A double frost forms when a layer of frost forms on top of an earlier frost.
If the conditions are right for frost to form then it will, normally overnight when the temperatures are lowest.
Then the sun rises (it is very likely to be a sunny day if there is a new frost, because clear skies lead to cooler temperatures). If the air temperature stays below zero during the day then frost may linger in many places. However, the sun will warm and thaw many surfaces that it can reach even if the air is below zero. This leads to some areas where the frost has melted and some areas, those that remain in shade all day, to stay frosty.
If there is another night of clear skies and low temperatures, then we will see the cycle repeat and this adds a second layer of frost to the ground. If there is frost lasting from the day before, then a second layer of frost accumulates there. This leads to two shades: a single frost (normally darker) where the sun reached during the day and a double frost in the shady but open areas. (The effect is also sometimes known as a ‘frost shadow‘.)
With practice, reading these patterns can help with navigation, when we use an understanding of the sun’s arc to read a ‘frost compass‘.
In the photo at the top of this page, we can see single frost near the top of the roof (the sun has reached that during the previous day). There is a double frost near the bottom of the roof, which remained in shade all of the previous day. We are looking south-west in this photo.
In the photo below, we are looking south-east towards the rising sun in winter. The sun never reaches the white strip nearest to the woods, leaving it in shade and creating a double frost there:
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