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Using the Brunt Equation to Estimate Minimum Temperature

Tom Oswalt, Larry Parsons and John Jackson - UF/IFAS

The temperature at which the water vapor in the air condenses as dew on grass, the roof of a car, or leaves on a tree is described as the dew point temperature. During cold weather, the dew point has a significant effect on the rate and amount of heat that can be lost to the atmosphere. Therefore, knowing the dew point can provide enough information to estimate the overnight minimum temperature. During a clear, calm night, heat is lost from the surface of Earth through radiation. Once the dew point is reached on one of these nights, heat released as water vapor condenses into water significantly slowing the rate of temperature decrease.

If, on a cold night, the sunset dew point and air temperatures are known at some location, the minimum overnight temperature can be estimated using the Brunt equation. The dew point temperature does not vary greatly over short distances, e.g., several miles. Therefore, the calculated sunset dew point from a nearby site can be used. The temperature, however, can vary greatly with distance depending on elevation and topography, and therefore, should be measured directly at the location of interest. The Brunt equation was designed to be used for a stable air mass of uniform moisture. Therefore, if dry air moves into the area or winds increase noticeably, significant errors could be introduced.

The FAWN Minimum Overnight Temperature calculator estimates the minimum overnight temperature using user-provided sunset air and dew point temperatures. Given the dew point does not vary greatly with distance, the sunset dew point from the closest FAWN site, and measured sunset air temperature at the site of interest can be used. These values can be inserted into the FAWN Minimum Overnight Temperature calculator to determine estimated minimum overnight temperature for that location. Actual minimum temperature can vary from the estimate by several degrees depending on nighttime conditions. Therefore, it should be noted that this method should be one of several tools used to manage a cold protection program.