High-pressure area

A high-pressure area is a weather system in which the air pressure at the center of the system is higher than the surrounding air. High-pressure areas are associated with sinking, drier air and tend to be responsible for sunny skies and more settled weather. The higher pressure is caused by air moving away from the system’s center. As this air, which we feel as wind, moves away from the system, air from higher in the atmosphere begins sinking to take its place. This falling air does not allow condensation to take place, so the air remains drier and free of clouds and precipitation. The winds around a high-pressure system rotate clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.

Background

Earth’s atmosphere is an envelope of air surrounding the planet. It is divided into several layers, with the lowest and thickest—the troposphere—stretching from about 4 to 12 miles (6.5 to 19.3 kilometers) above the planet’s surface. About 70 to 80 percent of the atmosphere’s mass is contained in the troposphere, which is thickest at the equator and thinnest near the poles. Almost all Earth’s water vapor is in the troposphere, and as a result, almost all the planet’s weather occurs there as well.

Air in the atmosphere moves around the planet due to heating from the sun and is directed by Earth’s rotation. Because the sun directly shines on the equator, that area of the planet receives more sunlight. Higher northern and southern latitudes receive less sunlight, causing the planet to be heated unevenly. As the sun warms the air near the equator, the air begins to rise and moves north or south toward the poles. North of the equator, the air moves toward the North Pole, while south of the equator, it moves toward the South Pole.

However, because Earth rotates, the rising air is directed by the planet’s spin. Rising air north of the equator turns toward the west, while air south of the equator turns toward the east. As the air moves north or south, it cools, sinks, and moves back toward the equator, where the process starts all over again. The planet’s rotation also creates three bands of moving air in in each hemisphere. The moving air near the equator is known as the trade winds. In the mid-latitudes, the winds move in the opposite direction—west to east in the north, and east to west in the south. In the Northern Hemisphere, these winds are known as the westerlies. Above 60 degrees North and South latitudes, the effect reverses, and the winds again move in the same direction as near the equator.

Overview

Air pressure is the weight of the air in the atmosphere at Earth’s surface. While pressure varies based on altitude and temperature, at sea level, normal pressure in about 14.7 pounds per square inch (psi). Meteorologists use units such as inches of mercury or millibars to measure air pressure. In these units, average air pressure is about 29.92 inches of mercury, or 1013 millibars.

A high-pressure system forms when downward air flows push more of the atmosphere into an area. Cooler air is heavier than warmer air and sinks toward the ground. As the air falls and cools, it shrinks, allowing the surrounding air to fill in behind it. This increases the mass of the air and the atmospheric pressure, a statistic measured by a meteorological instrument called a barometer. High-pressure areas can occur when an airmass is cooled, either by passing over cooler land areas below or interacting with cooler air in the upper troposphere. A high-pressure area can also form in the wake of a low-pressure system. In a low-pressure area, warm, moving air converges and rises. As the air rises, it cools and condenses, often resulting in precipitation. The rising air reaches the upper troposphere where it moves horizontally. The now-cooler air heads downward to the ground, forming a high pressure area.

As the moving air heads downward, it reaches the ground and spreads out in a clockwise flow in the Northern Hemisphere and a counterclockwise flow in the Southern Hemisphere. This wind movement is known as an anticyclonic flow. This falling air is dry and stable, typically resulting in sunny skies and calm weather. In the Northern Hemisphere, the clockwise flow around a high typically drags down cooler air on its eastern side. On its western side, the high brings up warmer, tropical air from the south. The surface airflow from a high always moves out along the ground toward a low-pressure area. On weather maps, a high is designated by a large blue “H,” while a low is shown as a red “L.”

Normally, highs north of the equator move from west to east, but on occasion, they can get “stuck” in place for an extended period of time. This often occurs in winter, when a system over the high northern latitudes can stall, forcing the jet stream farther southward. This can pull down frigid air into more southern latitudes, resulting in a brutal cold snap. Conversely, a summertime high pressure area—known as a Bermuda high—that stalls over the Atlantic Ocean can funnel hot and humid air over the Eastern United States.

The highest air pressures on Earth typically occur during winter over the large land masses in northern Asia and North America. The highest air pressure ever recorded was recorded on December 31, 1968, in Agata Lake, Siberia, in the former Soviet Union. Barometric readings hit 32.01 inches of mercury, or 1083.8 millibars. This figure was about 7 percent more than normal, meaning the atmosphere at that spot had 7 percent more mass.

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