Frost

Frost is a thin layer of ice that forms on solid surfaces like windows, grass, or car roofs. It appears when water vapor in the air turns directly into ice without becoming liquid first. This happens when the air is very cold and holds more water vapor than it normally can.

When the temperature drops below the dew point, the extra water vapor in the air settles on cold surfaces and forms tiny ice crystals. These crystals can grow in many shapes, from tiny specks to large, feathery patterns, depending on the temperature, humidity, and how calm the air is.

Frost is common in cold weather, especially at night when the ground cools down. It can look white and fuzzy or clear and smooth. Some plants, especially those from warmer places, can get damaged by frost when the water inside their cells freezes. Farmers sometimes protect their crops from this damage.

Formation

When a surface gets very cold, below the point where the air around it becomes its dew point, and is colder than freezing, ice can form on it. If water first turns to liquid and then freezes, it creates a coating that can look clear, cloudy, or like crystals. This process is sometimes called atmospheric icing.

Frost is different because it forms directly from water vapor in the air turning straight into ice. This happens when the surface is below the frost point, cold enough that ice forms without becoming liquid first. The air needs to have enough moisture but not too much, or else it will form liquid ice instead. The size of the frost crystals depends on the temperature, how much water vapor is present, and how long they grow undisturbed.

Frost usually forms when the surface is colder than the air around it. You might see frost around cracks in cold wooden sidewalks when warm, moist air comes up from below. Frost also often appears on objects that lose heat quickly, like rusty nails.

Types

Hoar frost

Hoar frost, also called hoarfrost, radiation frost, or pruina, is when white ice crystals form on the ground or on things like wires and leaves. This happens on cold, clear nights when heat moves away faster than it can come back. Objects can get very cold, even if the air a little higher up is above freezing.

The word "hoar" comes from an Old English word meaning "showing signs of old age". It describes how frost can make trees and bushes look like they have white hair.

Hoar frost can have different names based on where it forms:

• Air hoar forms on things above the ground, like tree branches and wires.
• Surface hoar is when ice crystals form directly on snow or ice.
• Crevasse hoar forms in cracks in glaciers where water vapor can collect.
• Depth hoar is when crystals grow inside dry snow.

Hoar frost can sometimes create risks for avalanches when it forms on snow. It can also create tiny ice particles that float in the air, known as yukimarimo, which has been seen in parts of Antarctica.

Hoar frost and white frost can also form in cold places like freezers or storage areas. If these cold areas are not well protected from moisture, frost can build up on pipes.

Advection frost

Advection frost (also called wind frost) is when tiny ice spikes form when very cold wind blows over tree branches, poles, and other surfaces. It usually forms against the direction the wind is blowing.

Window frost

Window frost (also called fern frost or ice flowers) happens when a window is very cold on the outside and has warmer, moist air inside. If the window does not keep heat well, water vapor can turn to frost on the glass. The shape of the glass can change how the frost looks.

If the indoor air is very moist, water may first turn to small droplets before freezing into clear ice.

White frost

White frost is a layer of ice that forms directly on snow from water vapor in the air.

White frost forms when the air is very humid and the temperature is below −8 °C (18 °F). It grows against the wind direction because the air coming from the windward side has more moisture. But the wind should not be too strong, or it can damage the frost as it forms. White frost looks like a heavy coating of hoar frost with large, needle-shaped crystals.

Rime

Main article: Rime ice

Rime is a type of ice that forms quickly in very humid and windy conditions. It is different from hoar frost because it usually forms from tiny water drops that are colder than freezing, not from water vapor. Ships in cold seas often get rime on their equipment. Unlike hoar frost, which looks feathery, rime looks solid and icy.

Black frost

Main article: Black ice

Black frost (or "killing frost") is when crops freeze and turn black because the air is too dry for regular frost to form, but the temperature gets very low. This kind of frost is called "killing frost" because white frost is usually less cold. The reason white frost is less cold is because when water freezes, it releases heat, which helps keep temperatures a little higher.

Effect on plants

Very cold temperatures, especially when frost forms, can hurt or kill many plants. Some plants, like beans, grapes, tomatoes, and peppers, are easily damaged even by a light frost. Others, such as carrots, onions, lettuce, and cabbage, can usually handle light frost better.

To protect plants from frost, farmers use different methods. They might use machines to move air around, cover plants with cloth, or spray water on the plants. The water freezes and helps keep the plants a little warmer. However, these methods must be used carefully to avoid hurting the plants further.

Frost-free areas

Frost-free areas are mostly found in warm, low-lying tropical regions. These places rarely get frost because they stay warm enough. Near the equator, areas higher than about 3,000 metres or 9,800 feet can have frost. Some coastal areas in warmer parts of the world are also frost-free.

In the contiguous United States, only southern Florida, around Miami Beach and the Florida Keys, along with the islands off the coast of California, stay frost-free. These places are in hardiness zones 11a and 11b.

Permafrost

Permafrost is ground that stays frozen all year, even in summer. It is made of dirt, sand, rocks, and plants, all held together by ice. You can find permafrost in very cold places like the Arctic and Antarctic, such as parts of Russia, Canada, Alaska, Norway, Greenland, and Antarctica. Permafrost can be just below the surface or much deeper. It helps store water and carbon and shapes the forests and wetlands in these cold areas.

It stops water from soaking deep into the ground, which changes the landscapes of northern areas, like the taiga and spruce bog environments.

Personifications

Further information: Ded Moroz and Father Frost (fairy tale)

In Russian culture, frost is often seen as a kind character named Ded Moroz. Some groups in Russia, like the Mordvins, also have stories about frost spirits.

In English tales, there is a fun idea that Jack Frost, a small magical being, makes the pretty frost patterns on windows on cold mornings.

On other planets

On the planet Mercury, special tools from a NASA mission found that very cold places in dark craters might have water ice.

On Mars, in 2024, two space machines found a thin layer of frost on the top of the tallest mountain, Olympus Mons. This frost appears for a few hours in the morning and then goes away.

In 2015, pictures from the NASA New Horizons mission showed that Pluto has frost made from nitrogen and methane on its surface.