Molecules are significant for measuring air pressure because if the number of air molecules above a surface increases, there are more molecules to exert pressure on a surface and total atmospheric pressure increases. By contrast, if the number of molecules decreases, so too does the air pressure.
Today, air pressure is measured with a mercury or aneroid barometer. A mercury barometer measures the height of a mercury column in a vertical glass tube. As air pressure changes, the height of the mercury column does as well- it drops when pressure falls and rises when it increases. An aneroid barometer uses a coil of tubing with most of the air removed. The coil then bends inward when pressure rises and bows out when pressure drops. Using instruments such as these, scientists have set the standard of normal sea level pressure at about 1013.2 millibars (force per square meter of surface area).
Low and High PressureAir pressure is not uniform across the Earth however. The normal range of the Earth's air pressure is from 980 millibars (mb) to 1050 mb. These differences are the result of low and high air pressure systems which are caused by unequal heating across the Earth's surface and the pressure gradient force.
A low pressure system, or "low," is an area where the atmospheric pressure is lower than that of the area surrounding it. Lows are usually associated with high winds, warm air, and atmospheric lifting. Because of this, lows normally produce clouds, precipitation, and other bad weather such as tropical storms and cyclones.
In addition, areas prone to low pressure do not have extreme diurnal (day vs. night) nor extreme seasonal temperatures because the clouds present over such areas reflect incoming solar radiation back into the atmosphere so they cannot warm as much during the day (or in the summer) and at night they act as a blanket, trapping heat below.
Conversely, a high pressure system, or "high," is an area where the atmospheric pressure is greater than that of the surrounding area. In some places highs are referred to as anticyclones. These move clockwise in the northern hemisphere and counterclockwise in the southern due to the Coriolis Effect.
High pressure areas are normally caused by a phenomenon called subsidence, meaning that as the air in the high cools it becomes denser and moves toward the ground. Pressure increases here because more air fills the space left from the low. Subsidence also evaporates most of the atmosphere's water vapor so high pressure systems are usually associated with clear skies and calm weather.
Unlike areas of low pressure, the absence of clouds means that areas prone to high pressure experience extremes in diurnal and seasonal temperatures since there are no clouds to block incoming solar radiation or trap outgoing longwave radiation at night. Thus such areas have higher high temperatures and lower lows.
Global Lows and HighsAcross the globe (diagram), there are several important consistently low and high pressure areas. They are as follows:
The Equatorial Low Pressure Trough: This area is in the Earth's equatorial region (0°-10° North and South) and is composed of warm, light, ascending and converging air. Because the converging air is wet and full of excess energy it expands and cools as it rises, creating the clouds and heavy rainfall that are prominent throughout the area. This low pressure zone trough also forms the ITCZ and trade winds.
Subtropical High-Pressure Cells: Located between 20° N/S and 35°N/S this is a zone of hot, dry air that forms as the warm air descending from the tropics becomes hotter. Because hot air can hold more water vapor, it is relatively dry. The heavy rain along the equator also removes most of the excess moisture. The dominant winds in the Subtropical high are called westerlies.
Subpolar Low-Pressure Cells: This area is at 60° N/S latitude and features cool, wet weather. The Subpolar low is caused by the meeting of cold air masses from higher latitudes and warmer air masses from lower latitudes. In the northern hemisphere, their meeting forms the polar front which produces the low pressure cyclonic storms responsible for precipitation in the Pacific Northwest and Europe. In the southern hemisphere, severe storms develop along these fronts and cause high winds and snowfall in Antarctica.
Polar High-Pressure Cells: These are located at 90° N/S and are extremely cold and dry. With these systems, winds move away from the poles in an anticyclone which descends and diverges to form the polar easterlies. They are weak however because there is little energy available in the poles to make the systems strong. The Antarctic high is stronger though because it is able to form over the cold landmass instead of the warmer sea.
By studying these highs and lows, scientists are better able to understand the Earth's circulation patterns and predict weather for use in daily life, navigation, shipping, and other important activities, making air pressure an important component to meteorology and other atmospheric science.