance from zero geopotential meters. Geopotential assumes the earth is perfectly flat and perfectly round. The difference between a geopotential meter and an actual meter is very small because the earth is close to being a perfect sphere and sea level is close to the elevation of zero geopotential if the earth was perfectly smooth. Geopotential can be thought of as the distance above sea level. An isohypse represents the distance from zero geopotential meters (at about sea level) to the pressure level of interest. For example, if the 700 millibar height at OKC is 2,970 geopotential meters, this means it is 2,970 gpm from sea level to the 700 millibar level. Rawinsondes can determine the geopotential height for each pressure level in the atmosphere above the point they are launched. From this information, points of equal geopotential height can be connected at each pressure level in the atmosphere that is plotted. This produces the isohypse contour pattern of troughs, ridges, shortwaves, etc. on an upper level plot.

The value of an isohypse is dependent on the average temperature of the air and the average moisture content of the air underneath the pressure level of interest. Low isohypses values indicate colder air (troughs) while high isohypse value (ridges) indicates warmer air (warm air expands thus produces higher heights) under the pressure level of interest. Moisture has a minor effect on height as compared to temperature. Moist air will have a little higher isohypse value than dry air at the same temperature because moist air is less dense than dry air.

How is the value of an isobar determined? Isobars are normalized to sea level. Normalization means each the city is given an isobaric value that represent the station pressure that city would have if it were at sea level. After pressures are normalized, low elevation regions can be compared to high elevation regions with respect to which locations have higher or lower pressure. If pressure is NOT normalized, high elevation regions would just about always have lower pressures than low elevation regions. Normalization makes it possible to know which locations are truly experiencing relative high or low pressure at the surface.