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Dew And Frost
When a jug full of
iced drink is taken out of the refrigerator, water droplets
condense on the outside of the container (provided the jug
is made of a material which is a good conductor of heat, such
as metal). This happens because the jug is at a lower temperature
than the dew-point of the air.
'Dew-point' is defined as the
temperature at which the air, when cooled, will just become
saturated. For example, on a summer's day when the air temperature
reaches 18°C, the dew-point might typically be 8°C. By sunset
the air temperature may have fallen to 12°C, but the dew-point
will still be around 8°C. During the night the temperature
continues to fall and if it reaches, say, 7°C the temperature
of the ground is below the dew-point of the air and droplets
of moisture begin to form - this is dew. Since the air is
now being 'robbed' of some of its water vapour, the dew-point
of the air will actually start to fall very slightly.
Next morning, as the incoming
solar radiation gathers strength, the dew will evaporate and
the grass will become reasonably dry (and suitable for sitting
on during the day). However, in winter, when calm conditions
prevail, the daytime evaporation may be so slow that dew may
persist all day.
Hoar-frost is composed of tiny
ice-crystals, 'feathery' in appearance when well developed.
It is formed by the same process as dew, but occurs when ground
temperatures are below freezing point. Consequently, when
the grass is covered in a white hoar-frost at dawn it cannot
be assumed that there has necessarily been an air frost.
Sometimes dew forms during the
evening and subsequently freezes to become hoar-frost with
globular ice on the grass.
Snow
Snowflakes
can be formed by the collision of ice crystals within clouds.
This is known as the process of aggregation and usually accounts
for the larger snowflakes that are seen to fall. Smaller snowflakes
are formed by the Bergeron-Findeisen process. Supercooled
water droplets (i.e. those with a temperature below freezing)
are 'picked up' by the falling ice crystals. The ice crystals
grow at the expense of the water droplets.
For snow to reach the ground the air temperature must be
no more than 2°C. One would expect the falling snow to melt
as soon as the temperature rises above freezing, but this
is not so. As the melting process begins, the air around the
snowflake is cooled. At temperatures above 2°C the snowflake
will melt to become 'sleet' or rain. In this country, the
heaviest falls of snow tend to occur when the air temperature
is between zero and 2°C.
- Individual ice crystals and snowflakes can be the shape
of prisms, plates or stars - but all have 6 sides.
- 30cm of fresh fallen snow has about the same water equivalent
as about 25mm of rainfall.
Hail
There are three different
phenomena which affect the British Isles that could loosely
be described as hail.
- Snow pellets are
beautifully
white but are easily crushable between the fingers. They
are occasionally called 'soft hail'.
- Ice pellets are
quite moderate in size and are composed of clear ice, sometimes
conical in shape.
- Hailstones are
whitish in appearance and vary greatly in size. If a hailstone
is cut open, a layered structure like and onion is sometimes
apparent.
Large hailstones fall
from deep cumulonimbus clouds. The cloud base may be 2,000
feet above the ground with tops at 30,000 feet. Much of the
cloud will be composed of supercooled water droplets. As the
hailstone falls it will collect tiny water droplets which
freeze and form a layer of ice. Perhaps the hailstone will
then be caught in a vigorous updraught. As it is carried back
higher into the cloud, it collects more minute water or ice
particles to form another layer of ice. Thus layers build
up on the hailstone (made of alternate layers of clear and
opaque ice) and the cycle may be repeated until the stone
is so big that it falls to earth.
Hail showers are quite
common over the British Isles in westerly and northerly airstreams
in spring, but really large hailstones tend to occur in the
south and are very much a feature of summer months.
The largest hailstone
recorded in the British Isles weighed 142 grams (5 oz) and
occurred at Horsham, West Sussex on 5th September
1958. Certainly anything approaching golf-ball size is remarkable,
but hailstones can grow large enough to dent cars, shatter
greenhouses and even injure people.
The USA, Canada, central
Europe, the southern parts of India and China all experience
large hail. So too do land areas in the southern hemisphere.
The world record (as quoted in the 1994 edition of The Guiness
book of Records) occurred in a hailstorm in the Gopalanj district
of Bangladesh on 14th April 1986. The hailstones
weighed up to 1kg (2lb 4oz) and were reported to have killed
92 people.
Related links
Winter's Weather - rain, drizzle
and fog
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