Atmosphere And Weather

A Guide to the different Forms of Frost



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 The different types of frost are dependent on temperature.  Most temperature readings use a “screen temperature”.  A screen temperature is when the sun warms the ground and the earth, in turn, heats the air.  Air, by itself, cannot be heated.  If there is no sun, then the ground cools and the air also cools. Screen temperatures vary with altitude, locality and time.  There are temperature lines on a map called isotherms.  Isotherms take into account latitude, elevation, wind temperature, ocean currents, whether inland or by the sea, ground to the sun, cloudiness, city warmth, and wetness of the ground.  The Stephenson screens are the most commonly used because they use a standard measure, which uses the same formula.  The following types of frost use the screen to measure its temperature and categorize it.

There is ground frost, which is when the ground’s minimum temperature is below 0 degrees C (32 degrees F).   This is the type of frost that settles during the night on car windows and  have to be scraped in the morning. 

The ground can have no frost (ice) and still be frozen. This black frost forms because the atmosphere is extremely dry and has no humidity.  It usually kills the weaker plants.  They turn black and die.  According to some meteorologists this is how black frost got its name.

A frosty night can be +2 C (35.6F), which means that it is cold, but not cold enough for the ice to remain. The moment the temperature rises, it disappears. 

A heavy frost occurs when the temperatures are 0 C (32 F), and below.

A killing frost (when most plants die from the cold) is below -2 C (28.4 F).

When tiny crystals form on surfaces, and colder than 0 C (32 F), is hoar frost.  This is the source of many avalanches when it forms on top of packed snow because the air can dissolve the crystals.  The winds blow and the air holes allow the wind to carry it.

A glaze occurs when the rain is frozen and falls on surfaces colder than 0deg C. but warmer than the raindrop.  This causes the raindrop to melt and spread.  It is immediately refrozen.  This is highly similar to removing an ice cream container from the freezer.  You will notice that there  is a glaze  of ice on top of the ice cream.  It means that the ice cream warmed up and ice crystals were able to stretch  and when refrozen, it forms the glaze.

Rime frost is very similar to glaze except the droplets are much smaller and they do not spread out.  They freeze too quickly.  Rime is more porous than glaze and has several air spaces as they build up a mass.  This makes rime appear white; whereas, glaze is transparent.

These are the different types of frost and how delightful old Jack began nipping at your nose.

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