Principles of Freeze Protection for Fruit Crops
WEATHER TERMINOLOGY RELATED TO FREEZES
Heat Transfer - Movement of heat is always from a warmer to a colder body. There are three basic forms of heat transfer.
Conduction is transfer of heat through solid bodies or bodies in contact. Heat is transferred from molecule to molecule as it moves through the body. For example, heat movement through soil or a metal rod is conduction.
Convection is transfer of heat through the movement of heated gas- like air or liquid. When a heater is operating in an orchard, cold air moves toward the heater, becomes warm and rises upward. A form of forced convection occurs as a helicopter or wind machine moves air past a heater bringing about a mixing of air in the orchard.
Radiation is the direct transfer of heat energy through space from one object to another. This is the form in which the earth receives the sun's energy. Radiation only travels in straight lines which means only those portions of fruit plants that are in direct line with nearby heaters receive radiant heat. Very little heat is absorbed by dry air. The sun's energy traveling through space creates heat as it strikes the earth's surface. Energy radiating from the soil surface as well as plants is lost to space (as on clear nights) or absorbed or reflected by water vapor (as in clouds) and water.
Differences in a Frost Versus a Freeze
The terms frost and freeze are often used interchangeably but refer to two different weather events. The term freeze is normally used to describe an invasion of a large, very cold air mass from Arctic or Canadian regions. This event is commonly called an advective or wind borne freeze. Wind speeds during an advective freeze are usually in excess of 5 mph. Clouds are commonly present during much or all of the event and air is usually quite dry (low dew points). Freeze protection systems are usually of limited value during this type of severe freeze.
A radiational frost (also called a radiational freeze) typically occurs when winds are calm (usually 0 to 3 mph) and skies are clear. Under such conditions, an inversion may form because of rapid radiational cooling at the surface. If a strong inversion forms, temperatures aloft (usually up to 100 to 200 feet) may become 10øF or higher than surface temperatures. Most people think of frosts as frozen moisture on plant surfaces. However, there are two types of frosts, a hoar or black frost and a white frost. Visible frost occurs when atmospheric moisture freezes (forms small crystals) on plant and other surfaces. Dew (free water) forms when air temperature drops below the dew point temperature. If temperatures continue dropping on cold nights, this dew may freeze (form frost) by sunrise. If the air temperature is below the freezing point of water (32øF) ice crystals rather than dew forms and the frost is called white frost. The temperature at which this occurs is referred to as the frost point. When the dew point temperature is below the freezing temperature of the air, neither frost or dew form. Such a condition is referred to as a black frost. The development of frost is dependent upon the dew point or frostpoint of the air. And the drier the air, the lower the dew point.
Damaging frosts seldom occur in Alabama through slow, seasonal lowering of air temperatures. The most common freeze event scenario is for a blast of Arctic or Canadian air to move rapidly southward across the state. The first night or two of the event usually features strong winds, clouds and rain followed by clearing skies and the continued importation of very cold, dry air. Thus advective or wind borne freezes are common the first one or two nights. With continued clearing skies the next night or two usually feature radiational frosts (or freezes) and the coldest temperatures.
Temperature and Humidity Measurements
Relative Humidity - This measurement is commonly given by radio and TV announcers as a percentage from 0 to 100. It is a measure of how much water vapor the air could actually hold at a given temperature. A relative humidity of 100% implies the air is fully saturated with water vapor at the reported temperature. The relative humidity changes quite rapidly as the temperature changes although the water vapor content (referred to as vapor pressure) changes very little over a 24-hour period. And because the relative humidity changes with changing temperatures, it is not a useful method for monitoring the moisture content of an air mass. The dew point, a much more useful index for determining moisture content of the air, may be ascertained at any time using the air temperature and relative humidity.
Air temperature which is so important during the freeze events of fall, winter and spring could be referred to as dry bulb temperatures. Official temperatures recorded by the National Weather Service (NWS) are taken at a five -foot elevation above the soil surface. When the NWS forecasts a frost or freeze, the temperatures used are for air temperature (dry bulb) at a five-foot elevation. This is important to remember because actual air temperature at the soil surface or on crop beds (such as strawberries) may easily be several degrees colder than at a height of five feet on calm, clear nights.
