The Earth gets its heat from the Sun. From sunrise to sunset, radiation from the Sun falls on the Earth heating the ground. The ground heats the air in contact with it which, in turn, heats the air above that. At the same time, the warm Earth is radiating heat out into space. During the day, the amount of radiation received greatly exceeds that being sent out so things warm up. As the Sun gets lower in the sky in the afternoon, the amount of radiation falling on the ground decreases until, about 3-4pm, the amount of radiation going out equals the amount coming in. That is the time of the maximum temperature. After that, the incoming radiation is less than the outgoing and the temperature falls. After sunset, the incoming radiation ceases but the outgoing continues and the temperature falls further. At sunrise, the incoming radiation starts again and a few minutes after sunrise the incoming and outgoing radiation are equal again. This is the time of the minimum temperature. Things start warming up again after that. The radiation out from the planet is from solid surfaces such as earth, rocks, plants, water droplets in clouds etc. These radiate more or less than each other. Some surfaces lose heat quickly while others do so more slowly. Smooth shiny surfaces lose heat faster than rough surfaces such as bark. As the ground cools overnight, it cools the air immediately in contact with it. This, in turn, cools the air above that and so on. As the ground continues to cool so the air near the ground is colder than the air above it. This is called a temperature inversion.
Water vapour in the air is in a constant state of flux with evaporation and condensation taking place all the time. If the rate of evaporation exceeds the rate of condensation, the air is clear. If the rate of condensation exceeds the rate of evaporation, the excess water is deposited as dew or forms fog or cloud. The temperature at which the rates of condensation and evaporation are equal is called the dewpoint. At this temperature the air is said to be saturated and there is 100% relative humidity. Above the dewpoint evaporation exceeds condensation. If the temperature falls below the dewpoint, condensation exceeds evaporation. In winter, the dewpoint can be close to or below 0°C. If the temperature falls below the dewpoint at these temperatures, the excess water vapour does not condense to form dew but changes from the gas, water vapour, to the solid phase, ice, without going through the liquid phase. These icy deposits are frost. Frost forms on the coldest surfaces first so smooth car windscreens, which radiate heat better than rough surfaces, get the first frosts. Sometimes there will only be frost on the car, not on the plants but once frost starts to deposit on the grass and the plants, it usually works from the ground upwards. This is because the coldest air is usually close to the ground. Air temperature as reported by the Bureau of Meteorology is measured in a thermometer screen. The bulbs of the thermometers are 1.5m above the ground. As the coldest air is below the thermometers, the air temperature can be 2-3°C while there are frosts on the ground. The Bureau considers an air temperature of 2.2°C in a thermometer screen to be the frost level minimum temperature. With that temperature in the screen, it is probable that there is frost on the ground. All this takes place on calm, clear nights. If there is wind, it mixes the cold air near the ground with warmer air above it (remember the temperature inversion) raising the temperature. If there is cloud, the cloud absorbs some of the outgoing radiation and re-radiates it in all directions including back to the ground thus raising the temperature. It is unlikely you will get frost on a windy or cloudy night. Even a full moon can provide enough radiation to lift the surface temperatures. We are only looking at frost here. Cold blasts from the Antarctic, cold enough to drop snow on the ranges are another matter altogether but, fortunately, these seldom occur in the subtropics. Sometimes the air is very dry. Temperatures fall as usual but the temperature does not fall below the dewpoint. This means there is no deposition of frost but with temperatures below freezing, plant material will freeze. This is often called a black frost and can be devastating even though no white frost has been seen. How can we minimise the effect of frost? There are various methods and I will discuss a few of them. Cold air is denser than warm air. If you are on the top or side of a hill, the cold air close to the hillside caused by the ground radiating is denser than the warmer air away from the hillside. The denser air rolls down the hill into the valley being replaced by warmer air. You can avoid many frosts by planting near the top of a hill rather than in the valleys. Unfortunately a really bad frost will hit the hilltops too so it isn’t 100% reliable. Remember the natural frost preventers, wind and cloud? Wind mixes the cold air with warmer air above raising the temperature. Farmers do the same by lighting fires in the corners of their fields to create convective currents that will mix the air. Asparagus farmers have been known to hire helicopters to fly up and down the fields mixing the air. It works. Large fires and helicopters might work on farms but might annoy the neighbours in a smaller suburban setting. If money is no object, those large space heaters would do a good job. Cloud prevents frost by absorbing and re-radiating the outgoing radiation. Greenhouses do the same thing. Build a greenhouse over your orchard and your frost problems will disappear – but that is impractical. Most fruit trees really only need protection when they are small. Once they are two or three metres high, they are away from the coldest air which is on the ground and they are protected at the lower levels by bark. For one or two trees it is quite practical to build portable greenhouses out of clear plastic sheeting and conduit pipe. Igloos and tepees are good shapes. Pop the igloo over the young tree when frosts are expected and take it off when the temperature rises in the morning. As the plastic is clear, it would be safe to leave it during the day but these things are light and a good wind will destroy them. Even so, they are inexpensive and it is worth experimenting with them until the plants are big enough to look after themselves. A really bad frost will defeat the greenhouse but it should protect the plants from most frosts. Frost prevention, therefore, falls into two areas. Either mix the cold air at the ground with warmer air above or prevent the radiative cooling. I have suggested some ideas, you might think of others.
Sheryl: I saw some clear plastic pallet covers which would cover your trees against frost $17.50 each.
Steve retired from the Bureau of Metrology - NSW a few years ago. He also has degrees in Anthropology and Linguistics.