Met Office College - Course Notes
Lifting Mechanisms that aid Cloud
Formation
Contents
1. What is a cloud?
2. Lifting mechanisms
2.1 Turbulence
2.2 Convergence
2.3 Topography
2.4 Frontal
2.5 Surface heating
3. Summary
4. Further reading
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1.What is a cloud?
Clouds are the visible manifestation of water or ice in the air. They are
both highly variable in shape and size and continually change with time.
Despite this variability, clouds all fall into two basic types - those formed
in a stable atmosphere and those formed in an unstable atmosphere.
Clouds generated in a stable atmosphere are of the layer type while
those formed in an unstable atmosphere are of the convective type.
Although we have discovered (in condensation) that water vapour in
the air can be brought to saturation by increasing the moisture content,
mixing or cooling. The most common way to form clouds away from the
surface is by the cooling method. Of the cooling processes, the adiabatic
reduction of pressure and expansion method is the normal way to
achieve cooling for cloud formation. Adiabatic expansion requires the
air to be lifted to lower pressures. How is that lifting generated?
2. Lifting mechanisms
A lifting mechanism raises air such that the air will cool. At height this
air will cool to its dew point allowing the vapour it contains to
condense. This height will depend on how close to saturation the air is
before it is lifted. Consider now the practical means by which air can be
lifted any vertical distance and assume there is sufficient moisture
present for cloud to form.
2.1
Turbulence
Winds that blow at about 1km (3000ft) above the ground are generally
unaffected by the surface topography. The wind speed at this height is
determined by measuring the distance between isobars, using the
geostrophic scale and making a correction for any curvature of the
isobars. However, below 1km the surface has the effect of slowing the
air due to friction between the air and the ground. Therefore, on most
days the wind increases rapidly with height in the first 1km. This
increase of wind speed with height leads to turbulence, see figure 1.
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Lifting Mechanisms That Aid Cloud Formation
Figure 1. Friction generating St or Sc type cloud due to lifting in turbulent air.
Changes of wind speed can occur away from the surface too. Often the
wind will continue to increase with height throughout the depth of the
atmosphere. However, if the increase stops for any reason, and more
especially if it starts to decrease with height, the same effect as surface
friction is seen. See figure 2.
Figure 2. Formation of cloud due to turbulence away from the surface. Note the
cloud need not be continuous as shown.
In a STABLE atmosphere surface turbulence can generate large sheets of
Stratus (St) or Stratocumulus (Sc). Turbulence away from the surface can
lead to sheets of Sc, Altocumulus (Ac) or Cirrus (Ci).
In an UNSTABLE atmosphere surface turbulence can lead to convective
clouds such as Cumulus (Cu) and Cumulonimbus (Cb). Away from the
surface it is possible for Ac Castellanus to be formed.
2.2
Convergence
Convergence is the meeting of airflows. This can occur in a number of
situations. Perhaps the most obvious is near the ground close to the
centre of a low pressure, where air is meeting from all directions, see
figure 3.
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Figure 3. Convergence near a low generating lift and cloud.
In figure 3, the air will either accumulate (in which case the low fills) or
will rise to allow room for more air to flow in towards the low centre.
This is not the only place that convergence occurs. Other notable places
are:

