How to Find the Maximum Pressure Tendency
Air pressure refers to the weight of the air pressing down on the ground. It can be used to predict changes in the weather. Pressure tendency is the characteristic and amount of change in air pressure over a specific period of time, usually three hours. You can calculate air pressure and find maximum pressure tendency using a barometer, which is why you'll sometimes see air pressure referred to as barometric pressure.
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Instructions
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Set your barometer outside and turn it on if you're using an electronic barometer. Note the air pressure. As an example, the initial air pressure might be 1,000 millibars.
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Note the air pressure at one-hour, two-hour and three-hour intervals. As an example, the pressure at one hour might rise to 1,001 mb, at two hours to 1,003 mb and at three hours to 1,005 mb.
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Subtract the initial barometer reading from the reading at the three-hour mark to calculate the maximum pressure tendency. In the example, subtracting 1,000 mb from 1,005 mb equals 5 mb.
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Interpret the pressure tendency. Pressure tendencies less than 0.1 mb indicate a steady character whereas those between 0.1 and 1.5 mb suggest a slowly rising or falling character. Those between 1.6 and 3.5 mb indicate a rising or falling character, and those between 3.6 and 6.0 mb suggest a quickly rising or falling character. Those above 6.0 mb indicate a rapid rising or falling character. In the example, the pressure tendency of 5 mb means that the air pressure is quickly rising, and this indicates that stormy weather is clearing. If the pressure tendency was -5 mb, it would mean that stormy weather was moving in.
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References
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