of vertical development
is essential that the mariner has a good knowledge of meteorology and
that he/she understands what weather conditions are existing. Cloud
formations will give a clear indication of this.
Clouds can occur at any level of the
atmosphere wherever there is sufficient moisture to allow
condensation to take place. The layer of the atmosphere where almost
all cloud exists is the troposphere, although the tops of some
severe thunderstorms occasionally pierce the tropopause.
Because of the large range in
temperatures and air movement in the troposphere, clouds vary in
structure and composition (a combination of ice crystal and water).
Consequently, clouds are classified into three main groups: lower,
middle and high level clouds.
Higher level clouds
Higher level clouds
represent the cloud in the highest levels of the troposphere. They
mostly appear brilliant white because of the ice crystals at that
level. They tend to develop at or just above the top part of the
troposphere. Higher level clouds can vary in shape, thickness and
Sunlight can be
observed passing through the higher level clouds most of the time.
The amount of light that penetrates depends on the density and
thickness of the layers. The thickness of such clouds are therefore
In most cases, the
direction of movement of the higher level clouds do not necessarily
represent the wind direction at the ground level. In fact, the wind
at upper and ground levels often differ.
There are three main
types of higher level clouds: cirrus, cirrostratus and cirrocumulus.
The bases of high clouds range from 16,500 feet
to 45,000 feet and average about 25,000 feet in the temperate
characteristics of cirrus and cirrostratus are similar, they can be
discussed together including any differences.
Cirrus clouds are
higher level clouds that develop in filaments or patches. They are
virtually brilliant white attributed to their ice crystal
composition. However, they lack in contrast between the top and
base. They occur in flat sheets with a low height to base ratio and
are usually isolated with large breaks of sky. Cirrus also vary
dramatically in 'shape' or patterns they portray but these represent
the fluctuating wind flow at that level both in the horizontal and
clouds that are more widespread than cirrus but containing some
similar features. Like cirrus, they are brilliant white and lack in
contrast. Sunlight can pass through cirrostratus but this again
depends on the varying thickness of the clouds.
Both cirrus and
cirrostratus clouds vary in thickness. The sun can easily be
observed through both types of clouds although the intensity of
light that is observed depends on the thickness of their layers. In
their thickest form, cirrus and cirrostratus will allow a similar
intensity of light to pass through to that of thin altostratus. They
do not only develop in one complete layer. It may be difficult to
observe because of the lack of contrast but these clouds can consist
of several thin layers.
cirrostratus tend to move in the direction of the wind at that level
which differ to that at the surface. The most common direction of
motion of these clouds are from a westerly direction. This varies
with factors such as the latitude, weather conditions and time of
the year. Their apparent velocities are relatively slow as compared
to lower clouds.
Both cirrus and
cirrostratus can occur in conjunction with any of the other cloud
types. Obviously, all the lower and middle level clouds will obscure
the view of the higher level clouds, appear to move faster and
appear less defined. They can only be observed above other clouds
when breaks in the clouds occur. Any type of higher level clouds can
Cirrus clouds tend to
develop on days with fine weather and lighter winds at the surface.
cirrostratus can develop on days with light winds but normally
increasing in strength. Although both cirrus and cirrostratus tend
to develop in fine weather conditions, they also acts as a sign of
approaching changes in the weather conditions. Such changes could
include any of the various types of cold front situations,
thunderstorms or developing and advancing troughs of low pressure,
normally with preceding cloud masses.
Except in the latter
case, cirrus and cirrostratus will typically precede any other types
of clouds as part of a cloud band. In fact cirrus normally precedes
cirrostratus. Nevertheless, the higher level clouds will persist
until the actual change in the weather occurs. The higher clouds can
develop from a few hours up to a few days before an actual change in
the weather conditions occurs. They may develop during one afternoon
and dissipate but redevelop the next day and so on until the actual
change occurs. If the amount of moisture in the lower layers of the
atmosphere increases, other lower clouds may also develop changing
the appearance of the cirrus or the cirrostratus clouds as well as
partially or totally hiding them from view. The same situation
occurs in the case where cirrus develop ahead of thunderstorms.