Wet bulb and dew point temperatures are also measurements that are quite valuable to growers during freeze events. Wet bulb temperature is a measurement of the evaporative cooling power of the air and may be measured with a sling psychrometer. The latter is an instrument comprised of two thermometers. The wet bulb thermometer has a gauze wick attached to the bulb end. To measure wet bulb temperature, the gauze wick is immersed in water and the instrument is slung in a circular fashion for a few moments. Moisture being removed from the gauze wick by evaporation causes a cooling effect and lowers the temperature of the bulb. The lower the moisture in the air, the lower this temperature will drop. The amount of the temperature drop is proportional to the rate of evaporation (which has a functional relationship to relative humidity and air (dry bulb) temperature). The two readings may be used to determine the dew point from a psychrometric table.
Measurements of wet bulb and dry bulb temperatures may be used to determine relative humidity from conversion tables. Wet bulb temperature is often computed by the NWS using other measurements. Knowing wet bulb temperature is especially important when using irrigation for freeze protection as with strawberries. Growers will use wet bulb temperatures (or estimates thereof) to turn irrigation systems on and off. This is useful because wet bulb temperature is the lowest temperature to which plant tissue will fall when water is first turned on, or insufficient water is being used, or if power or mechanical failure resulted in the irrigation system shutting down too early. Except under conditions where the air is saturated with moisture, the wet bulb temperature is normally less than air temperature but higher than dew point temperature.
Dew point is defined as the temperature at which moisture will condense out of the air. The dew point is considered the theoretical low that air temperature can drop to at any time. Although forecasters do not normally include dew point temperatures, it can easily be determined for the same hour if the air temperature and relative humidity are known. Warmer air holds more moisture than cold air. When the dew point is high the drop in temperature is much more gradual on a cold night. However, imported Arctic or Canadian air is usually quite dry and is characterized by low dew points. During some freeze events in the state, it is not uncommon for dew points to fall to zero or below.
Because the water vapor content of an air mass generally changes very slowly, dew point serves as a good indicator of how dry or moist the air is. Air is comprised of a mixture of gases, mainly, nitrogen, oxygen and argon. Water vapor only constitutes a fraction of 1% of the gases in the atmosphere but is the single most important absorber of heat radiated from the earth's surface. Consequently, the amount of water vapor in the air significantly affects the rate at which heat is lost during a typical radiational freeze night.
Windy, advective freezes with low dew points can desiccate peach and nectarine buds even when dormant. Further, low dew points allow rapid drop in temperature during radiational frost/freeze events, and growers using freeze protection systems use dew point information in planning for and carrying out freeze protection activities.
During daylight hours, the radiant energy of the sun warms the earth's surface including plants and soil. As plants and other objects warm, they in turn warm the air around them. The effect of this phenomenon is that layers of air near the earth's surface warm while much more elevated air in space remains cold. Thus air temperature decreases with height above the surface during the daylight (outer space is very cold). However, during the night solid objects such as plants and the soil lose heat to the sky by radiation. As the earth's surface cools, it cools the air around it. This results in a reversal of daytime conditions and is referred to as a temperature inversion because warmer air is now located above the cool air at the surface (Figure 1).
If an inversion forms the greatest amount of warmer air is usually some 25 to 200 feet above the surface although it may be several degrees warmer only 5 to 10 feet above the surface. The height of the warm air layer can vary from night to night as well as the temperature differences that develop. A strong inversion is one where temperatures in the "inversion zone" is at least 7 to 10 degrees warmer than temperatures at the surface. Even on calm, clear nights, an inversion may not form. However, measurements from remote weather stations in the state and from helicopters used for freeze protection have shown strong inversions of 6 to 8 degrees at 50 to 60 feet and 8 to 14 degrees at 100 to 200 feet. Large inversions afford an excellent source of heat which helicopters and wind machines can force downward to increase surface temperatures. Heating is also more effective when large inversions occur. Small inversions of only 1 to 2 degrees don't afford enough heat to be used for effective freeze protection except in very marginal freezes. On flat ground, an inversion that forms above 50 to 60 feet is not considered of real value because wind machines are unable to pull enough of the warmer air to the surface to make a temperature difference. However, if helicopters are used, they are able to force warmer air downward from a 100 to 200 foot level. More strongly sloping ground tends to give stronger inversions. And on sloping ground even wind machines are able to move the warmer air down the hill and warm lower areas. The warmer air aloft tends to reduce the upward movement of air warmed by heaters or irrigation. Without the warmer air above, the air warmed by heat or irrigation, being lighter than the surrounding colder air, will rise and continue to be lost from the orchard or field.
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