Forecasting frost

What conditions are needed for frost to occur?

The most common type of frost (called hoar frost) occurs on clear winter nights when the temperature of a surface falls below zero and there is enough moisture in the air to form the frost.

As a general rule of thumb, if the air temperature is forecast to fall between 0 °C and 4 °C on a night with little or no cloud and light winds, then you need to bear in mind there may be a frost outside in the morning. The closer it is to zero, the greater the chance of seeing frost. If the air temperature is forecast to be below zero, then the risk of seeing frost is much higher.

Why is this?

When you look for overnight temperatures on your app or during a weather broadcast, these are for air temperatures. However, the temperature of the ground, or perhaps your car can often drop a few degrees below this on a clear night. This is because of surfaces (e.g. tarmac, grass, metal car) cooling first overnight, and these then cool the air above them. This means that the surfaces usually get to a lower temperature than the air by the morning and is why you may notice a frost on the windscreen even if the car thermometer (which measures air temperature) reads above zero.

Different surfaces lose heat at different rates overnight. You may have sometimes noticed a frost on your car but not on the ground, or frost on the grass but not on the pavement. The grass tends to lose more heat than tarmac and so it is quicker to fall below zero overnight. Weather forecasters sometimes refer to this as a ‘rural frost.’

You must bear in mind that even patchy cloud can affect whether the frost forms though, as the cloud acts ‘like a blanket’ to stop the surfaces losing heat so quickly, and so on cloudy nights, the surface temperatures are closer to the air temperature.