Forecasting Temperature Trends during a Radiation Frost Night
The FFST_E.xls (English units) and FFST_M.xls (metric units) application, which is based on the method of Allen (1957), uses historical records of air and dew-point temperature at two hours past sunset and the observed minimum temperature during clear sky, calm, frost nights to develop the regression coefficients need to accurately predict the minimum temperature during a particular period of the year. Two hours past sunset is the model starting time (to), which corresponds to when the net radiation reaches its minimum. Assuming there is little or no cloud cover or fog during the night, the net radiation changes little from time to until sunrise the next morning. If there are clouds or fog, the model will predict temperature lower than observed. If a cold front passes or if there is cold air drainage, the predicted temperature may be too high. For use in the FFST.xls application program, select data only from radiation frost nights. And avoid nights with wind speeds greater than 2.0 m s-1 (5 mph) and nights with cloud cover or fog. Note that one can use data from nights when the minimum air temperature does not fall below 0 °C (32oF) as long as the night had clear skies and calm or little wind.
The FTrend_E.xls and FTrend_M.xls application use an empirical temperature trend model to predict how the temperature will change during a radiation frost night (with calm winds) from two hours past sunset to sunrise. Sunset and sunrise are determined from the input latitude, longitude and date. The model uses a square root function to predict the air temperature from two hours after sunset (at time to) until reaching the predicted minimum temperature (Tp) at sunrise (at time tp) the next morning. In addition to the air temperature, the application calculates the change in wet-bulb temperature based on temperature trend and initial dew-point temperature. The Title and Help worksheets provide information on the developers and instructions on how to use the program. The Input worksheet is used to input temperature data and to display the results of the trend calculations. The Wet-bulb worksheet is used to calculate the air temperature corresponding to the air and dew-point temperature at a given elevation. The Forecast worksheet is used to calculate an estimate of the minimum temperature at sunrise the next morning using an input of the air and dew-point temperatures measured at two hours past sunset.
The Plot in the FTrend.xls application is useful to help decide if the sprinkler should be used or not. For growers without soil water-logging problems, shortage of water or concerns about cost, it is best to start the sprinklers when the wet-bulb temperature approaches either 0°C (32oF) or the critical damage temperature, depending on the value of the crop and concern about losses. For growers who are concerned about these problems, using the FTrend.xls application will help to determine when to start the sprinklers.
If the plants are wetted by sprinklers, they should be started so that they are all operating when the wet-bulb temperature is above but approaching the critical damage temperature (Tc). However, note that the published critical damage temperatures are not always correct, so selecting a Tc that is 0.5°C (1.0oF) higher than the published value might be advisable. The choice depends on the risk one is willing to accept. If the minimum temperature (Tp) is forecast to be more than 1.0 °C lower than Tc, it is generally advisable to start the sprinklers as the wet-bulb temperature approaches Tc using the FTrend.xls application.
When Tp is forecast to be near Tc, the decision whether to protect and when to protect depends on the dew-point temperature and how long the air temperature is likely to stay below Tc. If the dew-point temperature is low, then it is often advisable to start the sprinklers before Tw falls below Tc. If there is only a short time prior to sunrise with the temperature falling below Tc, it might be wise to not start the sprinklers. The choice depends on the amount of risk the grower wants to accept and if there are problems with water-logging, water shortage, or cost.
Click here for FFST_E.xls
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Click here for FTrend_E.xls
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