FP001 Quick Answer
R. L. Snyder, Biometeorology Specialist
Department of Land, Air and Water Resources
University of California
Davis, CA 95616, USA
Copyright – Regents of the University of California
Created – June 22, 2000 – Last Revision – December 15, 2006
The decision about when to start and stop the sprinklers for frost protection should be based on both temperature and humidity in the orchard. When a sprinkler system is first started, the air temperature in the sprinkled area will fall to the wet-bulb temperature. Of course, this initial drop will be followed by an increase in temperature as the water freezes on the ground and plant parts to release heat and warm the air. However, if the dew-point temperature is low, then the wet-bulb temperature can be considerably lower than the air temperature and the initial temperature drop can lead to damage.
The upper instrument in this photograph is an aspirated psychrometer and the lower instrument is a sling psychrometer. The cotton wick on a wet-bulb thermometer is wetted with distilled or de-ionized water. The aspirated psychrometer is ventilated with a battery-powered fan inside the instrument. Swinging the instrument ventilates the sling psychrometer. When ventilated, the temperature of the wet-bulb thermometer drops because evaporation removes heat from the thermometer. The wet-bulb temperature is noted when the temperature of the wet-bulb thermometer stops dropping.
Starting and stopping sprinklers for frost protection should always occur when the wet-bulb temperature is above the crop’s critical damage temperature. Even if the sun is shining on the plants and the air temperature is above the melting point (0ºC or 32ºF), sprinklers should not be turned off unless the wet-bulb temperature is above the critical damage temperature. If soil waterlogging is not a problem, permitting the wet-bulb temperature to exceed the melting point (0ºC or 32ºF) before turning off the sprinklers adds an extra measure of safety.
The wet-bulb temperature can be measured directly with a psychrometer (see photo) or it can be determined from the dew point and air temperature. For direct measurements, the cotton wick on the wet-bulb thermometer is wetted with distilled or de-ionized water and it is ventilated until the temperature of the wet-bulb thermometer stabilizes. Ventilation is accomplished by swinging a sling psychrometer or aspirating with an electric fan using an aspirated psychrometer. If the temperature is below 0ºC (32ºF), the water on the cotton wick should be frozen and aspirated until the temperature stabilizes. Touching the wick with cold metal or ice will cause freezing. When the water on the wick is frozen, the temperature is called the “frost-bulb” rather than wet-bulb temperature. Both the frost-bulb and wet-bulb temperatures exist for temperatures below the melting point. The difference is that the saturation vapor pressure over ice is lower than over liquid water. This means that water vapor that strikes the surface from the air is more likely to attach to the ice than to the water surface. For a given water vapor content of the air, the frost-bulb will be slightly higher than the wet-bulb temperature.
The air temperature for a range of wet-bulb and dew-point temperatures can be selected from Table 1 or 3 for degrees Fahrenheit or Celsius, respectively. To determine the air temperature to start or stop sprinklers, first decide what is the critical damage temperature for your crop. Select a wet-bulb temperature that is at or above that temperature. Then find the wet-bulb temperature along the top of the table and the dew-point temperature along the left-hand side. Select the corresponding air temperature from the table. The sprinklers should be started before the air temperature falls below that value and they can be stopped after the air temperature upwind from the protected area exceeds that value. If the relative humidity and temperature are known instead of the dew-point temperature, then use Table 2 or 4, for degrees Fahrenheit or Celsius, respectively, to first determine the dew-point temperature and then use Table 1 to obtain the desired air temperature.
Generally, crop sensitivity to freezing temperature increases from first bloom to the small nut or fruit stages when a crop is most likely to be damaged. Sensitivity is also higher when warm weather has preceeded a frost night. Recommendations for starting and stopping temperature that are given here can be used for either over-plant or under-plant sprinklers. All sprinklers in a protection area should be on when the air temperature drops to the temperature selected from Table 1 or 3. This insures that the wet-bulb temperature will be above the critical damage temperature. Sprinklers can be turned off when the air temperature exceeds the value from Table 1 or 3.
Table 1. Minimum turn-on and turn-off air temperatures (oF) for sprinkler frost protection for a range of wet-bulb and dew-point temperatures (oF)*
| Dew-point Temperature | Wet-bulb Temperature (oF) | ||||||||||
| oF | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 | 32 |
| 32 | 32.0 | ||||||||||
| 31 | 31.0 | 32.7 | |||||||||
| 30 | 30.0 | 31.7 | 33.3 | ||||||||
| 29 | 29.0 | 30.6 | 32.3 | 34.0 | |||||||
| 28 | 28.0 | 29.6 | 31.2 | 32.9 | 34.6 | ||||||
| 27 | 27.0 | 28.6 | 30.2 | 31.8 | 33.5 | 35.2 | |||||
| 26 | 26.0 | 27.6 | 29.2 | 30.8 | 32.4 | 34.0 | 35.7 | ||||
| 25 | 25.0 | 26.5 | 28.1 | 29.7 | 31.3 | 32.9 | 34.6 | 36.3 | |||
| 24 | 24.0 | 25.5 | 27.1 | 28.6 | 30.2 | 31.8 | 33.5 | 35.1 | 36.8 | ||
| 23 | 23.0 | 24.5 | 26.0 | 27.6 | 29.1 | 30.7 | 32.3 | 34.0 | 35.6 | 37.3 | |
| 22 | 22.0 | 23.5 | 25.0 | 26.5 | 28.1 | 29.6 | 31.2 | 32.8 | 34.5 | 36.1 | 37.8 |
| 21 | 22.5 | 24.0 | 25.5 | 27.0 | 28.5 | 30.1 | 31.7 | 33.3 | 34.9 | 36.6 | 38.2 |
| 20 | 22.9 | 24.4 | 25.9 | 27.4 | 29.0 | 30.6 | 32.1 | 33.7 | 35.4 | 37.0 | 38.7 |
| 19 | 23.4 | 24.9 | 26.4 | 27.9 | 29.4 | 31.0 | 32.6 | 34.2 | 35.8 | 37.5 | 39.1 |
| 18 | 23.8 | 25.3 | 26.8 | 28.3 | 29.8 | 31.4 | 33.0 | 34.6 | 36.2 | 37.9 | 39.5 |
| 17 | 24.2 | 25.7 | 27.2 | 28.7 | 30.2 | 31.8 | 33.4 | 35.0 | 36.6 | 38.3 | 39.9 |
| 16 | 24.6 | 26.1 | 27.6 | 29.1 | 30.6 | 32.2 | 33.8 | 35.4 | 37.0 | 38.7 | 40.3 |
| 15 | 25.0 | 26.4 | 27.9 | 29.5 | 31.0 | 32.6 | 34.2 | 35.8 | 37.4 | 39.0 | 40.7 |
*Select a wet-bulb temperature that is at or above the critical damage temperature for your crop and locate the appropriate column. Then choose the row with the correct dew-point temperature and read the corresponding air temperature from the table to turn your sprinklers on or off. This table assumes a barometric pressure of 1013 millibars (101.3 kPa).
