Weather Fundamentals

Weather Almanac, (2004)

Station pressure

Pressure can be measured only at the point of measurement. The pressure measured at a station or airport is station pressure or the actual pressure at field elevation.

Pressure variation

Pressure varies with altitude and temperature of the air as well as with other minor influences.

Altitude

Moving upward through the atmosphere, weight of the air above becomes less and less. Within the lower few thousand feet of the troposphere, pressure decreases roughly 1 in of mercury for each 1,000-ft increase in altitude. At sea level, the average pressure is about 14.7 lb/in 2 . It has been found that the pressure will decrease by half for each 18,000-ft increase in altitude. Thus, at 18,000 ft, we could expect an average pressure of about 7.4 lb/in 2 and at 36,000 ft, a pressure of only 3.7 lb/in 2 , and so on.

Sea-level pressure

Since pressure varies with altitude, it is not easy to compare station pressures between stations at different altitudes. To make them comparable, pressure readings must be adjusted to some common level. Mean sea level seems the most feasible common reference. Pressure measured at a 5,000-ft station is 25 in; pressure increases about 1 in for each 1,000 ft or a total of 5 in. Sea-level pressure is approximately 25 5, or 30 in. The weather observer takes temperature and other effects into account, but this simplified example explains the basic principle of sea-level pressure reduction.

Sea-level pressure is usually expressed in millibars. Standard sea-level pressure is 1,013.2 mb, 29.9 in of mercury, or about 14.7 lb/in 2 .

Pressure analyses (using isobars)

Sea-level pressure is commonly plotted on a map and lines are drawn connecting points of equal pressure. These lines of equal pressure are isobars. Hence, the surface map is an isobaric analysis showing identifiable, organized pressure patterns. Five pressure systems are defined as follows:

LOW —a center of pressure surrounded on all sides by higher pressure; also called a cyclone. Cyclonic curvature is the curvature of isobars to the left when you stand with lower pressure to your left.

HIGH —a center of pressure surrounded on all sides by lower pressure, also called an anticyclone. Anticyclonic curvature is the curvature of isobars to the right when you stand with lower pressure to your left.

TROUGH —an elongated area of low pressure with the lowest pressure along a line marking maximum cyclonic curvature.

RIDGE —an elongated area of high pressure with the highest pressure along a line marking maximum anticyclonic curvature.

WIND

What causes wind?

Differences in temperature create differences in pressure. For example, local winds along lake and ocean shores are the result of the temperature differences between land and water, which cause a pressure difference and wind. These pressure differences drive a complex system of winds in a never-ending attempt to reach equilibrium. Wind also transports water vapor and spreads fog, clouds, and precipitation.