Blog · Agriculture
Frost risk explained: radiative and advective frost
There are two main kinds of frost, and they behave differently. Radiative frost forms on calm, clear nights when the ground loses heat to the sky, while advective frost arrives when a large mass of cold air moves in on the wind.
Knowing which kind you are facing matters, because the warning signs are different, the parts of the farm at risk are different, and several protection methods that help in one case do little in the other.
Frost and freeze damage
In everyday use, frost means ice crystals on surfaces. For crops, the concern is whether plant tissue falls below the temperature at which it is damaged. That critical temperature varies widely. It depends on the crop, the variety, the growth stage and how well hardened the plant is. Flowers and young fruit are usually far more sensitive than dormant buds.
Damage can also happen without any visible ice. When the air is very dry, the temperature can fall below freezing without reaching the dew point, so no white frost forms. This is often called a black frost, because the first sign is blackened tissue the next day.
Radiative frost
Radiative frost is the more common kind in many farming areas, particularly in spring and autumn.
During the day the ground absorbs heat from the sun. At night it gives heat back as infrared radiation. Cloud acts like a blanket and returns much of that heat. Under a clear sky the heat escapes, and the ground and the plants cool quickly. They in turn chill the air next to them.
If there is wind, it stirs the chilled air near the ground into the warmer air above, and the cooling is spread through a deep layer. If the air is calm, no mixing happens. The coldest air sits at the surface with warmer air above it. This reversal of the normal pattern is called a temperature inversion.
The typical conditions for radiative frost are:
- clear skies
- little or no wind
- dry air with a low dew point
- long nights
- a cool, dry air mass already in place
Cold air drainage and frost pockets
Cold air is denser than warm air. On a radiative frost night it flows slowly downhill, in the way water would, and collects in the lowest ground. It also builds up behind anything that blocks its path, such as a dense hedge, a wall, a bank or a belt of trees across a slope.
These places are called frost pockets or frost hollows. They can be several degrees colder than a slope a short distance away, and they are often the same places year after year. Mid-slope sites, where cold air can drain away, are usually the safest.
Two practical points follow. First, the minimum temperature from a weather station on open, level ground may be well above the minimum in your lowest field. Second, the height of the measurement matters. Standard air temperature is taken at about 1.25 to 2 metres above the ground. During an inversion, the air at crop height is colder than that. A forecast minimum just above freezing can still mean frost on the crop. Our guide to microclimate and weather stations explains why.
Advective frost
Advective frost happens when a cold air mass with temperatures below freezing moves into an area, usually behind a cold front. It comes with wind, it can happen under cloud, and it can last through the day as well as the night.
The key differences are:
| Radiative frost | Advective frost | |
|---|---|---|
| Cause | Heat lost from the ground to a clear sky | A cold air mass moving in |
| Wind | Calm or very light | Moderate to strong |
| Sky | Clear | Clear or cloudy |
| Inversion | Yes, warmer air above | No, cold through a deep layer |
| Coldest places | Low spots and sheltered hollows | Exposed sites and higher ground |
| Duration | Usually a few hours before dawn | Can last a day or more |
The two can also combine. A cold air mass arrives, the wind then drops and the sky clears, and radiative cooling takes an already cold night lower still.
Why the dew point matters
The dew point is the temperature at which the air becomes saturated. When the air cools to its dew point, water vapour starts to condense as dew, or deposit as ice, and that process releases heat, which slows the fall in temperature. Moist air also traps a little more outgoing heat.
So a high dew point tends to slow night-time cooling, and a low dew point lets the temperature fall fast and far. An evening with clear skies, falling wind and a dew point well below freezing is a classic warning sign.
Soil condition plays a part as well. Firm, moist, bare soil stores more heat in the day and releases more at night than dry, loose or heavily covered soil.
What protection methods rely on
This is a description of how common methods work, not a recommendation for any crop.
- Site selection. Avoiding frost pockets for sensitive crops, and leaving gaps so that cold air can drain through barriers, are the oldest methods and work passively.
- Wind machines. These pull warmer air down from the inversion layer and mix it with the cold air at the surface. They depend on an inversion being present, so they offer little in an advective frost.
- Heaters. They add heat directly and also work better under an inversion, which acts as a ceiling. In wind, the heat is carried away.
- Overhead sprinklers. Water releases heat as it freezes, which holds the coated tissue close to 0 °C. The water must keep flowing until the ice melts. If it stops early, or if wind and dry air cause strong evaporation, the tissue can become colder than it would have been with no water at all.
- Covers. Fleeces and similar covers reduce heat loss to the sky.
The critical temperatures for your crop and stage, and the choice and operation of any protection system, are matters for your agronomist or local extension service. Their guidance takes precedence over general explanations like this one.
Reading the forecast
A forecast minimum is a single number for a standard height at a general location. To turn it into a frost risk for a field, consider the sky, the wind, the dew point, the position of the field in the landscape and the height of the crop. The same logic of calm nights and inversions also matters for spray windows.
Where Prism fits
Prism Microclimate, built with FarmGrid, covers 961k fields across one whole state. It provides field-level weather, soil moisture, frost, spray and field-work windows, growing degree days and a morning brief per field. See our agriculture page for more, or browse the other industries we work in.