Pressure

The pressure differences found in the atmosphere are the prime indicator of the strength and direction of the wind. Atmospheric pressure is defined as the weight or force exerted by the atmosphere per unit area. Pressure varies with changes in temperature, moisture content and airmass motion. Pressure is measured by the weight of Mercury (Hg) or Water (H 2 O) that the air can support. As such pressure decreases approximately at 1 inch of Hg per 1000 feet of altitude at low altitudes (the decrease is NOT linear). For reference purposes a “Standard Pressure” has been defined at sea level to be 29.92 in. Hg (760 mmHg), 14.7 pounds per square inch (psi) or 1013.2 millibars ( mb). Station Pressure (alsocalled Field Level Pressure) is the measured pressure at a given altitude. Sea Level Pressure is the observed pressure corrected to Mean Sea Level (MSL). Isobars are lines on a weather map joining points of equal pressure, usually drawn in intervals of 4 mb. A Low is a centre of pressure surrounded on all sides by higher pressure; also called a cyclone. (Cyclonic curvature is the curvature of isobars to the left when standing with lower pressure to left). A High on the other hand is centre of pressure surrounded on all sides by lower pressure; also called an anticyclone. (Anticyclonic curvature is the curvature of isobars to the right when standing with lower pressure to left). A Trough is an elongated area of low pressure with lowest pressure along a line marking maximum cyclonic curvature and a Ridge is an elongated area of high pressure with the highest pressure along a line marking maximum anticyclonic curvature. A trough of warm air aloft is called a Trowal.

Wind is the term that describes the horizontal movements of air due to these pressure differences. Frictional force slows down the wind and counteracts the Coriolis effects such that the wind flows somewhat across the isobars. Frictional force slows down the wind and counteracts the coriolis effects such that the wind flows somewhat across the isobars. The wind veers or changes direction in a clockwise manner as you climb from the surface generally. Conversely the wind backs or changes direction in a counterclockwise manner as you descend. Near large bodies of water land breezes generally occur nighttime as the cooler air of the land flows out over the water. Conversely sea breezes are often a daytime phenomena when the reverse is true. Similar temperature affects are seen in the mountains and hills which warm up and cool down faster than the valleys. An anabatic wind flows up a mountain valley during the day, while a katabatic wind goes down a mountain valley during the evening.