Low-pressure area (weather system)

A low-pressure area is a weather system in which the air pressure at the center of the system is lower than the air around it. The lower pressure is caused by moving air, which people feel as wind, as it converges and rises into the atmosphere. With less air at the surface, the atmospheric pressure drops. This rising air causes water vapor in the air to cool and condense, forming clouds and often precipitation as well. Due to the rotation of Earth, the winds around a low-pressure system swirl counterclockwise in the Northern Hemisphere, while in the Southern Hemisphere, the winds swirl clockwise. This air movement around a low-pressure area is called cyclonic flow.

Background

Earth’s atmosphere is a multi-layered envelope of air surrounding the planet. The lowest and thickest of these layers is called the troposphere, which contains about 70 to 80 percent of the atmosphere’s mass. This layer extends from about 4 to 12 miles (6.5 to 19.3 kilometers) above Earth’s surface and is thickest near the equator and thinnest near the poles. Because the majority of the planet’s water vapor is contained in the troposphere, almost all of the planet’s weather also occurs there.

The movement of air around the globe is driven by the heating of the sun and directed by the planet’s rotation. Sunlight is strongest and more direct at the equator, resulting in an uneven heating of the atmosphere. As the sun heats the air near the equator, the air rises and begins to move toward the poles. Air north of the equator moves toward the North Pole, while air south of the equator moves toward the South Pole. When the air reaches the cooler upper region of the troposphere, it cools, sinks, and moves back toward the equator, where it repeats the process.

However, Earth also rotates, which directs the air to the west north of the equator and to the east south of the equator. This rotation also creates three bands of air that move in opposite directions depending on the latitude. The flow of air near the equator is known as the trade winds. In the Northern Hemisphere, these winds move from east to west. In the mid-latitudes, the winds begin to move in the opposite direction. For example, the westerlies, as the winds are called in the Northern Hemisphere, move from west to east. Above 60 degrees North and South latitude, the winds shift again, moving from east to west in the north and west to east in the south.

Overview

Air pressure is simply the weight of the air in the atmosphere at Earth’s surface. The pressure varies depending on height and several other factors, but at sea level, normal air pressure in about 14.7 pounds per square inch (psi). Meteorologists measure pressure using the units inches of mercury and millibars. In these units, average air pressure is about 29.92 inches of mercury, or 1013 millibars.

A low-pressure area forms when the upward flow of air moves some of the atmosphere away from Earth’s surface. This can occur at a frontal boundary where cold and warm air masses meet. As these air masses collide, the air is forced upward, creating a vacuum effect at the surface. The surrounding air flows toward the developing low. When the winds converge, they are pushed up into the higher levels of the troposphere. This resulting upward movement of air causes the pressure at the surface to drop, a statistic that meteorologists measure using a barometer. A thermal low occurs when the atmosphere is warmed by the sun, causing the air to rise. Thermal lows often form on summer afternoons and cause thunderstorms. As the air converges into a low pressure area, it moves in a cyclonic flow—a counterclockwise direction in the Northern Hemisphere and a clockwise direction south of the equator.

As air rises, it cools, causing water vapor inside the air to condense and turn to liquid water. This process results in cloud formation. If the air is moist enough, it can bring rain, snow, or other forms of precipitation. Because air flows counterclockwise around a low in the Northern Hemisphere, winds on the south and eastern sides of the low are usually from the south, resulting in warmer, wetter weather. To the north and west, the winds are usually cooler and drier.

As the air rises in a low-pressure area, it eventually reaches the upper levels of the troposphere, where it moves horizontally. The now-cooler air eventually begins to sink, forming an area of high pressure. Because the air is stopped by the ground, it moves along the surface towards a low-pressure area. On weather maps, a low is designated by a large red “L,” while a high is shown as a blue “H.”

The stronger the low-pressure area, the faster the air will move toward its center and up into the atmosphere. On land, very intense low-pressure systems can cause severe thunderstorms with damaging winds. In the tropics, intense lows that form over the warm oceans can result in powerful storms such as hurricanes, cyclones, or typhoons, depending on the area where they form. These storms typically produce very low barometric readings. The lowest air pressure reading ever recorded at sea level occurred in October 1979 during Typhoon Tip. The storm produced barometric readings of 25.69 inches of mercury and 870 millibars. Tip still holds records for the largest and most intense storm on record, producing 190 mile-per-hour (306 kilometer-per-hour) winds and reaching 1,380 miles (2,220 kilometers) in diameter. The typhoon weakened before it made landfall in Japan but still killed eighty-six people.

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