In our discussion on sea level in Session 1, we discussed the importance of the standard pressure at sea level. Because the surface of the earth is not flat and pressure decreases with height, it is hard to compare surface pressures taken at locations of different elevations. In order to standardize surface pressures we reduce them to sea level, meaning we calculate what they would be if the elevation at which they were taken were the same as the sea level. We use the term "reduce" whether the elevation of original pressure is obtained at a level above sea level or below.

Consider the surface pressure at the top of a mountain compared with the surface pressure at the base the mountain. We would expect the pressure at the base to be higher. Now suppose a low pressure system were moving in and the two pressures were equal. It is difficult to evaluate the low pressure system unless we have a standard point of reference. We can create a reference by reducing both pressures to sea level. Once this has been done we can compare the reduced pressures and determine the strength of the system. When we watch weather broadcasts on TV, the meteorologist reports the local surface pressure. That pressure is a real surface pressure, but the pressures that appear on the surface maps showing high and low pressure systems have been reduced so that pressure trends are easily discernible. The hypsometric equation enables us to easily reduce surface pressures. This equation can be written as

By reducing surface pressures to sea level we can evaluate pressure changes over large scales such as across the entire country or even globally. This enables us to recognize pressure trends that will indicate horizontal and vertical motion in the atmosphere. Session 4 will discuss how these pressure changes actually influence motion in the atmosphere.

Computing Sea Level Pressure

You can investigate the pressure reduction equation above using a Javascript calculator. This calculator will allow you to solve for many of the variables in the equation in order to see the relationships between the various parameters. Because this calculator is written in Javascript and uses frames, it requires Netscape 2.02 (or a more recent version).

On to Part 4: Where and How We Take Measurements

Back to Equivalent Potential Temperature

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