Cirrus normally precede cirrostratus which are then followed by the
anvil of the approaching thunderstorm. In fact, cirrus and
cirrostratus in this case are the remnants downwind of the weakening
Both cirrus and
cirrostratus can develop and persist after a change has passed
through a certain location. In this situation, cloud will decrease
within a few hours up to a few days following the change. If it
persists for longer periods, a jet stream cloud mass may be
where cirrus and cirrostratus can be observed is when lower cloud
breaks or clears during days with showers or rain. This case is far
less common but can indicate a few situations. The higher clouds may
be the remnants of the cloud mass that produced the actual wet
weather. They can also be developing ahead of other cloud masses
associated with another system, leading to the situation already
discussed above. It all depends on the weather situation at that
time but the observation of the movement of the higher level clouds
can be critical in determining what weather may follow.
Cirrus generally does
not produce precipitation except when it results from dissipating
thunderstorms. Precipitation from such cirrus usually consists of
larger droplets and the cloud normally dissipates and vanishes
completely. cirrostratus does not produce precipitation.
cirrostratus can develop and persist at any time of the day despite
the perception that it tends to occur during the day. This
perception arises because it is much easier to observe cirrus during
the day as compared to night time. The background darkness and the
fact that the stars can easily be observed through cirrus and
cirrostratus as thin layers allows them to camouflage from the view
of the observer.
Cirrocumulus is a
higher level cloud that is brilliant white but with a spotty
appearance created by the many small turrets. The turrets indicate
vertical turbulence within the cloud. Despite this spotty
appearance, cirrocumulus contains many features associated with
cirrostratus discussed above. It moves in directions similar to that
of the other higher clouds.
This cloud can develop
in conjunction with any other clouds as well as with cirrostratus
clouds. In Sydney, cirrocumulus is not as common as the other high
clouds and mainly develops during the winter times with west to
south westerly air streams. The development of cirrocumulus
sometimes occurs in conditions similar to those associated with the
development lenticular altocumulus. cirrocumulus clouds do not
produce precipitation and are normally associated with fine weather.
Middle level clouds
are those clouds that develop in the middle layers of the
atmosphere. These clouds are brighter and less fragmented in
appearance due to their distance from the ground and the higher
composition of ice crystals. Middle level clouds vary in thickness
from relatively flat sheets of cloud to a more cumuliform
appearance. In fact, the sun (and moon) can be observed through some
thin middle level clouds.
Middle level clouds tend to have
apparent speeds slower than the lower level clouds. (Recall the
larger radius and associated arc length that the higher level clouds
must undertake). They move in the direction of the wind at that
level which does not necessarily be the same of that at the surface.
There are 3 basic types of middle
level clouds: altocumulus, altostratus and nimbostratus.
The bases of
middle clouds range from 6500 feet to 23,000 feet.
As the name suggests,
altocumulus refers to the middle level cloud that exhibit to some
extent the features normally associated with cumulus. This includes
cumuliform tops and bases that are usually relatively darker than
the tops. This cloud type can be widespread or patchy depending on
the conditions. It can vary in appearance from broken to smooth, and
vary in thickness.
In its broken form,
altocumulus can be confused with stratocumulus. To distinguish
between them requires examining how defined the cloud appears,
whether there other forms of middle level or upper level clouds
present above the layer and the difference in brightness. Like
stratocumulus, the breaks become more visible at a steeper angle of
If conditions are
unstable in the middle level of the atmosphere, the air will tend to
rise in currents allowing areas of cumuliform turrets to develop. In
fact, altocumulus can develop from dissipating thunderstorms during
the morning and then redevelop during the day if the air remains
unstable. Altocumulus clouds therefore in this form indicate
unstable or unsettled conditions.
Altocumulus can vary
in its apparent movement (speed) depending on the wind and direction
at that level. However, since altocumulus (like most other cloud
types) represents an ever changing system, an observer must be
careful in determining cloud motion. On some days, altocumulus
continuously develop as it moves in the direction of the wind.
Upstream, more altocumulus may develop giving the impression that
the cloud is progressing slower than its actual speed. This process
can occasionally create an illusion in terms of direction.
Considering an example of altocumulus observed moving to the south
east, because of development on the north and north-eastern side of
the cloud band, the apparent direction may be more to the east.
develop in more than one layer and also in conjunction with other
cloud types. The lower layer will obscure part or all of the higher
altocumulus cloud layer. This situation also applies to higher level
clouds. Higher level clouds will be obscured by the altocumulus.
