Download CHAPTER 4 Moisture in the atmosphere

CHAPTER 4
Moisture in the atmosphere
Moisture, or water vapour, is an extremely important constituent of the atmosphere.
Through heat released or absorbed when it changes state from solid or liquid or gas, it is
vital in transferring energy from the surface to the atmosphere. It is also involved in the
horizontal transfer of energy from surplus to deficit latitudes.
Perhaps even more important, it is through the condensation of moisture into droplets that
we obtain clouds – our source of precipitation. In this chapter we explore the way in
which microscopic water droplets and ice crystals in clouds can grow large enough to
reach the ground surface as precipitation when circumstances are suitable.
Clouds give us a useful indication of the state of moisture levels in the lower atmosphere.
A cloud will form when air is cooled sufficiently to reach its condensation point. Cooling
usually takes place through the rising of air. If the air is humid, air will soon reach the
condensation level and clouds will form. If the air is dry, uplift may be insufficient for
clouds to form and we have a clear blue sky. In general, the higher is the cloud base of
convection-type clouds, the drier is the atmosphere.
Mist and fog also provide information about the moisture levels in the lowest part of the
atmosphere, though their formation is also linked with the degree of night-time cooling.
Experiments can be done to determine the dew point temperature of air. Put some warm
water in a glass beaker, stir well, then record its temperature. Add ice and keep stirring
until condensation starts to form on the outside of the beaker. The temperature recorded
when the first sign of condensation takes place is close to the dew point temperature. In
cooler climates, discussion about condensation on windows, and freezing on grass, can be
useful.
The effects of heating and cooling in the atmosphere
•
A parcel of air when heated will rise, through its decreased density.
•
As the air parcel rises it will expand as it enters areas of lower pressure.
•
From the physics of the gas laws, as air expands it will cool.
•
Heating and cooling in the lower atmosphere are linked with the processes of
evaporation, condensation and precipitation formation.
•
The change in temperature with height in a well mixed atmosphere is known as
the Environmental Lapse Rate. It is variable and can be positive when
temperatures increase with height in an inversion of temperature; more frequently
temperatures decrease and its value is negative. Its value averages to about
6°/1000 m.
•
A parcel of air will cool at 9.8°/1000 m as it rises away from the ground surface.
This figure is known as the Dry Adiabatic Lapse Rate, as no condensation is
taking place and no heat exchange with the air’s surroundings is assumed.
•
The parcel of air will rise only if it is warmer and less dense than its surroundings.
•
If the parcel rises sufficiently it may reach its condensation point, and water
droplets form.
•
The latent heat released on condensation reduces the rate at which the parcel
cools. It now cools at the Saturated Adiabatic Lapse Rate. Its value varies because
different amounts of water will be condensed depending upon air temperature;
warm air will result in a lot of latent heat release, cool air will produce less latent
heat. The rate of cooling is therefore less in tropical than in polar areas.
•
If the air is less dense than its surroundings, and able to rise, the atmosphere is
said to be unstable. If the parcel is cooler and therefore denser than its
surroundings, the atmosphere is said to be stable.
•
If the air is only unstable once saturation is reached, the atmosphere is said to be
conditionally unstable.
Causes of condensation
•
Condensation occurs when air reaches its saturation point. It may do this by water
being added or by cooling of the air.
•
In the atmosphere there are always sufficient nuclei on which the water droplets
can condense once saturation point is reached.
The effects of condensation
•
Fog is formed when air near the ground is cooled to dew point temperature. The
cooling may be through radiational loss or by contact with a cold ground surface.
•
Clouds take a variety of forms, depending upon the rates and areal extent of uplift,
atmospheric temperature and humidity, and winds.
Formation of precipitation
•
Precipitation is the result of a complex series of micro-physical processes within a
cloud.
•
Precipitation may form as a result of collision and coalescence within a cloud.
•
Precipitation may form where ice crystals and water droplets coexist within a
cloud; this precipitation mechanism is known as the Bergeron–Findeisen process.
•
Precipitation-size droplets do not form instantly. It takes time for the droplets to
grow in size. Only if the conditions favourable to droplet growth last for a
sufficient length of time will precipitation reach the ground.
Types of precipitation
•
Precipitation formed in an unstable atmosphere by convectional uplift is called
convectional precipitation.
•
When the atmosphere is very unstable, thunderstorms or hailstorms may develop.
