atmospheric pressure

The atmosphere that surrounds Earth has weight and pushes down on anything below it. The weight of air above a given area on Earth’s surface is called atmospheric pressure.  It is an important factor influencing Earth’s weather and climate.

Atmospheric pressure can be measured with an instrument called a barometer and this is also known as barometric pressure. In a barometer, a column of mercury in a glass tube rises or falls as the weight of the atmosphere changes. Meteorologists describe the atmospheric pressure by how high the mercury rises.

 It is usually measured in millibars (mb) or kilopascals (kPa).

An atmosphere (atm) is a unit of measurement equal to the average air pressure at sea level at a temperature of 15 degrees Celsius (59 degrees Fahrenheit). One atmosphere is 1,013 millibars or 760 millimeters (29.92 inches) of mercury.

Atmospheric pressure changes at different altitudes. As the pressure decreases, the amount of oxygen available to breathe also decreases. At very high altitudes, atmospheric pressure and available oxygen get so low that people can become sick and even die. The highest pressure is at sea level where the density of the air molecules is the greatest.

Mountain climbers use bottled oxygen when they ascend very high peaks. They also take time to get used to the altitude because quickly moving from higher pressure to lower pressure can cause decompression sickness. Decompression sickness also called “the bends”, is also a problem for scuba divers who come to the surface too quickly.

Aircraft create artificial pressure in the cabin so passengers remain comfortable while flying. As you go up in an airplane, the atmospheric pressure becomes lower than the pressure of the air inside your ears. Your ears pop because they are trying to equalize, or match, the pressure. The same thing happens when the plane is on the way down and your ears have to adjust to a higher atmospheric pressure. 

Atmospheric pressure is an indicator of weather. When a low-pressure system moves into an area, it usually leads to cloudiness, wind, and precipitation. High-pressure systems usually lead to fair, calm weather.

Sea level air pressure – The greatest air pressure pressing down on our bodies is at sea level. Scientists use the term one atmosphere to describe the pressure at sea level. Normal pressure at sea level is 14.7 psi (pounds per square inch). Normal pressure at sea level measures 29.9213 inches (760 mm) on barometers. This means that on every square inch of our body. The reason we are able to move our hands back and forth is that the pressure is equal to the pressure inside and out of our bodies also.

Low-Pressure Systems

A low-pressure system, also called a depression, 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. Under these conditions, lows normally produce clouds, precipitation, and other turbulent weather, such as tropical storms and cyclones.

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. As a result, they cannot warm as much during the day (or in the summer) and at night they act as a blanket, trapping heat below.

High-Pressure Systems

A high-pressure system, sometimes called an anticyclone, is an area where the atmospheric pressure is greater than that of the surrounding area. These systems move clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere 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 temperature since there are no clouds to block incoming solar radiation or trap outgoing long-wave radiation at night.

Atmospheric Regions

Across the globe, there are several regions where air pressure is remarkably consistent. This can result in extremely predictable weather patterns in regions like the tropics or the poles.

• Equatorial low-pressure trough – This area is in the Earth’s equatorial region (0 to 10 degrees 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 clouds and heavy rainfall that are prominent throughout the area. This low-pressure zone trough also forms the Inter-Tropical Convergence Zone and trade winds.
• Subtropical high-pressure cells – Located between 20 degrees and 35 degrees north/south, 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 – These areas are at 60 degrees north/south latitude and feature 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 much of 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 degrees north/south and are extremely cold and dry. With these systems, winds move away from the poles in anticyclones which descend and diverge to form the polar easterlies. They are weak, however, because little energy is 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 the weather for use in daily life, navigation, shipping, and other important activities, making air pressure an important component of meteorology and other atmospheric science.

Isobars

Detailed weather maps show the atmospheric pressure by means of curved lines called isobars. As with an isotherm for temperature, an isobar connects all points with the same atmospheric pressure. However, there is one difference between isobars. The pressure at the land surface is less where the elevation of the surface is high, so the pressure is “corrected” to sea level. The corrected pressure is what you would measure at the place if you could dig a very deep mine all the way down to sea level and put your barometer at the bottom of the hole. The corrected pressure is used on weather maps.