Met Office College - Course Notes
Water in the atmosphere
Contents
1. Introduction
2. Dew and frost
3. Raindrops
4. Snow
5. Hail
6. Lightning and thunder
7. Fog
8. Drizzle and snow grains
 Crown Copyright. Permission to quote from this document must be obtained from The Principal, Met Office College
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1. Introduction
It need hardly be said that water is important – we would not be here if
it were not for its existence. However, it is not only in sustaining life that
it plays a prominent role but also in meteorology. The formation of
clouds, precipitation processes, and the release or absorption of latent
heat due to phase changes are integral to the atmospheric system. (By
‘water’ we are not restricting attention solely to the liquid phase but
referring generally to H2O in its liquid, solid and gaseous forms.)
Much of the weather we experience (particularly in the UK) is directly
affected by water. But water is no ordinary substance – its behaviour is
distinctive, even peculiar, particularly when phase changes are
involved. It is these manifestations of water as we experience it that is
the subject of these notes. In the following sections, each of the main
contexts in which water makes its presence known will be discussed.
2. Dew and frost
When a jug full of iced drink is taken out of a refrigerator, water
droplets condense on the outside of the container (provided the jug is
made of 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 (at constant pressure and constant
‘relative humidity mixing ratio’).
On a summers day when the air temperature reaches 18 °C, the dewpoint might typically be 8 °C. By sunset the air temperature may have
fallen to 12 °C (see Fig. 1) but the dew-point will still be around 8 °C.
During the night the ground temperature continues to fall and if it
reaches, say, 7 °C the temperature of the ground is below the dew-point
(the saturation point) of the air and droplets of moisture begin to form this is dew. Due to the release of latent heat when the dew is formed, the
fall in temperature slows down.
Next morning, as the incoming solar radiation gathers strength, the dew
will be evaporated. The grass will become reasonably dry and suitable
for sitting upon during the day. However, in winter in calm weather 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; the crystals are especially feathery when the dew
point is below 0°C. It is formed by the same process as dew, but occurs
when ground temperatures are below freezing-point. A ground frost
may occur when the air temperature does not get down to freezing-
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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.
Temperature
Dew forms
when ground
temp. falls to
dewpoint
Dew evaporates 2-3
hours after sunrise
Midday
Sunset
Midnight
Sunrise
Midday
Fig. 1. A simplified diurnal temperature curve. Possible points at which dew
may form and evaporate are indicated. The actual time of dew formation and
evaporation depends upon the amount of moisture in the air, the nature of the
underlying surface, and other factors.
Sometimes dew forms during the evening and subsequently freezes to
become hoar frost with globular ice on the grass.
There is a weather saying of some value: “Dew at night, the day will be
bright”. To a certain extent this is self-evident, for dew tends to form on
clear, calm nights which occur in anticyclonic (high pressure) weather.
Such conditions also tend to give sunny days.
3. Raindrops
The windscreen of a car is a superb place to view raindrops, and indeed
all precipitation particles. Every second or two the windscreen wipers
provide a fresh screen on which to view the next sample.
Water drops larger than 0.5 millimetres in diameter are classed as rain,
whereas smaller drops are described as drizzle. The difference is purely
one of drop size rather than the 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.
In the early days of meteorology it was known that rising air would cool
and its water vapour would condense into tiny droplets of water to form
a cloud. Within the cloud the large drops would tend to grow by
colliding and merging with the smaller ones (a process called
coalescence) - this appeared to provide the explanation of how rain was
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produced (see Fig. 2). During the 19th and early 20th centuries scientists
became aware that the rate of growth of water droplets by this
mechanism was too slow to account for the formation of large raindrops,
though it did explain drizzle.
Large drops:
fall speed ~10m/s
Small drops:
negligible fall speed
Fig. 2. A large rainfall falls rapidly and, with its large cross-sectional area,
‘sweeps up’ small droplets from its path.
In 1933 Bergeron demonstrated that ice crystals could play an important
part in the formation of raindrops. In certain conditions ice crystals form
in clouds and rapidly increase in size at the expense of the neighbouring
water droplets. Collisions between the ice crystals then lead to
snowflakes. It is then possible to get large raindrops when the
snowflakes melt as they fall through air which is above freezing. This
process is responsible for most of the rain and snow over the British
Isles.
In the summer months large raindrops are often particularly noticeable
when they fall from medium-level clouds on a hot afternoon. The first
few drops may spread out on a path to the size of a 2p piece. Such “fullsize” raindrops are sometimes the precursor of thundery weather.
