Barometric Pressure

Often referred to as atmospheric pressure, barometric pressure is the weight per unit area of the entire mass of air above a certain point of Earth’s surface. Decreasing with altitude and measured in inches, millibars (mb) and kilo-Pascals (kPa) by an instrument called a barometer, the gradations of barometric pressure are based on a bar, or the unit of pressure that at sea level equals the pressure of 29.53 inches (1,000 mb) of mercury.

The result of differences in air temperature and humidity levels within individual air masses, barometric pressure forms one of the major mechanical processes by which the world’s weather systems are formed and sustained. For instance, an area of high atmospheric pressure (>29.60 inches or 1,002 mb)—a high—generally fosters those calm, dry conditions most frequently associated with anticyclones, while a low—an area of low barometric pressure, >29.53 inches (1,000 mb)—gives Earth its rain squalls, gales, blizzards, extratropical cyclones, mesocyclones, and tropical cyclones.

In a tropical cyclone, where the mechanical principles of atmospheric pressure are further applied to the behavior of the system’s pressure gradients, inverse barometer, and central barometric pressure, the drop in barometric pressure begins slowly at first and then quickens as the storm’s point of lowest pressure draws closer to the observing barometer. Concurrently, wind speeds caused by the system’s steepening pressure gradient sharply increase, while wind shear zones within the eyewall may spawn tornado-like miniswirls of considerable destructiveness. Survivors of some of history’s most severe tropical cyclones have spoken of how abrupt changes in barometric pressure made their eardrums pop and their eyes involuntarily fill with tears.

The substantial changes in barometric pressure that accompany a tropical cyclone have been linked to recorded occurrences of earthquake activity during landfall. Under normal atmospheric conditions, sea-level barometric pressure exerts approximately 2,160 pounds per square foot of pressure on Earth’s surface, or 14.7 pounds per square inch. For every inch (33 mb) that the barometric pressure falls, some 70 pounds per square foot of weight is released.

When multiplied across the 200-mile-wide (322- km) girth of an average hurricane or typhoon, the decrease in downward thrust on Earth’s crust is measured in the millions of tons, thereby allowing for the possibility of shifting between nearby faults. This seismic phenomenon has been particularly pronounced in Japan and the Philippines, both of which lie in active earthquake zones that are subject to an extensive number of tropical cyclone strikes. 

Barometric pressure in tropical cyclones is measured through a number of different technological systems, including dropsondes, barometers, satellite radiometers, and satellite estimates based on the

Dvorak Scale (see Dvorak Technique) as well as experienced intuition. In the United States, barometric pressure—in particular, the central barometric pressure of tropical cyclones—is measured in either millibars (mb; or mbar), inches (Hg), millimeters (mm) and hectopascals (hPa). Barometric pressures for Super Typhoon Tip, for instance, can be rendered as 25.69 inches or Hg; 870 mb or mbar; of 87.0 hectopascals, or hPa.