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Frost

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Formation        Locations       Rime..        Hoar..       Fern..        Jack Frost
 

On cold winter mornings, we often see frost lying on roads, paths and even on leaves and trees. It can in fact form on any cold surface, such as windows. Have you had to scrape ice off car windscreens before going to school before?

Formation...  

Frost forms when water vapour freezes into ice crystals on cold surfaces. In winter, temperatures are usually low as the sun is low in the sky during the day and the nights are long. On clear nights, when there is no blanket of clouds to keep the warmth in, then any heat received during the day quickly escapes. The temperature will therefore drop considerably and as the moisture in the air freezes, the ground will be covered with frost.

Locations...  

Frosts occur more often in some areas of the world than others. The tropics rarely, if ever, get frosts, whereas at the Poles, they occur almost continuously. In mid-latitudes (areas between the Poles and the Equator, such as Great Britain), frosts occur whenever the conditions are right. This happens more often inland than near the coast, because the sea retains heat and therefore stays warmer for longer, so making it harder for frosts to form. (See ‘Land and Sea’ in the temperature section for more information).

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There are three types of frost that occur in different ways:

 

Rime frost... rime frost

Rime frost.

Rime is ice formed when a damp, icy wind blows over flowers, branches and other surfaces. Rime frost looks like icing around the edge of petals and leaves, and only occurs when the temperatures are very low.

Hoar frost... hoar frost  

Hoar frost.

Hoar frost occurs when water vapour touches a very cold surface and freezes on it instantly. This can happen to the leaves and branches of plants, and will cover them with ice crystals that look like spiky fingers.

It can also occur on other freezing surfaces such as soil and metal, and so can often be seen on cars. Hoar frost can occur at higher temperatures than rime frost – usually when the air temperature is around 0C (32F). However, the ground is usually much colder, and the air must be moist for the ice crystals to form.

 

Fern frost... fern frost

Fern frost.

In particularly cold weather, fern frost may appear on windows. This happens when tiny water droplets (dew) first form on the cold glass.

These then turn into ice and more moisture freezes on top.  As this process continues, more ice crystals are formed and the frost develops into what looks like feathery fingers. Fern frost can create beautiful patterns of ice crystals, which often look leaf or fern like – hence the name.
Jack Frost...  

 

Frost factsjack frost

  • Frost is white because the crystals contain air
  • Jack Frost – frost is often characterised by the evil Jack Frost. People used to say that this spiky character left his icy finger marks on every window pane.
 

What else is cold and similar to frost?  That's right - ice!  Let's have a look at ice in more detail on the next page.

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