This lists the logos of programs or partners of NG Education which have provided or contributed the content on this page. Powered by. The air around you has weight, and it presses against everything it touches.
That pressure is called atmospheric pressure , or air pressure. It is the force exert ed on a surface by the air above it as gravity pulls it to Earth. Atmospheric pressure is commonly measured with a barometer. In a barometer, a column of mercury in a glass tube rises or falls as the weight of the atmosphere changes. Meteorologist s describe the atmospheric pressure by how high the mercury rises.
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. So how does atmospheric pressure relate to daily weather patterns? What is that all about? Basically, in a nutshell, every day the heat of the sun varies all over the Earth. Because of unequal solar heating, temperatures vary over the entire globe; the air at the equator is much warmer than at the poles.
So the warm, light air rises and spreads toward the poles and the colder, heavier air sinks toward the equator. But we live on a planet that rotates, so this simple wind pattern is distorted to such a degree that the air is twisted to the right of its direction of motion in the Northern Hemisphere and to the left in the Southern Hemisphere.
Today we know this effect as the Coriolis Force and as a direct consequence, great wind spirals are produced which we know as high and low pressure systems. In the Northern Hemisphere, the air in low pressure areas spirals counterclockwise and inward — hurricanes, for instance, are Coriolis mechanisms, circulating air counterclockwise. In contrast, high pressure systems the air spirals clockwise and outward from the center.
In the Southern Hemisphere the direction of the spiraling of the air is reversed. So why do we generally associate high pressure with fair weather and low pressure with unsettled weather? Since cool air has less of a capacity to hold water vapor as opposed to warm air, clouds and precipitation are caused by cooling the air. Any droplets that might lead to the formation of clouds would tend to evaporate.
The end result tends to be a clearer and drier environment. Conversely, if we decrease the air pressure, the air tends to rise into the higher levels of atmosphere where temperatures are colder. As the capacity to hold water vapor diminishes, the vapor rapidly condenses and clouds which are composed of countless billions of tiny water droplets or, at very high altitudes, ice crystals will develop and ultimately precipitation will fall.
Of course, we could not forecast zones of high and low pressure without employing some sort of device to measure atmospheric pressure. Atmospheric pressure is the force per unit area exerted by the weight of the atmosphere.
To measure that weight, meteorologists use a barometer. This is called cyclonic flow. On weather maps, a low pressure system is labeled with red L. A high pressure system has higher pressure at its center than the areas around it. Winds blow away from high pressure. Swirling in the opposite direction from a low pressure system, the winds of a high pressure system rotate clockwise north of the equator and counterclockwise south of the equator. This is called anticyclonic flow. Air from higher in the atmosphere sinks down to fill the space left as air is blown outward.
On a weather map, you may notice a blue H, denoting the location of a high pressure system. How do we know what the pressure is? How do we know how it changes over time? Today, electronic sensors in weather stations measure air pressure. These sensors are able to make continuous measurements of pressure over time. In the past, barometers were used and measured how much air pushed on a fluid, such as mercury. When you inflate a balloon, the air molecules inside the balloon get packed more closely together than air molecules outside the balloon.
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