When a jug full of iced drink is taken out of the
refrigerator, water droplets soon begin to condense
on the outside of the container (provided the jug is
not made of an insulating material). This happens because
the jug is at a lower temperature than the dew point
of the air. Should the air be very dry and the temperature
of the outside of the container does not fall below
its dew point, then no condensation forms.
Fig 1: Dew on a
leaf
'Dew point' is defined as the temperature at which
the air, when cooled, will just become saturated. Let
us take as an example a day in which the air temperature
reaches 18 °C with a dew point of 8 °C. Late
in the afternoon, the air temperature begins to fall,
but the dew point will still be around 8 °C. However,
the air temperature is measured at 1 metre above the
ground and, under a clear sky, the temperature of some
objects may be significantly lower, due to loss of
heat by radiation. Once the temperature of the object
has fallen below the dew point, water vapour begins
to condense on to it in the form of dew. This is particularly
noticeable on the surfaces of cars that have been parked
for some time.
Dew also forms readily on grass because (a) the temperature
falls more rapidly nearer to the grass and (b) the
grass leaves produce water vapour, which raises the
dew point of the air immediately in contact. Dew does
not form as readily on other surfaces, such as soil,
brick or stone. This is because these materials absorb
heat from the sun which is then slowly emitted during
the evening, causing the temperature of air immediately
in contact to stay above the dew point for much longer
than over grass.
Fig 2: Dew formation
Next morning, as the incoming solar radiation gathers
strength, the dew will evaporate. Metal surfaces, such
as car bodies, will dry relatively quickly whereas
grass stays damp for considerably longer. In fact,
from late autumn to early spring, in some places shaded
from the sun, grass may remain damp all day after a
heavy dew.
Hoar frost is composed of tiny ice crystals and is
formed by the same process as dew, but when the temperature
of the surface falls below freezing point. The 'feathery'
variety forms when the surface temperature reaches
freezing point before dew begins to form on it. A 'white'
frost, composed of more globular ice, occurs when the
dew forms first, then subsequently freezes. A ground
frost may occur when the air temperature does not get
down to freezing point. Consequently, when the grass
is covered in a white hoar frost at dawn it cannot
be assumed that there is or has necessarily been an
air frost.
Raindrops show up well on a clear glass window. It
is immediately noticeable that they vary considerably
in size. A spattering of rain will show up as individual
drops, but a downpour soon develops a stream of water
down the glass.
Fig 5: Raindrops on
a window
Fig 6: Heavy rain
Water drops larger than 0.5 mm in diameter are classed
as rain, whereas smaller drops are described as drizzle.
The difference is purely one of drop size rather than
intensity of precipitation. Usually, drizzle comes
from sheets of low shallow cloud, whereas rain is more
likely from deeper clouds. Drizzle, with its many small
drops, will cut down the visibility more than the equivalent
amount of water falling as rain. Also heavy drizzle
is more wetting than slight rain.
Fig 7: A large raindrop
falls rapidly and sweeps up small droplets in its
path
When air rises, it cools and its water vapour condenses
into tiny droplets of water to form a cloud. Condensation
usually occurs around small particles called cloud
condensation nuclei. The motion of air within the cloud
causes the water drops to collide and larger drops
tend to grow at the expense of the smaller ones (a
process called coalescence). If water droplets continue
being developed within the cloud, such as in moist
air rising over a hill, they eventually start falling
out as drizzle. In deeper clouds, where the updraughts
are more vigorous, the water droplets become larger
before entering a region of the cloud where there is
a compensating downdraught and fall as rain.
This explains precipitation from cloud that is composed
entirely of water, but another process is at work when
a cloud contains ice crystals. In 1933 Tor Bergeron
demonstrated that these ice crystals are important
in the formation of raindrops.
The water inside a cloud does not start to freeze
at 0 °C, but at a much lower temperature.
In the meantime, it exists as supercooled water. When
the temperature falls to -40 °C, all water turns
to ice, but between about -10 °C and -40 °C,
the cloud consists of a mixture of supercooled water
and ice crystals. Bergeron demonstrated that water
vapour condenses more readily (a process known as sublimation)
on to ice crystals than on to supercooled water.
Aggregation of the ice crystals occurs as they move
into areas of cloud where the temperature is above
-25 °C. Accretion also occurs as water droplets
crystallize on coming into contact with the ice crystals.
These snowflakes eventually begin to fall, being precipitated
out as rain when the air temperature is above about
3 °C.
