A chart of the atmospheric pressure over a large area of the Earth's surface at any given time tells you which way different air masses (masses of air which have common temperature and humidity characteristics) are moving. Some air masses originate in the cold polar regions; some in the tropics. By the time they reach you, some air masses have moved from large bodies of water (called maritime air masses). Others (called continental air masses) have grown up over more or less dry land.
Air masses carry along with them the temperature and humidity characteristics of the areas they crossed. Where distinctly different air masses touch, the boundary between them is called a front and is marked by cloudiness and precipitation.
PRESSURE AREAS
The atmosphere can produce weather in other ways, but frontal weather, can be violent, and can be predicted from a chart of the pressure systems. Atmospheric pressure is reported in inches of mercury or millibars. One atmosphere equals 14.696 psi, a bar equals slightly more than 0.98 atmosphere, and a millibar equals 1/1000 of a bar. On weather charts pressure usually is indicated in millibars. The atmosphere can produce weather in other ways, but frontal weather, which usually is violent can be predicted from a chart of the pressure systems. Atmospheric pressure is reported in inches of mercury or millibars. One atmosphere equals 14.696 psi, a bar equals slightly more than 0.98 atmosphere, and a millibar equals 1/1000 of a bar. On weather charts pressure usually is indicated in millibars. Isobars never join or cross. Some may off the chart, but others may close, forming irregular ovals that define the areas of highest and lowest pressure. Air flows from high-pressure areas to low pressure areas areas. The strength of the wind depends on two things, the amount of difference in pressure and the distance of the high-pressure area from the low-pressure area. All these factors combined are called pressure gradients. The greater the gradient, the stronger wind. Isobars can give a rough indication of the amount of wind. The closer an isobar is to another, the stronger the wind in that area.
Widely separated isobars indicate light winds,isobars closer together mean greater wind velocity. Isobars are always tied-out curves, usually making irregular ovals about the high- or low-pressure center. The greatest pressure is at the system center.
