Frost is formed overnight when the air temperature drops below 0°C (32°F) and the dew freezes. Clouds in the sky act as insulation, preventing the heat from the Sun that has built up in the land, sea and air during the day from escaping. This means that the temperature is less likely to drop below freezing. When the sky is clear, the day’s heat is able to escape easily, and a frost is likely.
There are several factors that influence whether frost will form on a given day or not. These factors include temperature of the surface (e.g. grass, window) on which it forms, clearness of the sky during the evening and night, wind speed during the night and humidity levels.
Surface temperature is one main factor of which frost forms. Within a certain area, there can be a difference of temperature readings. For example, when the weather personnel read the current temperature, they are receiving these readings from an outdoor weather station or what is called the Stevenson Screen (instrument shelter). These weather stations are sensors which are placed 4 feet 11 inches (1.5 meters) above ground level. The actual ‘ground or surface temperature’ could be lower than what the local weather personnel state as the ground is almost 5 feet lower than the sensor. If the temperature of the item’s surface is below freezing [32? F (0? C)], the water droplets or dew will turn into frost.
A clear or cloudy sky also will influence the formation of frost. If the sky is clear and calm while the temperatures continue to fall into the evening, the chance for frost increases. If the sky is cloudy as the evening approaches, the clouds will help contain the heat emitted from the earth’s surface. This will keep the ground warmer making frost a less likely possibility.
Another factor assisting in the formation of frost is wind. If there is no wind, the air is still and the colder air will settle on the ground. However, if there is a slow, gentle wind, the colder air will be pushed along and not have the chance to settle on the ground making frost a bit difficult to form. The most damaging wind type is one which is very cold and has below freezing temperatures. If frost forms during this time, it can be severely damaging to plants and other surfaces.
Finally, an important factor in the formation of frost is the humidity in the air. Air holds water vapor and can contain up to 100% water vapor saturation. This is known as 100% relative humidity. At this point, the air can no longer hold any additional water vapor so the water begins to condense into a liquid form. If the temperature is above freezing, dew will form leaving droplets of water on many different types of surfaces.
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