WRAL WeatherCenter Blog

Is the atmospheric pressure as reported over the TV the actual reading or is it somehow adjusted? For instance, the reported readings in Denver are in the same range as those here, but you have to boil things longer due to the lower pressure.

Posted September 12, 2007 11:46 a.m. EDT

MIKE MOSS SAYS:     Eugene,     There are three primary types of pressure readings used at weather reporting stations. The pressure exerted by the weight of the atmosphere at and above the location of the barometer is called "station pressure." This value is then adjusted to produce two different estimates of what pressure would be at sea level. One of these, "altimeter setting," is typically used in public weather reports. The purpose of the adjusted readings is to allow for a reasonable comparison of pressure at one location to another in a way that allows for analysis of the horizontal gradients of pressure and the locations of troughs, ridges and centers having high or low pressure relative to surrounding areas. Because pressure varies much more rapidly with height than it does horizontally, if one tried to use station pressure for a "surface" map you would end up with the low pressure centers and troughs always surrounding mountains and ridges due to the higher elevation of the stations at those places. For that reason, when we refer to a surface pressure map, we're usually referring to a map on which all the pressures have been adjusted to their approximate sea level values.

As you implied regarding the temperature at which water boils in Denver, station pressure there will just about always be lower than in Raleigh because of the elevation difference.