Table 2. Dew-point temperatures (oF) for a range of air temperature and relative humidity*.
| Relative humidity | Temperature (ºF) | |||||
| % | 32 | 36 | 40 | 44 | 48 | 52 |
| 100 | 32 | 36 | 40 | 44 | 48 | 52 |
| 90 | 29 | 33 | 37 | 41 | 45 | 49 |
| 80 | 27 | 30 | 34 | 38 | 42 | 46 |
| 70 | 23 | 27 | 31 | 35 | 39 | 43 |
| 60 | 20 | 23 | 27 | 31 | 35 | 39 |
| 50 | 16 | 19 | 23 | 27 | 30 | 34 |
| 40 | 10 | 14 | 18 | 21 | 25 | 28 |
| 30 | 4 | 8 | 11 | 15 | 18 | 22 |
| 20 | -4 | -1 | 2 | 6 | 9 | 12 |
| 10 | -18 | -15 | -12 | -9 | -6 | -3 |
*Select a relative humidity in the left column and an air temperature from the top row. Then find the corresponding dew point in the table.
Table 3. Minimum turn-on and turn-off air temperatures (oC) for sprinkler frost protection for a range of wet-bulb and dew-point temperatures (oC)*
| Dew-point Temperature | Wet-bulb Temperature (oC) | ||||||
| oC | -5.0 | -4.0 | -3.0 | -2.0 | -1.0 | 0.0 | |
| 0.0 | 0.0 | ||||||
| -1.0 | -1.0 | 0.7 | |||||
| -2.0 | -2.0 | -0.4 | 1.3 | ||||
| -3.0 | -3.0 | -1.4 | 0.2 | 1.9 | |||
| -4.0 | -4.0 | -2.5 | -0.9 | 0.8 | 2.4 | ||
| -5.0 | -5.0 | -3.5 | -1.9 | -0.4 | 1.3 | 2.9 | |
| -6.0 | -4.5 | -3.0 | -1.5 | 0.1 | 1.8 | 3.4 | |
| -7.0 | -4.1 | -2.6 | -1.0 | 0.6 | 2.2 | 3.9 | |
| -8.0 | -3.6 | -2.1 | -0.6 | 1.0 | 2.6 | 4.3 | |
| -9.0 | -3.3 | -1.7 | -0.2 | 1.4 | 3.0 | 4.7 | |
*Select a wet-bulb temperature that is above the critical damage temperature for your crop and locate the appropriate column. Then choose the row with the correct dew-point temperature and read the corresponding air temperature from the table to turn your sprinklers on or off. This table assumes a barometric pressure of 1013 millibars (101.3 kPa).
Table 4. Dew-point temperatures (oC) for a range of air temperature and relative humidity*.
| Relative humidity | Temperature (ºC) | |||||
| % | 0.0 | 2.0 | 4.0 | 6.0 | 8.0 | 10.0 |
| 100 | 0.0 | 2.0 | 4.0 | 6.0 | 8.0 | 10.0 |
| 90 | -1.4 | 0.5 | 2.5 | 4.5 | 6.5 | 8.4 |
| 80 | -3.0 | -1.1 | 0.9 | 2.8 | 4.8 | 6.7 |
| 70 | -4.8 | -2.9 | -1.0 | 0.9 | 2.9 | 4.8 |
| 60 | -6.8 | -4.9 | -3.1 | -1.2 | 0.7 | 2.6 |
| 50 | -9.2 | -7.3 | -5.5 | -3.6 | -1.8 | 0.1 |
| 40 | -12.0 | -10.2 | -8.4 | -6.6 | -4.8 | -3.0 |
| 30 | -15.5 | -13.7 | -12.0 | -10.2 | -8.5 | -6.8 |
| 20 | -20.2 | -18.6 | -16.9 | -15.2 | -13.6 | -11.9 |
| 10 | -27.9 | -26.4 | -24.8 | -23.3 | -21.7 | -20.2 |
*Select a relative humidity in the left column and an air temperature from the top row. Then find the corresponding dew point in the table.