along sea breeze fronts

at air mass frontal surfaces

where air is funnelled by high ground

the region between high and low pressures, see fig 4.
Figure 4. Convergence around pressure centres leading to lift and cloud.
Some of these can of course happen away from the surface. In this
situation the air need not rise, but may sink. In this case to determine if
the air will rise or sink the stability of the air needs to be considered.
The clouds formed by convergence can be either layered or convective
or a combination of the two. Near low pressure centres both Cu and Sc
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are commonly formed by this method. Ac and Ci can be formed too.
Along sea breeze fronts convective cloud is common such as Cu and
occasionally Cb as the air is often unstable. Funnelling around high
ground or the flow around pressure centres can give rise to convective
cloud or layer cloud, usually in the form of Cu or Sc.
2.3
Topography
Air can be lifted over high ground, although the shape of the high
ground is important. Stability is an important factor in determining
whether air goes around rather than over obstacles. Only if it is more
difficult to flow around, (such as around an elongated ridge), will air
take the route over high ground. Clearly if sufficient cooling takes place
on this ascent cloud will form. The cloud that forms depends much on
the strength of the wind, shape of the hill, the moisture content of the air
either side of the hill, and the stability of the air. Some of the possibilities
are shown in figures 5, 6 and 7.
Figure 5. Orographic cloud forming due to general ascent of air over high
ground.
Orographic cloud is a common feature of exposed coasts where high
ground is present. In the UK, orographic cloud is often found over
western facing hill slopes when the low ground is clear of cloud. It can
be considered a form of upslope fog but its cloud type is usually St,
although it can be Sc.
Orographic cloud disperses when the air descends the lee side of high
ground and begins to warm, evaporating the cloud. Because some of the
condensed water droplets are lost in precipitation on the upslope side,
less latent heat of evaporation is required to disperse the cloud on the
lee side. This resulting cloud base being higher on the lee-side compared
to the windward side.
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Figure 6. Banner clouds form in the updraught to the lee of high ground.
Banner cloud is not a common cloud type. They are most frequently
found at the top of the lee side of isolated peaks whose lee slopes are
steeper than the windward. Banner clouds can be seen attached to the
Rock of Gibraltar and the Matterhorn mountain in the Alps.
Lenticular clouds form in the ascending part of lee waves. They are a
fairly common occurrence and do not require very large hills over which
to form. Some particular criteria do need to be met though:





a stable layer of atmosphere starting just above hill top height
the stable layer to be topped by an unstable layer
the wind should not change direction much with height
the wind may increase in strength
the wind should be roughly at right angles to the ridge of hills
Now given enough moisture, as the stable layer is pushed up by the air
going over the hill cloud will form. Some distance from the hill the
stable air will return to its original level and the cloud will disperse. The
air once it is started ‘bouncing’ often continues to bounce downstream.
Each successive bounce producing cloud in the rising part. Although
these clouds appear to be stationary, individual cloud droplets move
through the cloud with the wind. New cloud droplets constantly form in
the updraught and old ones disperse in the down draught.
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Figure 7. Lenticular clouds forming in lee waves triggered by high ground in a
suitable atmosphere.
2.4
Frontal
Frontal surfaces force air to rise due to density variations. This results in
large masses of air being lifted, giving rise to the familiar range of cloud
types we have come to expect from fronts.
2.5
Surface heating
Surface heating can occur in two ways. Either by the sun heating the
land or by cold air moving (advecting) over a warm surface. In both
cases the lowest layers of the atmosphere are heated by contact with the
warm surface which causes the air to become less dense and it will
therefore rise. Both are classic examples of creating an unstable
atmosphere. Cloud types that form in this unstable atmosphere tend to
be isolated convective clouds. Although these convective clouds can be
numerous they do not normally merge to produce large areas of
convective cloud.
While layer cloud types are not directly formed by surface heating they
can be spawned indirectly by it. When growing convective clouds reach
a part of the atmosphere which is less dense than itself (i.e. warmer) it
will stop rising but may well start spreading horizontally. In this case
layer cloud is being generated at a height. Sc, Ac and Ci can be formed
in this way.
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Figure 8. Schematic of isolated convective clouds forming in an unstable
atmosphere. The sinking air between each cloud suppresses lifting.
3. Summary
Clouds: These are visible manifestations of water and ice in the
atmosphere.
Cloud formation: Primarily due to cooling brought about by adiabatic
expansion and reduction of pressure. This requires a lifting mechanism.
Lifting mechanisms for clouds:
Turbulence
Convergence
Topography
Frontal
Surface heating
Cloud types: These depend solely on the stability of the air not the
lifting mechanism.
STABLE air allows only layer cloud to form
UNSTABLE air encourages convective cloud to from.
There can of course be a large variety of layer and convective clouds in
each case.
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4. Further reading
A Course in Elementary Meteorology, Chapter 5 by D E Pedgley
A Short Course in Cloud Physics, Chapters 3 and 4 by R R Rogers and
M K Yau
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