4. Snow
Snow forms typically at temperatures well below freezing, say -10 °C ,
inside a cloud where ice crystals grow at the expense of tiny supercooled
water droplets (i.e. droplets of water with temperature below freezing).
This is the Bergeron-Findeisen process - the end result is snow. When
snowflakes fall out of the bottom of the cloud, their rate of descent is
slower than rain (perhaps by a factor of 5).
If the snow reaches a level where the air is above freezing then
obviously it will melt, but at the same time the air will be cooled. In
practice, snow will often reach the surface with a temperature of 1 °C or
2 °C. Partly melted snow, called sleet, is likely at temperatures around
2°C or 3 °C. The notion that it can be too cold for snow is erroneous,
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although heaviest falls do tend to occur with temperatures around
freezing.
Individual ice crystals can be the shape of prisms or plates or, with
typical snowfalls, 6-pointed stars. The medium to large snowflakes
normally seen in the British Isles are composed of many ice crystals
which have collided and stuck together (aggregated) inside the cloud,
and thus snowflakes may be 3 cm across.
If 10 cm of fresh fallen snow is collected in a tall glass and allowed to
melt, it is found that the water equivalent is about 1 cm. However, the
result does vary with different snowfalls and also depends on how
carefully the snow is collected.
On the Scottish mountains almost any front or depression can give some
snowfall in winter. The lowlands of England, though, need perhaps an
approaching warm front or occluded front, or an active depression
passing just to the south of the observer (see Fig. 3). A showery
northerly is another possibility, with snow showers coming in from the
sea. Cold fronts which, by their name, sound as if they should bring
snow tend to be disappointing in this respect. However, sometimes rain
will turn to snow behind the front before the precipitation ceases.
On low ground in the British Isles, October is very early for snow and
June is very late - though notable falls in eastern England occurred on
2nd June 1975 delaying cricket matches and causing raised eyebrows on
the farm. Summer observations of “snow” usually turn out to be falls of
Rain and
possible snow
Low
Rain
soft hail, but in meteorology virtually nothing is impossible.
Fig. 3. If an active depression crosses the British Isles in winter then snow is
possible to the north of its track, with rain to the south.
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5. Hail
There are three different phenomena that could loosely be described as
hail which affect the British Isles.

Snow pellets are beautifully white but easily crushable between the
fingers. They are occasionally called “soft hail”.

Ice pellets are quite moderate in size and are composed of dear 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 (see Fig. 4).
Fig. 4. A large hailstone may consist of several layers of clear and opaque ice.
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. The way in
which hailstones are formed is still the subject of some controversy. One
theory is that they grow to a large size by repeated recirculation within
the cloud and where updraughts are strong enough to support them. 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 the summer months. Some
dramatic temperature falls accompany hailstorms with the transition
from sunshine to hail-covered ground taking less than an hour. Strong
gusts of wind may also occur, and so too may lightning and thunder.
The largest hailstone recorded in the British Isles weighed 141 grams
and occurred at Horsham, Sussex on 5 September 1958. Certainly
anything approaching golf-ball size is remarkable, but hailstones can
grow large enough to dent cars, shatter greenhouses and even injure
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people. The USA, Canada, central Europe, the southern parts of the
USSR, India and China all experience large hail. So too do land areas in
the southern hemisphere. The world record quoted of 758 grams was
from Kansas, USA with a diameter of 190 millimetres.
6. Lightning and thunder
Lightning is one of the most impressive displays of atmospheric energy
and yet - despite all the studies that have been carried out - the
mechanism by which the electrostatic potential develops in the
atmosphere is not fully understood.
Lightning occurs when a charge builds up within a cloud (see Fig. 5). A
difference in charge then exists between that region of the cloud and the
ground, or between the cloud and another nearby. The charge is thought
to build up on ice particles or water droplets. If the difference in
potential becomes sufficiently great, “the spark jumps the gap” between
cloud and ground or between neighbouring clouds. Intense heating
along the discharge path causes very rapid expansion of the air —the
‘explosive expansion’ causes the thunder. When the path of the
discharge can be seen the lightning is described as “forked”, but “sheet”
lightning occurs when the flash is obscured by cloud.
+
+
+
+
+
+
+
+
_
_
_
_
_
_
_
_
+
+
_
_
_
+
+
_
_
_
Fig. 5. The distribution of charge within a typical cumulonimbus cloud. The
discharge may be cloud-to-cloud or from cloud to ground.