Snow
Fig 8: A snowflake
Precipitation will fall as snow when the air temperature
is below 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 six sides.
Thirty centimetres of fresh fallen snow has about
the same water equivalent as 25 mm of rainfall.
If rain falls continuously through air with a temperature
as high as 6 °C, it may cause the air temperature
to fall low enough for the rain to turn to snow.
This is due to latent heat being absorbed by the
evaporation of water vapour from the raindrops as
they fall, leading to the reduction in temperature.
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 an onion is sometimes apparent.
Large hailstones fall from deep cumulonimbus clouds.
The cloud base may be 3,000 feet (900 m) above the
ground with tops as high as 60,000 feet (18,000 m).
Much of the cloud will be composed of supercooled water
droplets. As the hailstone falls it will collect water
droplets which instantly freeze and form a layer of
ice. It may then be caught in a vigorous updraught
and, as it is carried back higher into the cloud, it
collects more water droplets 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 showery airstreams in spring, but really large hailstones
tend to occur in thunderstorms that have originated
from hot, continental air and are very much a feature
of summer months.
The largest hailstone recorded in the British Isles
weighed 141 grams (5 oz) and occurred at Horsham, West
Sussex on 5 September 1958. Certainly anything approaching
golf-ball size is remarkable, but hailstones can grow
large enough to dent cars, shatter greenhouses, injure,
and perhaps even kill people.
The USA, Canada, central Europe, the southern parts
of the CIS, India and China all experience large hail.
So too do land areas in the southern hemisphere. The
heaviest hailstone (as quoted in the Guinness Book
of Records) occurred in a hailstorm in the Gopalanj
district of Bangladesh on 14 April 1986. The hailstones
weighed up to 1 kg (2 lb 3 oz) and were reported to
have killed 92 people.
Fig 9: Hail
Fig 10: Cross-section
through hail
Fog
The official definition of fog is a visibility of
less than 1,000 metres. This limit is appropriate for
aviation purposes, but for the general public and motorists
an upper limit of 200 metres is more realistic. Severe
disruption to transport occurs when the visibility
falls below 50 metres. Useful labels for these three
categories are aviation fog, thick or motoring fog
and dense fog. The reduction in visibility is due to
tiny water droplets suspended in the air. The thickest
fogs tend to occur in industrial areas where there
are many pollution particles acting as nuclei for the
water droplets. This is no longer the case in most
of Europe following Clean Air Regulations and a reduction
in heavy industry.
Away from coasts, the most common type of fog is 'radiation
fog'. It forms overnight when the ground loses heat
by radiation, and cools. The ground, in turn, cools
the nearby air to saturation point, thus forming fog.
Often the fog remains patchy and is confined to low
ground, but sometimes it becomes more dense and widespread
through the night. Ideal conditions for the formation
of this type of fog are light winds, clear skies and
long nights. Consequently, the months of November,
December and January are most prone to foggy conditions,
particularly the inland areas of England and the lowlands
of Scotland in high pressure conditions.
Freezing fog is composed of supercooled water droplets
(i.e. ones which remain liquid even though the temperature
is below freezing point). One of the characteristics
of freezing fog is that rime - composed of feathery
ice crystals - is deposited on the windward side of
vertical surfaces such as lamp posts, fence posts,
overhead wires, pylons and transmitting masts.
Fig 12: Burning off
of radiation fog over France, 18 March 2005 0930-1330
After dawn, fog tends to disperse because it is 'burnt
off' by the incoming solar radiation, some of which
penetrates the fog and begins to heat the ground. This
then heats the layer of air immediately above, causing
the minute fog droplets to evaporate. This improves
the visibility and, if the fog is thin enough, it soon
clears. Thicker fogs sometimes lift into low cloud
before they clear. An area of fog will also contract
as the solar radiation raises the temperature of ground
at the edge more quickly than under the fog itself,
see Figure 12. However, in
winter, when solar radiation is low, fogs can be very
persistent if they become widespread. In such cases,
clearance is often the result of increasing wind or,
sometimes, drier air being advected from elsewhere.
Some coastal regions of the British Isles suffer from
'sea fog' which forms when moist air is cooled to saturation
point by travelling over a cooler sea. The wind may
then take the fog into coastal regions. This type of
fog tends to occur in spring and summer, and particularly
affects south-western and North Sea coasts. It is not
cleared by solar radiation since the sea-surface temperature
changes little, even on a sunny day. Sea fog is cleared
by the advection of drier air into it.