•
In temperate and polar latitudes, cyclonic eddies or depressions produce much of
the precipitation. The air converges towards the centre of the depression and rises,
causing the air to cool slowly over a widespread area. It is terms cyclonic or
convergent precipition.
•
Forced uplift over mountains can generate precipitation. This type is called
orographic precipitation. It only occasionally forms in isolation but often adds to
precipitation generated by convection or cyclones.
CASE STUDY Use of Clouds as Indicators of the State of the Atmosphere
Clouds are one of the best indicators of the current state of the atmosphere. They can give
some idea of such features as stability, wind, and moisture levels and even the possibility
of predicting what weather may occur within the next few hours.
Clouds form when air is cooled sufficiently for it to reach saturation. The cooling is
nearly always the product of the air rising either by mountains (orographic), by dynamic
convergence (cyclonic) or by convection. The appearance of the cloud will be
determined by the interactions between the factors favouring the formation of the cloud
and those causing its destruction through evaporation. Where air is forced to rise up a
mountain slope it may cool sufficiently to reach condensation. Mountain slopes can also
be heated preferentially when facing the sun and so act as a source of rising air. In both
cases the clouds are determined by the position of the mountain range so the cloud
positions may remain relatively stable. They are, of course, one of the sources of
orographic precipitation (see Chapter 5).
Figure 1 The clouds in the centre of the image that look like a pile of plates are the result
of layers of moister air blowing over the island in the distance. Where the air is rising it
has cooled sufficiently to form cloud, then as the air descends it warms and the cloud
evaporates. As a result, the cloud is static above the island despite air moving through
the cloud from right to left. It has a very smooth outline because the air is stable and only
rises where forced to do so by the ground surface. It is called altocumulus lenticularis
because of its lens-shaped appearance. Above these clouds there are some cirrostratus
and some cumulus below.
Source: Peter Smithson
Figure 2 Cumulus clouds forming over Snowdonia. These clouds are composed of
water droplets, as shown by the clear outlines of the cloud tops, and are relatively thin.
Convection is not strong because the air is not very unstable. This picture shows that you
do not need strong surface heating to produce convection cloud. The presence of the
snow shows the ground is cold but the atmosphere is sufficiently unstable in its lower
layers to allow the clouds to form. In the distance there is a layer of cirrostratus cloud
approaching as shown by the uniform spread of greyish cloud. This often indicates the
approach of a cyclonic system bringing rain or even snow to high ground.
Source: Peter Smithson
Figure 3 Convectional clouds following the mountain ridge on the left bank of the
Urubamba River in Peru. In this instance the clouds follow the line of the ridge very
closely, unlike those in Figure 1. Again they have clear outlines indicating liquid water
droplets but are unlikely to build up sufficiently to give precipitation.
Source: Peter Smithson
Dynamic convergence tends to operate over a large area but with relatively slow rates of
uplift compared with orographic or convectional clouds. As a result the clouds formed
by this method are often of vast extent, with long lifetime but with small vertical
thickness. They are best seen on satellite images as they may fill the whole observable
sky for a single observer.
Figure 4 Cirrostratus and cirrocumulus clouds. These are high altitude clouds
composed of ice crystals and are often associated with jet streams. The wave-like forms
are ripple effects between air of different speeds, directions or stability.
Source: Peter Smithson
When cirrostratus clouds cover the whole sky and appear to thicken upwind it is often the
sign of the approach of a cyclonic system and precipitation (see Chapter 7). The outline
of the sun (or moon) may be seen through the cloud to give the appearance of looking
through opaque glass. When the cloud is thick enough to obscure totally the sun then it is
classified as altostratus cloud. By then it will have become deeper and at a lower altitude.
Figure 5 Cirrostratus and altostratus clouds approaching from the west and a
forewarning of precipitation. The clouds have little clear structure as the streams of ice
crystal cloud have become so widespread as they ascend ahead of the centre of the low
pressure system.
Source: Peter Smithson
The easiest clouds to see for an individual are convectional clouds where instability of
the atmosphere allows air to rise in a column to give what meteorologists call cumulus or
cumulonimbus clouds depending upon their depth. As the air rises it is often affected by
the stronger winds at higher levels of the atmosphere causing the clouds to bend over
downwind. Eventually the rising mass of liquid water droplets of the cloud may
experience sufficiently low temperatures that they become ice crystals. Surprisingly this
is not at 0ºC but, because of supercooling, it can be as low as about -35ºC. As the cloud
becomes ice crystals, the appearance of the cloud becomes more fibrous or wispy. Ice is
less easily evaporated than liquid water so they can stream away from the cloud in a
downwind direction. This is often observed in condensation trails from aircraft.
