Oksarisu

Oksarisu is a Finnish term that translates to "night frost" or "black frost." It refers to a type of frost that forms when the ground is not frozen. This occurs when the air temperature drops below freezing, but the ground remains warmer, often due to a layer of insulation like snow or leaf litter.

Unlike white frost, which typically forms on surfaces exposed to the open sky and often has a crystalline structure, oksarisu forms on surfaces below this insulating layer. It is called "black frost" because the ice crystals, when they do form on plant tissues, can cause damage by freezing the cells. This freezing and subsequent thawing can lead to the discoloration and death of the affected plant parts, giving them a blackened appearance.

Oksarisu is particularly concerning for agriculture and horticulture because it can damage crops and plants even when white frost is not observed. The hidden nature of this frost and its detrimental effect on plant tissues make it a challenging phenomenon to predict and prevent. The conditions for oksarisu are often met during clear, calm nights when radiative cooling from the ground is significant.