The atmosphere surrounding Earth has weight, and atmospheric pressure is simply the force exerted onto the surface by the column of air directly above it. This pressure is not uniform across the globe; areas where the air column is heavier are called high-pressure systems, and regions where it is lighter are low-pressure systems. These differences in pressure are the fundamental drivers of weather, causing air to move from high to low areas, which we experience as wind.
Defining Characteristics and Air Movement
The fundamental difference between the two systems lies in the vertical movement of air within the column. A high-pressure system is characterized by a process called subsidence, where air descends toward the surface. As this air sinks, it compresses and warms adiabatically.
At the surface, the dense, sinking air must diverge, or spread out, away from the center of the high-pressure system. This outward movement of air is influenced by the Earth’s rotation, a phenomenon known as the Coriolis effect. In the Northern Hemisphere, this effect causes the air diverging from the center of a high-pressure system to rotate in a clockwise direction.
Conversely, a low-pressure system is defined by air rising from the surface in a process known as uplift. As the air ascends, it expands because the surrounding atmospheric pressure decreases with altitude, and this expansion causes the air to cool.
This vertical motion draws air inward at the surface, a process called convergence. The converging air spiraling into the center of a low-pressure system is also subject to the Coriolis effect. In the Northern Hemisphere, this force deflects the moving air to the right, creating a distinct counter-clockwise rotation as the air flows toward the low-pressure center.
Associated Weather Conditions
The vertical motion within each pressure system is the primary determinant of the associated weather conditions. In a high-pressure system, the sinking and warming air mass leads to highly stable atmospheric conditions. This warming inhibits the condensation of water vapor, which prevents the formation of clouds and precipitation.
Consequently, high-pressure systems are strongly associated with fair weather, clear skies, and lower humidity. The lack of cloud cover allows for greater incoming solar radiation during the day, but it also permits more heat to escape at night, often leading to a larger difference between daytime high and nighttime low temperatures.
The rising and cooling air in a low-pressure system has the opposite effect on weather stability. As the ascending air cools, water vapor condenses into liquid droplets, leading to the formation of clouds. If the uplift is strong and sustained, the amount of condensation can increase enough to result in precipitation, such as rain or snow.
Low-pressure systems are therefore the common source of unsettled, stormy, or dynamic weather, often accompanied by increased humidity. These systems are frequently associated with the development of weather fronts, which intensify the uplift and precipitation. The convergence of air at the surface also means these systems often produce stronger winds compared to the light winds typically found near the center of a high-pressure area.
Mapping and Tracking
Meteorologists track these systems using specialized weather maps that feature lines called isobars, which connect all points on the map that share the exact same atmospheric pressure at a given time. The centers of high-pressure systems are typically marked on a map with a large blue ‘H,’ while low-pressure systems are denoted with a red ‘L’.
The spacing between these isobars provides a direct visual indicator of wind strength, a concept known as the pressure gradient. When isobars are packed closely together, it signifies a steep pressure gradient, resulting in high wind speeds. Conversely, widely spaced isobars indicate a weak pressure gradient and suggest much calmer wind conditions.
Air naturally flows from areas of high pressure toward areas of low pressure. In the mid-latitudes, both high and low-pressure systems generally follow a path from west to east across the continent. By observing the movement and intensity of these pressure centers and their associated isobars, forecasters can predict changes in wind, cloud cover, and precipitation for the days ahead.