Lower level clouds, however, will obscure part or all of the
altocumulus cloud layer. In fact, it may be impossible to observe
altocumulus above a full stratocumulus, stratus or lower level
nimbostratus cover. If a break occurs, altocumulus can only be
distinguished by its different (slower) speed and direction of
develop within the structure of cumulonimbus (thunderstorm
producing) clouds. The appearance of altocumulus within
thunderstorms vary depending on the structure, severity and the
amount of moisture drawn into the thunderstorm. The altocumulus
usually develops after the anvil (consisting of cirrostratus and
altostratus) develops and becomes darker as the precipitation
cascade approaches. However, on days where thunderstorms develop
with widespread altocumulus conditions, the altocumulus obscures the
thunderstorms and its development observed only through breaks in
develops into thicker layers, precipitation can develop. The
intensity of rainfall most often expected from altocumulus is light
to moderate rainfall. If large cumulus develop amongst the rain
bearing altocumulus, then heavier rainfall will develop. On days
when precipitation from altocumulus becomes widespread and
continuous, the cloud forms a smooth lighter-grey shaded sheet and
becomes known as nimbostratus (at the middle layers).
altocumulus can develop rapidly at the rear even though the cloud
may be moving fairly rapidly. This will obviously influence the
duration of rainfall as well as the normally large cloud base. This
situation often occurs before a cold front with unstable conditions.
Thunderstorms can develop amongst the altocumulus band or they may
develop after the cloud band clears well ahead of the actual change.
As discussed in the
case of other clouds, lower clouds may be present below the
altocumulus layer but not producing the rain. The observer again
must consider which cloud is producing the rain to determine in
which direction it is moving.
Another form of
altocumulus is the lenticular type where the altocumulus appears in
the form of a lens. They appear very smooth and flat, often
displaying two or more layers. This occurs due to a wave effect in
the air flow. This wave effect normally develops as a result of a
mountain range on windy days. The wave effect forces air to rise
above the condensation level and hence allows cloud to form. Due to
the rise and fall effect (peaks and troughs), the cloud may only
exist in areas of peaks and therefore appear patchy. The most
striking feature of this cloud is that it tends to remain relatively
stationary compared with the associated wind at that level. What is
actually happening is that as the air begins to rise above the level
of condensation, cloud forms. When the air falls below this level,
dissipation occurs and the cloud disintegrates back to clear air. So
long as the peak of the air wave remains stationary as compared to
the ground, the cloud will develop and dissipate almost in the same
position whilst the wind conditions persist.
The direction of the
wind associated with lenticular altocumulus can be determined by
considering the sharpest edge as the end of the cloud where the air
is flowing in and the opposite end where the air is flowing out.
Sometimes this will be the longest span of cloud. The most efficient
method of determining the direction of the wind is by closely
examining the direction that the patterns and ripples within the
cloud base move. The cloud will also be moving in the direction of
the wind within the cloud region.
is generally not associated with precipitation. The conditions
associated with the development of this cloud involves more
horizontal rather than vertical flow. The air masses are also more
stable and drier.
mostly develop during the day when the atmosphere is most lively in
terms of strong winds at that level. The wind conditions at the
surface are often very similar to the direction of wind at the cloud
level. In the case of Sydney, lenticular altocumulus tend to develop
during the morning period and clear off the coast during the
evening. Almost all lenticular altocumulus in Sydney develop under
the influence of south westerly, westerly or north westerly air
streams associated with cold fronts.
Altocumulus can also
develop in the form of ripples. In this case, the altocumulus cloud
appears broken but lined as a result of minor wind wave ripples. In
fact it develops in conditions associated with the development of
lenticular altocumulus. This type of cloud obviously does not
develop from the spreading out of the tops of cumulus. The spreading
out occurs as the tops of the cumulus grows until it reaches an
inversion layer (or stronger winds that cause divergence) situated
in the middle levels of the atmosphere. Because the cumulus
updraughts are not strong enough to pierce this layer, the tops
begin to spread in the form similar to that of an anvil facing in
the direction of the wind at that level. Occasionally, this
situation may further develop into thunderstorm or thundery shower
Altocumulus with a
turreted appearance. Instability is a characteristic.
Altocumulus castellanus may develop into cumulonimbus.
Altostratus refers to
middle level cloud that appears as a flat, smooth dark grey sheet.
These clouds are most often observed as large sheets rather than
isolated areas. However, in the process of development, altostratus
may develop in smaller filaments and rapidly develop to larger
sheets. These types of clouds in certain conditions normally
indicate an approaching cloud mass associated with a cold front, a
trough system or a jet stream.
develop into a thick or thin layer. As a thin layer, the sun can be
observed through the cloud. In its thinner form, the developing
altostratus can sometimes be confused with approaching cirrostratus.
