FAWN Cold Protection Toolkit

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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.

The Forecast Tracker provides a visual display which shows if the National Weather Service's point forecast is indicating that damage could occur during the next twenty four hours. The Forecast Tracker is intended to provide increased confidence in the forecast and allow producers to make better informed decisions.

One of the key elements of a cold protection program is having a minimum temperature forecast that can be utilized to determine the potential for economic freeze damage. Not all forecasts are exactly on target, as each cold night has unique features which must be analyzed correctly by forecasters. The FAWN Forecast Tracker plots the actual temperature and the forecasted temperature for the previous twenty four hours showing how well the forecast has tracked the actual temperature. The forecasted temperature is displayed for the next twenty four hours along with an optional critical temperature line to give an idea as to whether or not the temperature will drop below the critical temperature during the night.

FAWN does not make weather forecasts, but utilizes the National Weather Service products, especially the pin point forecasts. For more information see J ETSTREAM, an online weather school, Pinpoint Forecasts.

Using Wet Bulb Temperature to Determine When to Turn Off Irrigation Systems Used for Cold Protection

John Jackson, Larry Parsons, & David Martsolf

Shutting down irrigation based on the wet bulb temperature can save a substantial amount of water and money!

The wet bulb temperature can be used to determine when it is safe to shut down irrigation systems used for cold protection. When the wet bulb temperature is 32°F or higher, the irrigation system can be stopped without danger to any part of the plant (Harrison, Gerber, and Choate, IFAS Circular 348).

There is always a risk when using irrigation systems - micro-sprinkler or conventional sprinkler - for cold/frost protection. Low humidity and wind can produce weather conditions that result in a wet bulb temperature - the lowest temperature to which air can be cooled by evaporating water into it - 5°F to 6°F lower than the air temperature. Therefore, plant surfaces that have been wetted by irrigation systems, and subsequently exposed to evaporation, can end up 5°F to 6°F cooler than the air temperature. This may result in plant damage if the wet bulb temperature is below the critical minimum temperature (CMT) for your particular crop. However, when the wet bulb temperature is equal to or above your CMT, there is no risk of cold-air damage due to evaporative cooling. Therefore, irrigation systems can be safely turned off when the wet bulb temperature reaches your CMT or higher.

The Wet Bulb-Based Irrigation Cutoff Temperature calculator calculates an air temperature at which the wet bulb and your CMT would be equal. It is based on a user-selected critical temperature (see Determining Critical Temperature for Freezing Plants in Florida for more information on critical temperature) and the difference between the observed air and wet bulb temperatures, and is calculated for each FAWN site.

Once the cutoff temperature has been reached and irrigation systems have been turned off, it is not necessary to continue irrigation until all ice has melted from the plant.

Below are links to UF Electronic Data Information Source (EDIS) searches for publications that may be useful for learning more about this topic

EDIS Search – Evaporative Cooling

EDIS Search – Wet Bulb Temperature