Frost is a localised phenomenon which is fairly common in parts of southern Australia.
Frost is defined as a deposit of soft white ice crystals or frozen dew drops on objects near the ground; formed when the surface temperature falls below freezing point.
There are two main ways that frost can form, with radiation being the most common method of formation in Australia.
- Radiation frost: Radiation is the most common mechanism for frost formation in Australia. Frost occurs when the ground and ambient air cools down by the loss of heat to the atmosphere. This most commonly occurs under clear skies and with little or no wind. Radiation frost begins at ground level and gradually rises to higher objects.
- Advection frost: Advection frost (also know as "freeze") can occur at any time, day or night. Frost forms when a mass of very cold air moves over an area, replacing the warmer air in that area. It is not affected by cloud cover. This type of frost is generally not seen in Australia, as the air masses in our region are very rarely cold enough to produce a freeze.
Frost can also take several forms, including:
- White frost (or hoar frost): White frost is a deposit of ice crystals formed by direct deposition on objects exposed to the air. Water vapour in the air freezes upon contact with an object that has a surface temperature below 0°C.This is the kind of frost, common in Australia, that we see on tree branches, grass stems, and car windscreens.
- Black frost (or dry freeze): Black frost occurs when the temperature drops to freezing point, but the adjacent air does not contain enough moisture to form white frost on exposed surfaces. This causes an internal freezing of the vegetation, leaving it with a blackened appearance and killing it. Black frost is fairly uncommon in Australia.
- Killing frost: When a frost period is sufficiently severe that it ends the growing season (or delays the beginning of the season) it is referred to as a killing frost. Killing frosts are normally black frosts and are uncommon in Australia.
- Rime: A deposit of ice formed by the rapid freezing of super-cooled water droplets. This type of frost is rare on the surface in Australia, however it regularly affects aircraft flying at higher levels in the atmosphere where the temperature is much cooler.
Some areas, on a regional scale, are particularly prone to frost. These are sometimes referred to as frost hollows, or frost pockets if they are very small. Frost hollows often occur in valleys due to cold air drainage. As the air at the top of a hill cools at night, it becomes dense and heavy compared to surrounding air, and will drain to lower levels. This is referred to as a katabatic wind, and can result in frost forming in valleys when surrounding areas remain frost-free.
Frost formation is affected by a series of factors including cloud coverage, humidity, surface winds, topography and location. In Australia, frost is more likely to form under a clear sky, with low humidity and light surface winds.
- Cloud cover: Clear skies favour the escape of radiation (heat) from the earths surface to space. Clouds reflect the outgoing radiation, slowing the cooling at the surface. Crop covers and tree wraps can be used to manage the risk of frost by preventing the loss of heat during the night.
- Humidity: When the air is more humid, internal processes relating to the change of state of water between vapour-liquid-solid causes the release of heat (latent heat). This slows down the cooling, decreasing the likelihood of frost. Spraying water on crops can help to prevent the development of frost. As the liquid water becomes ice, it releases heat, slowing down the cooling process. Care must be taken with this method of frost prevention however, as excessive ice build-up can also harm a crop.
- Surface winds: At night, wind can act to mix the cooler air near the surface with warmer air just above it. This slows the radiative cooling at the surface, making frost formation less likely. Wind machines and helicopters are often used to stir up the air around a crop, reducing the likelihood of frost. Heaters also generate a movement of air, helping to enhance their effectiveness in frost prevention.
Temperature is measured by standard instruments which are located in a shelter (Stevenson screen) at a height of approximately 1.2 m above the ground. These observations are then used to approximate the conditions at surface level. An observed temperature of 2.2°C at screen level indicates that the temperature at the surface is approaching 0°C.
A set of frost potential climate maps has been produced to give you an indication of areas that may be affected by frost. These maps show the average number of days (annually and monthly) where the minimum temperature falls below a given threshold.
The occurrence of frost can have a devastating effect on vegetation and crops. Frost potential maps and frost warnings are used by the agriculture industry to manage the risks associated with frost. Frost information is also used by the home gardener to minimise damage to gardens.
The impact that a particular frost event will have on a crop or garden depends upon the severity of the frost and the crop or plants in question. Frost is a localised phenomenon, the severity of which can vary significantly over a very small area. Different crops and plants have different tolerances to frost. Crop type, variety, sowing date, nutrition and stage of growth can all determine the extent to which the crop is affected by a frost.