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The 7 Types of Weather in a High Pressure System

Understanding Weather Forecasts When a High Moves Into the Area

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Learning to forecast the weather means understanding the type of weather associated with an approaching high pressure zone. A high pressure zone is also known as an anticyclone. On a weather map, a blue letter H is used to symbolize a zone of pressure that is relatively higher than the surrounding areas. Air pressure is typically reported in units called millibars or inches of mercury.

  1. The origin of a high pressure zone will determine the type of weather to come. If a high pressure zone moves in from the south, the weather is usually warm and clear in the summer. However, a high pressure zone originating from the north will usually bring cold weather in the winter months. One common mistake is to think all high pressure zones bring warm and nice weather. Cooler air is dense and has more air molecules per unit of volume making it exert more pressure on the surface of Earth. Therefore, the weather in a high pressure zone is generally fair and cool. An approaching high pressure zone doesn’t cause the stormy weather associated with low pressure zones.
  2. Winds blow away from a high pressure zone. If you think of the wind like a squeezed balloon, you can imagine that the more pressure you put on the balloon, the more air will be pushed away from the source of the pressure. In fact, wind speeds are calculated based on the pressure gradient produced when air pressure lines called isobars are drawn on a weather map. The closer the isobar lines, the higher the wind speed.
  3. The column of air above a high pressure zone is moving downwards. Because the air above the high pressure zone is cooler higher in the atmosphere, as the air moves downward, a lot of the clouds in the air will dissipate.
  4. Due to the Coriolis effect, winds in a high pressure zone blow clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. In the United States, the prevailing winds move from West to East. Looking on a weather map, you can generally predict the type of weather heading your way by looking towards the west.
  5. Weather in a high pressure system is usually drier. As the sinking air increases in pressure and temperature, the number of clouds in the sky decreases leaving less chance for precipitation. Some avid fishermen even swear by a rising barometer to get their best catches! Although the scientific community has had no luck in proving this tidbit of weather folklore many people still believe fish will bite better in a high pressure system. Still other fisherman think fish bite better in stormy weather making buying a fishing barometer a popular addition to a tackle box.
  6. The speed at which air pressure increases will determine the type of weather an area can expect. If the air pressure rises very quickly, the calm weather and clear skies will generally be over as quickly as they came. An abrupt rise in pressure can indicate a short-lived high pressure zone with a stormy low pressure zone behind it. That means you can expect clear skies followed by a storm. (Think: What goes up, must come down) If the rise in pressure is more gradual, a persistent period of calm may be seen for several days. The speed at which the pressure changes over time is called the pressure tendency.
  7. Reduced air quality is common in a high pressure zone. The wind speeds in a high pressure zone tend to decrease because, as discussed above, the winds move away from a high pressure zone. This can cause pollutants to build up near the area of the high pressure zone. Temperatures will often increase leaving behind favorable conditions for chemical reactions to occur. The presence of fewer clouds and warmer temperatures make the perfect ingredients for the formation of smog or ground-level ozone. Ozone Action Days are also often common during periods of high pressure. Visibility will often decrease in an area as a result of increased particulate pollution.
High pressure systems are usually called Fair Weather Systems because the 7 types of weather in a high pressure zone are generally comfortable and clear. Keep in mind that high and low pressures mean the air is under higher or lower pressure relative to the surrounding air. A high pressure zone can have a reading of 960 millibars (mb). And a low pressure zone can have a reading of 980 millibars for example. The 980 mb is clearly a greater pressure than the 960 mb, but it is still labeled a low when stated in comparison to the surrounding air.

So, when the barometer is rising expect fair weather, decreased cloudiness, possible reduced visibility, reduced air quality, calmer winds, and clear skies. You may also want to learn more by checking out how to read a barometer.

Sources:
Newton BBS Ask-a-Scientist Program
The Environmental Protection Agency

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