In its thicker form, the sun can only occasionally be observed
through the thinner sections if not at all. Obviously, the thicker
the altostratus, the darker it becomes. When observed closely, it
becomes apparent that altostratus is not just one complete layer but
a composition of several thin layers.
produce precipitation. It will normally develop and then thicken.
The precipitation is observed as relatively thick dark sections
since precipitation cascades are very difficult to observe with the
same colour in the background. In this situation, rain will develop
as a light shower and gradually increase to showers, light rain or
moderate rain. If the precipitation becomes persistent, the cloud
then becomes known as nimbostratus. The duration of the
precipitation is influenced by factors similar to those discussed
with other types of clouds.
In certain conditions,
altostratus will develop during the afternoon period and increase to
cover most or all of the sky. By late afternoon, evening or during
the night, precipitation will develop. This situation is the most
common observation that occurs in Sydney. However, altostratus can
develop at any time as well as the associated precipitation.
As discussed above,
altostratus can develop in conjunction with other clouds such as
cirrostratus, altocumulus and stratocumulus. Obviously, the lower
clouds will obscure the view of altostratus, appear to move faster
and appear less defined. Although altocumulus is a middle level
cloud, it will develop below altostratus. Sometimes, altocumulus can
be observed developing from dissipating altostratus. cirrostratus
can often be observed above altostratus when it does not cover the
sky. On days where altostratus is observed above a stratocumulus
cover, it may indicate a trough with possible rain or even
thunderstorms either during the afternoon or within the next few
altostratus also forms part of thunderstorms normally within or
below the lower part of the anvil region. Of course this depends on
the height of the thunderstorm anvil. Different structures of
thunderstorms display various forms of altostratus. As the anvil of
the thunderstorm passes overhead, the altostratus begins to appear
normally with a grey base but becoming increasingly dark.
develop in situations similar to the development of lenticular
altocumulus. Altostratus in this form develops in large sheets and
has a patchy base appearance. The cloud seems to be moving rapidly
but because of its development at the rear actually progresses very
slowly in the direction of the wind at that level. This type of
cloud does not produce any rain.
Nimbostratus can be
described as a widespread light grey or white sheet of cloud that
produces persistent rain or showers. Because of its light colours,
nimbostratus lacks contrast and in fact is quite difficult to
photograph. Being sufficiently thick to produce precipitation, the
sun or moon can rarely be observed through nimbostratus.
The cloud may be more than
15,000 feet thick. It is generally associated with warm
Because of its lack of
contrast, it is difficult to determine the apparent speed and
direction of nimbostratus. This speed can sometimes be determined by
observing the movement of a break in the cloud or observing the
cloud's motion against the occasional glimpse of the sun or the moon
that is relatively motionless. Another method involves the
observation of approaching intermittent showers although patterns of
precipitation can sometimes change dramatically.
precipitation associated with nimbostratus is long in duration. The
intensity can vary from light to heavy depending on the associated
conditions. Normally, light to moderate rain is associated with
nimbostratus. However, the passage of strong lows and cold fronts
can produce moderate to heavy precipitation. In Sydney, weather
associated with flooding rains often contains thick nimbostratus
As discussed in
earlier cases, nimbostratus can develop or occur with most other
types of clouds. Stratus and stratocumulus will often develop below
nimbostratus in its middle level form and obscure the view of the
whole cloud base. With approaching precipitation regions, the lower
clouds may appear darker or lighter than the nimbostratus creating
some contrast. This depends on the intensity of the background
nimbostratus. The movement of the lower clouds do not necessarily
have to be the same as the nimbostratus.
clouds can develop below nimbostratus, they can also thicken to
develop into a nimbostratus layer with precipitation. This refers to
nimbostratus in its lower levels of the atmosphere. It can be
difficult to distinguish this from nimbostratus in the middle levels
of the atmosphere. It often requires observation of the initial
cloud (stratocumulus or altostratus) or the cloud that follow.
