Understanding Atmospheric Pressure
Atmospheric pressure is very important to meteorology. The distribution of pressure in the atmosphere controls the winds and to some considerable extent the occurrence of clouds and weather. Atmospheric pressure is defined as the weight of a column of air rising vertically above that level to the top of the atmosphere. For example, an atmospheric pressure of 1025 millibars at the surface correlates to a weight of 14.7 pounds of air in a 1 inch square 'tube' rising to the top of the atmosphere.
The actual atmospheric pressure measured at a station is known as the Station Pressure. This pressure decreases with height (shorter 'tube' of air over a higher station). In order to compare pressures at various stations, the pressures are reduced to a Mean Sea Level (MSL) pressure. This is done by taking the mean temperature over the past 12 hours and adding/subtracting an imaginary column of air to the Station Pressure. This reduces the pressures at the stations to a common 'level' allowing forecasters to analyze the pressure differences horizontally.
On a weather chart isobars are lines connecting points of equal MSL pressure. The patterns that are generated allow the forecasters to make a prediction of the wind speed and direction, locate fronts and other synoptic features and to forecast clouds and weather.
Low Pressure Areas
Low pressure areas may vary in size from a few kilometers (or even hundreds of meters - tornadoes) to many hundreds of kilometers in extent. As wind blows from high pressure areas to low pressure areas this means that air will 'pile up' in a low and have nowhere to go but up creating a column of rising air at the center of the low. When this air rises it expands and cools with the associated creation of condensation and cloud development. As a result lows are associated with areas of moist, cloudy weather.
Troughs are low pressure areas that are stretched out into a line which may be several hundred kilometers long. Should a front exist, it will have formed in a trough.
High Pressure Areas
High pressure areas are almost always many hundreds of kilometers in diameter (or greater). As the wind flows out of a high, it creates a 'vacuum' in the lower atmosphere that draws down air from higher levels. This sinking air column (subsidence) compresses and warms. Any cloud that may be present to dissipated. Highs are generally areas of clear or almost clear skies.
Ridges are high pressure areas that are stretched out into a line which may be several hundred kilometers long.
Pressure Gradient
Looking at a weather chart you will notice that the spacing of the isobars (lines of equal MSL pressure) are not always evenly spaced. The change in pressure in the horizontal is known as the Pressure Gradient. The effect that different gradients has on the wind is that the closer the isobars are spaced, the stronger the wind velocity will be.
Understanding Wind
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