What is Surface Pressure?

Surface pressure, also known as atmospheric pressure, is the pressure that the air exerts on the surface of the Earth. Air has mass, and the effects of gravity on that mass cause air pressure. Air pressure differs by geographical region and by season. Changes in air pressure, throughout the year and over different parts of the Earth, are largely responsible for changes in weather. Atmospheric pressure is typically measured with a barometer, and it is most often measured in units called Pascals (Pa).

As a whole, the atmosphere of the Earth exerts pressure on the surface of the Earth equal to 100,000 Pascals or 100 kilopascals (kPa). This number represents the average surface pressure of the atmosphere on the Earth. In reality, surface pressure can vary widely over different geographical areas. Variances in air pressure depend largely on the seasons of the year, and they are considered responsible for most weather phenomenon.

From the months of December to February, areas of of high atmospheric pressure typically exist over Northern Africa and Spain, in the Pacific Ocean off the Californian coast, over central Asia, over central North America, and over subtropical ocean waters in the Southern Hemisphere. The southernmost regions of Greenland and the Aleutian Islands can usually be expected to experience low surface pressure during these months of the year.

From the months of June through August, these pressure systems typically change. The high pressure systems over central Asia, as well as the low pressure systems over Greenland and the Aleutian Islands, generally vanish. The high pressure systems of the Pacific Ocean, Spain, North Africa, and subtropical Southern Hemisphere waters, generally grow stronger and extend to the north. High pressure systems tend to appear over Antarctica and Australia during these months, while central and southwest Asia develop the low surface pressure said to contribute to that continent's rainy season.

Weather usually occurs when air moves from an area of high surface pressure to an area of low surface pressure. The movement of air is commonly known as wind, and it typically moves in spiral-shaped patterns caused by the Earth's rotation. Air subjected to higher levels of surface pressure is usually more dense than air subjected to low levels of surface pressure; this difference in density normally contributes to the movement of air from high to low pressure areas. Air generally becomes less dense as it moves into areas of low atmospheric pressure. This change in air density often contributes to the development of atmospheric depressions, which are characterized by clouds and precipitation.

Surface pressure is the force per unit area exerted by the weight of the air above a given point on Earth's surface. It's measured in units such as millibars (mb) or hectopascals (hPa). Variations in surface pressure are indicative of different weather patterns, with high pressure often leading to clear skies and low pressure associated with clouds and precipitation.

Surface pressure is measured using an instrument called a barometer, which can be either mercury-based or aneroid. Mercury barometers measure pressure by the height of mercury in a column, while aneroid barometers use a small, flexible metal box called an aneroid cell. Changes in air pressure cause the cell to expand or contract, which is then translated into a pressure reading.

Surface pressure changes due to the heating and cooling of the Earth's surface, which causes air to rise and fall, respectively. This movement results in areas of high and low pressure. Additionally, the rotation of the Earth and the uneven distribution of land and water masses contribute to variations in surface pressure, influencing weather systems globally.

Surface pressure is a key factor in determining weather conditions. High-pressure systems are typically associated with descending air, leading to clear skies and stable weather. Conversely, low-pressure systems involve ascending air, which can result in cloud formation and precipitation. Meteorologists closely monitor pressure changes to forecast weather patterns and potential storms.

Yes, rapid changes in surface pressure can impact both human and animal health. For instance, some people may experience joint pain or headaches when the pressure drops, a phenomenon known as a barometric pressure headache. Animals, particularly those sensitive to changes in the environment, may also display altered behavior prior to weather changes influenced by surface pressure shifts.

Surface pressure is crucial in aviation for calculating aircraft performance and ensuring flight safety. Pilots need accurate pressure readings to determine their altitude above sea level, as altimeters are calibrated to pressure levels. Additionally, understanding pressure patterns helps pilots anticipate weather changes and make necessary adjustments to their flight plans.