Farmers use a variety of techniques to help protect their vulnerable crops against frost damage when temperatures get near 32 F, including irrigation, large fans or other heating methods. But it is hard to know when to start the frost prevention measures. If you wait until it gets to 32 F, then it is often too late to prevent damage.
In a recent paper in early release in the Bulletin of the American Meteorological Society this month, my husband Dr. John Knox, graduate student David Nevius and I have published an easier way to calculate wet-bulb temperature, the key component in determining when to turn on irrigation. Thanks to Renee Allen of Bacon County for suggesting some of this work!
Wet-bulb temperature is the temperature to which a thermometer bulb will cool if covered by a wet “sock” from which the water is allowed to evaporate. When you step out of your shower in the morning and feel cold, essentially you are feeling the wet-bulb temperature. Wet-bulb temperature is between the actual air temperature and the dewpoint temperature, and in our paper we show that for temperatures near freezing, the wet bulb temperature is 1/3 of the way from temperature down to the dewpoint temperature. So, for example, with a temperature of 32 F and a dewpoint temperature of 23 F, the wet bulb temperature would be 32 F – 1/3 x (32-23 F) or 29 F.
The idea of using irrigation to help prevent frost is that the water that is sprayed in the fields freezes and releases heat to the air as the liquid water changes to ice. The goal is to keep the air temperature in the area at 32 F by adding heat in this process. Once you start spraying, the temperature in the fields will initially go down as evaporation occurs, but as water is continually added, it will keep the temperature up even as the surrounding air temperature drops overnight.
Unfortunately, this technique has limitations which may come into play this week in the hard freezes which are predicted for Wednesday and Thursday mornings. If the dewpoint and wet-bulb temperatures are too low, then you can’t spray enough water to keep the temperatures at or above freezing–either you will weigh down the bushes, causing breakage and loss of flowers and fruit, or your irrigation equipment just can’t keep up with the amount of water needed, especially if there is any wind. In that case, you will just waste water and fuel to pump water that will not help anyway.
In this week’s case, the second morning may be worse than the first because the dewpoint temperature will be lower, which means you will have to pump more water for longer to keep the temperatures near freezing, and you could end up just wasting the water from the first night just to see the frost get your fruit anyway the second night.
The forecast below is for near Macon, Ga., for Wednesday March 15 and Thursday March 16. Dewpoints on the morning of the 15th will be falling from the low 20s on Wednesday morning into the teens by late in the day and will remain in the mid-teens throughout the day on Thursday.
If you are considering irrigating for frost prevention this week, make sure you watch the forecasts carefully and keep in mind that you need to consider conditions for both Wednesday and Thursday morning. You can get graphical forecasts like the one above for each NWS office (here is the one for Peachtree City GA FFC at http://forecast.weather.gov/gridpoint.php?site=ffc&TypeDefault=graphical). When you click on a location, you will get hourly graphs of temperature, dewpoint and other variables out to several days ahead. This will allow you to determine how long you may have to pump water or if it is even worth it to try.
Table 4 below to shows when you should start irrigation to keep the wet-bulb temperature at 32 F. First two columns: Dewpoint temperature and dry-bulb thresholds for irrigation of blueberries and strawberries to prevent frost damage, adapted from Longstroth (2012). Third column: wet-bulb temperature calculated from the first two columns, using the one-third rule (3).