Another useful method is measuring the apparent speed of the cloud
if it can be observed. Of course, the lower the cloud, the less
likelihood that lower clouds will be observed below the
develop from altostratus if it becomes sufficiently thick to produce
precipitation. In fact, increasing altostratus cloud tends to lead
to nimbostratus. Generally, the altostratus will become darker and
gradually rain will develop. This sometimes leads to a lighter
appearance of the cloud base although the cloud still remains
nimbostratus can develop below altostratus and partially or
completely obscure it from view. However, if the altostratus layer
develops into nimbostratus itself, the lower level nimbostratus will
most probably become difficult to see especially if precipitation
begins to fall.
The weather conditions
that produce middle level (and sometimes lower level) nimbostratus
also lead to the development of higher level clouds. Nimbostratus
developing or occurring below higher level clouds will obscure most
or all of it from view. The higher clouds can only be observed
through breaks of the nimbostratus if and when they occur. These
breaks often occur when the cloud is decreasing in intensity and
conditions are beginning to clear.
Lower level clouds
consist of those clouds in the lower layers of the atmosphere.
Because of the relatively low temperatures at this level of the
atmosphere, lower level clouds usually reflect lower amounts of
light and therefore usually exhibit low contrast. The clouds at this
level also appear not as well defined. When observed closely, it is
easy to observe the turbulent motions and hence the ever-changing
Being closer to the
ground, lower level clouds appear to move or progress faster than
other clouds. The clouds generally move in the direction of the wind
very similar to the direction of the wind on the ground.
The most efficient
method used to recognise lower clouds is when observed in
conjunction with other clouds. The lower clouds will obscure part or
all the view of the upper level clouds if they pass in between the
observer's line of sight. In other words, all the observer can see
is the lower clouds as well as parts of the higher level clouds
through breaks of the lower clouds. What is observed will vary due
to the different directions and relative wind speeds associated with
the different layers of clouds.
There are 3 main types
of lower level clouds: cumulus, stratocumulus and stratus.
The bases of low
clouds range from surface height to about 6500 feet.
Stratus is defined as
low cloud that appears fragmented and thin. It can also occur in the
form of a layer or sheet. The sun, moon and generally the sky can
usually be observed through stratus clouds, especially at a steep
angle of elevation. Stratus lacks the vertical growth of cumulus and
stratocumulus, and therefore lacks the contrast. This is more
evident when observed as one layer as compared to patchy stratus.
Being closest to the ground, stratus clouds normally move fairly
rapidly in the direction of the wind depending of course on the wind
stratus develops under several conditions or weather situations.
Stratus mostly develop under the influence of wind streams where
moisture condenses in the lower layers of the atmosphere. Wind
changes during the summer months often lead to the development of
stratus as the wind evaporates moisture from the ocean and
condensing as turbulence mixes the surface air with the cooler air
above. In these conditions, stratus develop in patches and gradually
may become widespread forming into stratocumulus.
with nimbostratus and rain, stratus cloud develop simply due to the
amount of moisture in the air. With light winds, stratus are
normally observed in sheets. In stronger wind conditions, stratus
develops in patches, similar in appearance to stratocumulus. Both
the direction and appearance of stratus can change rapidly with
changing weather conditions. It can clear and redevelop several
times during certain conditions usually appearing when rain
approaches, and clearing as the rain clears. Being the lowest cloud
layer, it obscures at least partially the view of stratocumulus or
other types of clouds above.
Stratus, like stratocumulus,
can develop in weather conditions associated with thunderstorms and
thunderstorm development. In this case, stratus is observed moving
rapidly towards the storms and thickening in the region of the
updraughts, especially those of severe thunderstorms. The stratus is
only the visible condensed water vapour feeding into the
thunderstorm. One good example of a thunderstorm illustrating this
behaviour is the violent hailstorm that occurred on the 18th of
March, 1990 in Sydney (This storm is not illustrated here). Earlier
in the day, stratus had developed with a south to south-easterly
change and was moving rapidly with the air stream. As the
thunderstorms developed and approached, the stratus thickened to
form stratocumulus. As the storm (which was a supercell) with the
updraught region moved almost overhead, the stratocumulus cleared
rapidly. The major rain band then moved through with strong winds,
heavy rain and medium to large hail in some areas.
can develop in the various types of weather conditions associated
with stratocumulus discussed above. However, the characteristics of
stratus do not vary as much as stratocumulus and therefore they are
easily distinguishable. Therefore, there is no real need to discuss
further the weather conditions associated with stratus clouds.
Stratocumulus are low
clouds that generally move faster than cumulus and are not as well
defined in appearance (recall the techniques of observing clouds).
