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.
