Frost is water vapor, or water in gas form, that becomes solid. Frost usually forms on objects like cars, windows, and plants that are outside in air that is saturated, or filled, with moisture. Areas that have a lot of fog often have heavy frosts.

Frost forms when an outside surface cools past the dew point. The dew point is the point where the air gets so cold, the water vapor in the atmosphere turns into liquid. This liquid freezes. If it gets cold enough, little bits of ice, or frost, form. The ice is arranged in the form of ice crystals.

Frost is most common in low-lying areas. Warm air rises, and cool air sinks—cool air is denser than warm air. That means there are usually more water molecules in cool air than in warm air. As cool air collects in valleys, frost forms.

Frost usually forms at night, when the air temperature is cooler. Once the sun rises and warms the air around the frosted object, frost melts quickly.

Types of Frost

There are different types of frost. The most common are radiation frost (also called hoarfrost), advection frost, window frost, and rime.

Radiation frost is frost in the form of tiny ice crystals that usually shows up on the ground or exposed objects outside. Hoarfrost also forms in refrigerators and freezers.

Advection frost is a collection of small ice spikes. Advection frost forms when a cold wind blows over the branches of trees, poles, and other surfaces.

Window frost forms when a glass window is exposed to cold air outside and moist air inside. Window frost is familiar to winter residents of cold climates. Indoor heat and cold outdoor temperatures form this type of frost. Window frost was much more common before people began using double-paned windows.

Rime is frost that forms quickly, usually in very cold, wet climates. Rime also forms in windy weather. Rime sometimes looks like solid ice. Ships traveling through cold places like the Arctic Ocean often end up with rime covering at least part of the exposed part of the ship.

Frost and People

Frost can severely damage crops. It can destroy plants or fruits. Plants with thin skins, such as tomatoes, soy, or zucchini, can be ruined. If frost is bad enough, potatoes will freeze in the ground. Farmers have had entire fields destroyed in just a few frosty nights.

Farmers typically consult almanacs and maps to predict frost. Maps and information from almanacs (such as previous days of frost) tell farmers what areas usually receive frost, how often it occurs, and how long it lasts. These tools are reliable but are not always accurate.

Although farmers still rely on the weather, many spray their crops to reduce frost damage. This spray usually contains a genetically modified organism (GMO), an organism whose genes have been altered by people. The GMO that reduces frost damage to crops is called ice-minus bacteria. Ice-minus bacteria make it difficult for ice crystals to form. Ice-minus bacteria do no damage to the plant.

Many farmers also protect their crops by using the selective inverted sink (SIS) method. The selective inverted sink is a large fan that draws cold, moist air up into a chimney. The chimney expels the cold air far above the crops. This protects crops from frost without spraying them.

Sturdier plants are not destroyed by frost, but frost will stop them from growing. Evergreen trees, such as pine and spruce, will stop growing during a frost, but they won't die.

Roads can also be damaged by frost. Frosty roads are slippery and, exposed to the heat from cars, quickly become wet. Many drivers have trouble navigating frosty or wet roads.