Atmospheric pressure describes the pressure exerted by the atmosphere on any given surface as a result of its weight. The unit of measurement for atmospheric pressure is normally the millibar, 1,000 millibars being one bar or one million dynes per square centimeter. A dyne is a measurement of force.
Most people become familiar with atmospheric pressure in the artificial environments of balloons. In simple terms, the more air is forced into a balloon, the higher will be the pressure of the air inside it. If the balloon is then burst, the high pressure air will rush out through the hole in an attempt to equalise the air pressure on the inside with that on the outside.
In the natural atmospheric environment there are areas of relatively higher and lower pressure. As the earth’s atmosphere constantly moves from areas of higher to lower pressure, as in the case of the burst balloon, so winds are created.
It has been established that the average atmospheric pressure over the earth as a whole is 1013.25 mbs (millibars). This represents the force required to push a column of mercury, at 0 degrees Celsius, up 29.92 inches, or 760 mms. Put another way, the average pressure represents the equivalent of a weight of 1033.3 grammes pushing down on one square centimeter.
Atmospheric pressure is measured by a barometer, and there are two types, aneroid and mercury. The second is the easier of the two to explain, as it relates to the 29.92 inch column of mercury in average conditions. As the atmospheric pressure increases, so the column of mercury will rise, so that pressure in this situation can be measured in inches; relatively high being more than 29.92, and relatively low being less than 29.92.
An aneroid barometer comprises a metal box inside which there is a virtual vacuum. When the atmospheric pressure outside the box increases, it pushes in the box’s flexible sides, and conversely, when the pressure outside decreases, the sides move outwards. These movements of the box are connected by a spring to a needle on a calibrated dial.
This type of barometer, only liable to very slight errors, is much more convenient than one requiring a column of mercury at a constant temperature, and is therefore by far the more common of the two. It will be familiar to anyone measuring the height of land, or using an altimeter in an aircraft.
As atmospheric pressure represents a weight of air, it decreases away from the earth’s surface. Quite simply, at a higher level there is less air, and therefore less weight. This is why a barometer can be used to measure altitude.
When the sun heats an area of the earth, the air immediately above it will receive radiated heat. It will therefore expand and rise. This will reduce the atmospheric pressure, just as the air moving out of a balloon reduces the pressure within it, but more gradually. This vertical movement of the atmosphere because of heat, particularly as it occurs in Equatorial regions, reduces atmospheric pressure at ground level, causing air from the north and south of the Equator to move in, as it were, to fill the space. The vertical and horizontal movements of air which this process generates are the cause of winds.
Atmospheric pressure rarely remains the same for long periods of time. When it does, weather conditions become particularly stable. What is more normal is for the pressure to vary and so for winds to be generated and blow. The greater is the gradient of pressure between higher and lower pressure areas, the stronger will be the wind. Wind is therefore air which is caused to move from areas of high atmospheric pressure to low, as in the case of the burst balloon. Therefore, an appreciation of likely winds in a weather forecast requires a study of changing atmospheric pressure, as one causes the other.