Since light travels at 300,000 kilometres per second the flash is seen
almost instantaneously. Sound, on the other hand, travels 1 kilometre in
3 seconds (1 mile in 5 seconds). To find out how far away the thunder is,
count the seconds as soon as the lightning is seen and continue until the
thunder is heard. Then divide by 5 to get the storm distance in miles.
In England and South Wales, thundery days occur mostly in the
summer half-year when southerly or south-westerly winds at medium
or high levels bring storms up from France. Such storms often occur at
night. Inland areas of eastern England tend to be the most prone to
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thunderstorms where they occur on 10 to 20 days per year. In Scotland,
Northern Ireland and North Wales thunderstorms are less common and
generally occur at any one site on fewer than 10 days per year. Also such
storms may be one flash and a bang and they have gone. Lightning
accompanied by snow can occur when big cumulonimbus clouds come
racing in from the sea on to our northern coasts in winter.
Thunderstorms tend to be quite frequent in parts of the tropics where
deep cumulonimbus clouds are common.
7. Fog
The official definition of fog is a visibility of below 1000 metres. This
limit is sensible for aviation purposes but for the general public and
motorist 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 fog and dense fog
(see Table 1).
The reduction in visibility is due to tiny water droplets suspended in the
air. In industrial areas where there are many pollution particles on
which the water droplets can grow the thickest fogs tend to occur.
Freezing fog is composed of supercooled water droplets (i.e. ones which
remain as liquid water even though the temperature is below freezingpoint). One of the characteristics of freezing fog is that rime - composed
of feathery crystals of ice - is deposited on the windward side of vertical
surfaces such as lamp-posts, fence posts, overhead wires, pylons and
transmitting masts.
Dense fog
Thick fog
Aviation
fog
Mist or
haze
Severe
disruption
to most
transport
Road, rail
and aircraft
on the
ground
delayed
All aircraft
landings
affected to
some extent
Shipping
and light
aircraft
affected
Zero
50 m
200 m
1000 m
5000 m
Table 1. Difficulties for transport caused by various categories of poor
visibility.
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. Liquid water
droplets form as continued cooling occurs, but instead of falling out as
dew it is held in suspension by light winds in the form of fog. Often the
fog remains patchy and is confined to low ground, but sometimes it
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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 inclined to fogginess, particularly the inland
areas of England and the lowlands of Scotland in high pressure
conditions.
After dawn, thin fog tends to disperse because it is “burnt off’ by the
incoming solar radiation. Some of the solar radiation penetrates the fog
and reaches the ground. The ground heats up, as does the layer of air
near the ground. Eventually the air reaches a temperature where the
minute fog droplets evaporate and the visibility improves. However, in
winter thick fogs can be very persistent due to the lack of insolation.
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
coasts in the south-west and the North Sea coasts. Meteorologists often
call this “advection fog” because it is blown by the wind.
8. Drizzle and snow grains
Drops of drizzle usually fall from a low-level sheet of stratus or
stratocumulus. They tend to form by small water droplets in the cloud
merging. When the droplets are too heavy to be supported by the
updraught in the cloud they fall as drizzle.
The mechanism causing the air to rise and to form cloud is turbulence
rather than convection, so generally there is a good wind blowing when
drizzle occurs. Beneath the clouds the air needs to be moist to prevent
any drizzle evaporating before it reaches the ground. Thus “drizzle
weather” is associated with very moist air, low cloud (usually with a
cloud base less than 800 feet) and probably at least a moderate to fresh
breeze.
In airmass terminology, drizzle can occur in a tropical maritime
airstream, so it is especially likely on south-west facing coasts. An
alternative to the turbulence mechanism is uplift over a hill or two, so
south-west facing hills tend to be affected by drizzle. Cornwall, Devon,
western Wales and north-west Scotland tend to receive more than their
share of drizzle. Also in the cold season drizzle sometimes occurs in an
easterly airstream coming in from the North Sea or up the Thames
estuary.
With a typical cloud base of, say, 400 feet and cloud tops at 2,000 feet,
subtle changes in the depth of this relatively shallow cloud layer can
make a substantial difference to the weather. To put it another way,
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drizzle is difficult to forecast in detail, particularly in an easterly
airstream.
The mid-winter equivalents of drizzle are snow grains; these are small
compared with snowflakes. Snow grains are often angular in shape and
so small that accumulations on the ground are usually negligible. The
breeze may well blow an hour’s worth of snow grains into the roadside
gutter, but it still does not amount to much. However, an outbreak of
light snow grains can turn to “proper” snow.
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