Atmospheric pressure is a large determining factor in the Earth’s weather patterns. There are low and high pressure systems as well as parts of the earth that commonly experience one type of pressure or the other. The atmosphere does not exert equal pressure on all part of the world. High pressure weather systems are areas where the air pressure is greater than that of the surrounding area. Areas of the world prone to high pressure system have greater temperature extremes with higher high temperatures and lower low temperatures. Subtropical regions often have consistently high pressure. The highest pressures though occur over Asia and North America in the winter.
High pressure weather systems are caused by a process known as subsidence. Air pressure in general is a measure of how hard the air of the atmosphere is pressing down on the Earth. Simply put, high pressure areas are made up of more air molecules thus the atmosphere weighs more and exerts more pressure. When subsidence occurs air is cooled either from below or above. Often cooling from below is caused by cool ocean waters such as those off of the coast of California and parts of South America and Africa. Infrared cooling causes most cooling from above and it happens when the air cools faster than the air can be warmed by the sun. Infrared cooling occurs during winter months.
Regardless of the method of cooling, as the air molecules experience the temperature decrease they get closer together. The air becomes more dense while the air masses shrink in size. The increasing density causes the air to then move toward the Earth’s surface, due to gravity. Surrounding air fills in the space left behind by the sinking, cooled air and the process continues. High pressure areas are subsequently caused by an increases the overall amount of air in that area and more air equals more pressure.
Subsidence evaporates water vapor as well cooling off the air, causing high pressure systems to be associated with clear skies and calm weather. To contrast, low pressure systems are associated with storms. Highs do often present light winds caused by pressure differences between the air pressure system and its lower pressure surroundings. Air tends to flow from higher to lower pressure and as it does so, some wind is generated. The Earth’s rotation causes the winds of the system to rotate clockwise in the northern hemisphere and counterclockwise in the southern hemisphere.
