Waking up to the first frost of the year is generally a wonderful experience, aside from having to scrape the ice off of the car windows. It is also a signal that winter is near. This weather phenomenon has a beauty of its own, and often creates beautiful patterns on windows and other surfaces. Like hard snow, it crunches beneath a person’s feet when it is walked on, and generally when it occurs it presents almost no danger to people. It is certainly one of the more docile weather phenomena.

Facts about Frost

  • There are a minimum of 3 types of frost: Hoar, Advection and Window.
  • Four factors aid in the formation of frost – surface temperature below 32 degrees, clear skies, high relative humidity levels, and low wind speeds.

How does Frost form?

There are several factors that influence whether frost will form on a given day or not. These factors include temperature of the surface (e.g. grass, window) on which it forms, clearness of the sky during the evening and night, wind speed during the night and humidity levels.

Surface temperature is one main factor of which frost forms. Within a certain area, there can be a difference of temperature readings. For example, when the weather personnel read the current temperature, they are receiving these readings from an outdoor weather station or what is called the Stevenson Screen (instrument shelter). These weather stations are sensors which are placed 4 feet 11 inches (1.5 meters) above ground level. The actual ‘ground or surface temperature’ could be lower than what the local weather personnel state as the ground is almost 5 feet lower than the sensor. If the temperature of the item’s surface is below freezing [32⁰ F (0⁰ C)], the water droplets or dew will turn into frost.

A clear or cloudy sky also will influence the formation of frost. If the sky is clear and calm while the temperatures continue to fall into the evening, the chance for frost increases. If the sky is cloudy as the evening approaches, the clouds will help contain the heat emitted from the earth’s surface. This will keep the ground warmer making frost a less likely possibility.

Another factor assisting in the formation of frost is wind. If there is no wind, the air is still and the colder air will settle on the ground. However, if there is a slow, gentle wind, the colder air will be pushed along and not have the chance to settle on the ground making frost a bit difficult to form. The most damaging wind type is one which is very cold and has below freezing temperatures. If frost forms during this time, it can be severely damaging to plants and other surfaces.

Finally, an important factor in the formation of frost is the humidity in the air. Air holds water vapor and can contain up to 100% water vapor saturation. This is known as 100% relative humidity. At this point, the air can no longer hold any additional water vapor so the water begins to condense into a liquid form. If the temperature is above freezing, dew will form leaving droplets of water on many different types of surfaces.

Types of Frost

There are several different types of frost including hoar frost, surface hoar, window frost and advection frost.

Hoar frost forms differently than other types of frost. When the air is over saturated with water vapor and the air becomes colder than freezing. The water vapor does not have a chance to form into drops of water first; the water vapor freezes directly on surfaces that are colder than the air surrounding the surfaces. Hoar frost can become different forms such as a feather-like form, needles and fern-like forms. The term ‘hoar’ comes from an old English definition which means ‘signs of old age’ and relates to when the frost forms on surfaces, it looks like white hair.

Surface hoar frost is a different variation of hoar frost. Surface hoar frost forms like hoar frost; however, it usually forms on snowy surfaces. Surface hoar frost is a very thin, weak type of frost and even a bit of wind blowing could create pockets within the frost. If there is a layer of snow, then a weak layer of surface hoar frost and then another layer of snow, the surface hoar frost layer will be a weak layer in between two layers of snow. When this occurs, especially in the mountainous regions, an avalanche could occur. Surface hoar frost looks different than hoar frost. Surface hoar frost appears as feathery, flat, sparkly crystals.

Window frost occurs on windows or glass panes when the temperature outside is below 32⁰ F (0⁰ C) and the air inside is moist. By combining the moist inside air with the below freezing windows or glass panes, frost will form on the surface. These frost formations can grow into many different shapes depending on the surface. Factors which will determine the frost formation include streaks on the surface, scratches on the surface, finger prints, etc. Window frost does not occur as much today as it has in the past. Many houses now have double paned windows which help in suppressing window frost.

Advection frost appears as tiny, icy spikes which forms around tree branches, poles and other surfaces. A cold wind blows over these surfaces and the advection frost occurs at this point. Advection frost appears as a border around the surface.

Effects of Frost

The effects of frost on many plants depends on whether the frost is a ‘light frost’ or a ‘hard frost’. A light frost occurs when the temperature is between 28⁰ and 32⁰ F (-2⁰ to 0⁰ C). A hard frost occurs when temperatures fall below 28⁰ F (-2⁰ C). A light frost will damage many vine plants such as beans, pumpkins, grapes, tomatoes, peppers, and melons. There are some plants which will withstand a light frost. These plants include many root vegetables such as beets and onions. Most of the evergreen plants also withstand frost. In Florida, there have been many years when frost occurred and destroyed most of the citrus plants.

Where is Frost found?

Many tropical areas are frost free areas as the temperatures in this region remain above freezing. However, when the altitude close to the equator is above 9,800 feet (3,000 meters), frost could occur. Other tropical areas or subtropical areas could or could not be affected by frost depending on a number of additional factions such as ocean breezes.

Frost in Literature, Film and Culture

  • Jack Frost is a borderline mythological being that represents snow, ice, frost and other winter weather. It was also the name of a 1998 film starring Michael Keaton in which a father came back to life as a snowman.
  • Robert Frost was the name of a very famous poet who, somewhat coincidentally, wrote poems about nature. One of his most famous poems was titled ‘Stopping by Woods on a Snowy Evening’.
  • Some of the giants in Norse Mythology, known as the Jotun, were Frost Giants. The Jotun would battle against the gods of Asgard, which included Thor and Odin.