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Rime is ice that accumulates on the ground, on bushes and trees and also the roofs of houses and other elevations. Like dew it occurs because of humidity during the early morning hours, in other words at the time of day when it is the coldest. This is the case when air cools down to a temperature lower than that of the dew point.
When it is dew, it is above freezing and turns to water. When it is rime, temperatures are below freezing so that it becomes ice immediately.
Rime is a crystalline covering that looks like snow. It can occur when the temperature of the surface of the ground falls and also as advection rime on a horizontal air movement.
Equally it can occur as a precipitation during freezing air, when the moisture in the air measured approx. 2 m above ground, freezes.
The most frequent way rime occurs is as radiation rime. This is due to the fact that in winter the earth radiates warmth faster as a result of the small amount of clouds or clear skies that are free of clouds and when it cools down faster than when the sky is covered with clouds. So rime can form even when the temperatures are above freezing because the ground temperature is significantly lower than freezing.
Not only the ground but also cars that are parked outside can cool down considerably over night and then, if there is enough humidity, turn into a "frosted" object, which then necessitates scraping the ice and the frost in the morning.
Rime can lead to slippery roads, which is referred to as slippery frost. This can occur, even if the rest of the road is dry, if there are places that are particularly cold, such as bridges. When temperatures are around freezing one should expect slippery patches and adjust one’s way of driving.
Hoar frost looks similar but occurs under different conditions. It is precipitation which forms from very cold water drops or directly from the water vapor contained in the air. This condition necessitates humidity of more than 90% and an outside temperature of less than - 8° C. Wind – which may not be too strong – contributes to hoar frost occurring.
Rime also resembles glazed frost, a precipitation that consists of freezing fog which forms when there are high winds and temperatures that are below freezing, and which accumulates on branches and telephone poles. These accumulations can reach a considerable weight and can cause damage when the branches break or the supports of the overhead lines snap. The effects are relatively minor in countries such as Germany, where many overhead lines have been replaced by underground lines.