The smallest convectional clouds are termed cumulus humilis as they indicate weak uplift
and probably an inversion of temperature preventing them rising further.
Figure 6 A good example of fair weather cumulus clouds randomly scattered around the
landscape with no preference for formation over the hills. They have a firm outline
confirming liquid water though there is evidence of evaporation from the upper parts of
the cloud.
Source: Peter Smithson
Figure 7 When convection and instability are greater, the cloud can build up to a greater
depth within the atmosphere and allow precipitation to form. In this figure we can see
rain falling from the base of the cloud as the grey streamers in the centre. In the right
distance there are a few cumulus clouds but none deep enough to give rain.
Source: Peter Smithson
Figure 8 Where the convection is more extensive and the atmosphere moister, the
clouds may form an almost complete cover with only a few clear areas. In this picture of
the Lake District, one of the wettest parts of England, a number of small showers can be
seen falling from the base of cumulus clouds. Note in the cool air of spring the cloud
base is only slightly above the level of the peaks at around 600-700m. If the air was
more unstable, the clouds would become deeper and develop into cumulonimbus clouds.
Such clouds usually give heavy showers and even thunderstorms.
Source: Peter Smithson
There are a number of excellent books giving more information and images about cloud
appearance and cloud formation. It would be useful to spend some time just looking at
the sky and following the constantly changing shapes and appearance of clouds. See if
you can relate these to other features of the atmosphere such as surface pressure patterns,
as well as recognizing them from book pictures. If you live in a hilly area, try to relate
the cloud features with those of the ground surface. Estimate the height of the cloud base
from where it hits the ground surface.
Essay and discussion questions
1 What makes convectional clouds produce their characteristic shape? Over a period of
several days, describe the different cloud shapes that you can see. Can you relate these
cloud types to weather conditions?
2 Describe the main processes that cause air to rise.
3 For temperatures below freezing, explain the consequences of the saturation vapour
pressure over water being greater than that over ice.
4 In what ways are the methods of formation of fog and cloud similar and in what ways
are they different?
5 Not all clouds give rain. Why don’t they?
6 Is the standard sub-division of the origins of precipitation into convectional, convergent
(or cyclonic) and orographic satisfactory?
Further reading
Mason, B.J. (1975) Clouds, Rain and Rain-making, Cambridge: Cambridge University
Press. A combination of elementary and advanced ideas about rain formation. Only for
those who want to go into more detail especially about the microphysics of clouds and
precipitation.
McIlveen, R. (1992) Fundamentals of Weather and Climate, second edition, London:
Chapman and Hall. Elementary to intermediate-level text that provides a useful
explanation of stability and moisture with only limited maths and physics – chapters 5
and 6.
Schaefer, V.J. and Day, J.A. (1981) A Field Guide to the Atmosphere, Boston, Mass.:
Houghton Mifflin. A very interesting book which stresses the visual approach to the
atmosphere, hence the emphasis on clouds. Strikes a nice balance between description
and explanation.
Web Resources
http://www.bbc.co.uk/weather/features/weatherbasics/cloud_types.shtml
For anyone wanting to know more about cloud types and their appearance and
significance, this BBC website provides a number of images and explanations about each
type of cloud. Has other weather related information.
http://www.metoffice.gov.uk/education/secondary/students/water.html
Part of the Met Office website’s educational section, this URL is concerned with water in
the atmosphere. Each type of precipitation is discussed. An associated link to clouds as
available.
http://www.metoffice.gov.uk/education/higher/lapse_rates.html
If the concept of lapse rates proves difficult, the Met Office have tried to simplify the
concepts of cooling in the atmosphere on this site, and how they affect vertical movement
and cloud formation.
http://www.cloudappreciationsociety.org/gallery/index
If you are interested in cloud appearance, this site has a wide range of good examples of
different types. Not all images have a full explanation of what is visible, but there is an
impressive range of clouds categorized into the main cloud groups.
http://www.cloudman.com/
Another site with a wide range of cloud photographs and some explanation of the nature
of the cloud and where it was taken. Also includes a mini-atlas of the main cloud types
together with a description of those illustrated.