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When most people think
of a frost, they think of Jack Frost, a white coating on the
ground after a cold night, but this is only one type of frost.
There is also ground frost and air frost, but how do they
differ?
The temperatures recorded
by the Met. Office are those that are measured in Stephenson
screens. This is a standard screen that ensures all temperatures
are measured similarly. The screen is just over a metre above
the ground and air at this height can have a different temperature
from that of the ground.
When the temperature
in the Stephenson screen reaches zero, there is said to be
an ‘air frost’. Even with an air frost, the ground can still
stay above freezing. This often happens in early autumn, when
the soil still retains some of its summer heat. Equally
the ground can fall below freezing when the air does not.
Sometimes the air temperature
at night dips to 3 or 4 degrees, but the forecaster still
warns of a ‘ground frost’ overnight and the need to de-ice
your car in the morning. This is because the ground can reach
freezing without the air.
Neither a ground nor
an air frost are the white crystals seen on the grass on a
cold morning. This is ‘hoar frost’, which forms when the air
cools and water condenses onto the grass. It is actually dew
if it forms at temperatures above freezing, but if it forms
below zero degrees it is hoar frost.
Hoar frost is very
different from frozen dew. Frozen dew is dew that has frozen
after it has formed. They are very different to look at, frozen
dew looks like frozen water droplets, and hoar frost is a
delicate icy structure. Clear skies help all kinds of frost
to form. When the night is cloud free, heat can escape from
the earth quicker, without being ‘blanketed’ by the clouds,
allowing the temperatures drop enough to form a frost. It
is also the origin of the proverb ‘clear moon, frost soon’.
The difference between
the different types of frosts are very important to some people
including farmers, who need to know when their plants are
at risk, and motorists, who need to know when there are icy
patches on the roads. To others though, all types of frost
mean that it will be very cold!
Other features in
the Weather Basics series
Air Pressure
Clouds
Day and Night
Fog and Mist
Fronts
High Pressure
Humidity
Jet Streams
Lightning
Low
Pressure
Mirages
Northern Lights
Rain or Showers?
Thunder
Watercycle
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