They tend to spread more horizontally rather than vertically. Like
cumulus, the bases of stratocumulus are normally darker than the
tops. However, they can vary in terms of characteristics.
Depending on the
weather conditions, stratocumulus can appear like cumulus since
stratocumulus can develop from cumulus. They may also appear as
large flat areas of low, grey cloud. Sometimes stratocumulus appear
in the form of rolling patches of cloud aligned parallel to each
other. Stratocumulus can also appear in the form of broken clouds or
globules. The sun, moon and generally the sky can be observed
through the breaks in broken stratocumulus clouds. Of course, this
depends on how large the breaks are, how high the clouds rise and
the angle of elevation of the breaks with respect to the observer.
This generally applies to all clouds but is more notable with clouds
in broken form.
develop in wind streams moving in the direction of the wind similar
to the direction of the wind at the surface. The friction created by
the earth causes turbulence in the form of eddies. With sufficient
moisture, condensation will occur in the lower layers of the
atmosphere visible as clouds. The amount of stratocumulus covering
the sky depends on the amount of moisture concentrated at that level
of the atmosphere. The speed that the cloud moves varies according
to the wind speed at that level.
also can develop in the form of lenticularis. The only method that
can be used to distinguish between these clouds is that
stratocumulus will not appear as well defined, will tend to move
more quickly. Sometimes they develop below cumulus or cumulonimbus
which means that it must be low cloud.
A low layer of
uniform, dark grey cloud. When it gives precipitation, it is in the
form of continuous rain or snow. The cloud may be more than 15,000
feet thick. It is generally associated with warm
occurs in stratus. The low cloud bases and poor visibility
make VFR operations difficult to impossible.
clouds of vertical
The bases of this type
of cloud may form as low as 1500 feet. They are composed of water
droplets when the temperature is above freezing and of ice crystals
and supercooled water droplets when the temperature is below
cauliflower-shaped low level clouds with dark bases and bright tops.
When observing cumulus, you are actually observing the condensation
process of rising thermals or air bubbles at a certain level in the
atmosphere known as the condensation level.
The air rising above
this level condenses and cloud is observed. Since the height of this
level is fairly constant at any particular time, then the bases of
cumulus are usually flat.
The appearance of
cumulus like other clouds can be illusive. If stratus formed at the
same level as cumulus, the cumulus will appear different observed
from different perspectives with respect to the sun's position. (If
light from the sun must reflect to get to the observer, then the
cloud will tend to appear brighter and display more contrast than
cloud reflecting very little direct sunlight. In fact, the latter
case indicates that the shadow area of the cloud is facing the
observer). A similar situation may occur when observing cumulus
below a much darker background such as a thunderstorm. The cumulus
clouds appear as a uniform white or at least much lighter with
little or no contrast. The same cumulus clouds observed away from
this cloud band will appear darker, with more contrast.
With practice, an
observer can easily determine the size of cumulus clouds (or any
clouds in general) by considering the following factors; their
apparent distances, coverage of the sky (density), their angle of
elevation (how much of their base can be observed), how much
overlapping occurs, and their base to height ratio. Cumulus often
occurs in conjunction with other clouds and may vary in appearance.
If cumulus is observed below other clouds, the shadow effect of
other clouds can decrease contrast of the cumulus.
Cumulus clouds that
build up into high towering masses. They are likely to develop
into cumulonimbus. Rough air will be encountered underneath
this cloud. Heavy icing may occur in this cloud type.
Heavy masses of
cumulus clouds that extend well above the freezing level. The
summits often spread out to form an anvil shaped top that is
characteristic of thunderstorm. (below)
motion of moist air is a prerequisite for cloud formation, downward
motion dissipates it. Ascending air expands, cools adiabatically
and, if sufficiently moist, some of the water vapour condenses to
form cloud droplets. Fog is likely when moist air is cooled,
not by expansion but by contact with a colder surface.
The diameter of the
condensation nuclei is typically 0.02 microns but a relatively small
number may have a diameter up to 10 microns. Maritime air contains
about one billion nuclei per cubic metre, polluted city air contains
many more. The diameter of a cloud droplet is typically 10 to 25
microns and the spacing between them is about 50 times diameter,
perhaps one mm, with maybe 100 million droplets per cubic metre of
cloud. The mass of liquid in an average density cloud approximates
0.5 gram per cubic metre.
Above the freezing
level in the cloud some of the droplets will freeze if disturbed by
contact with suitable freezing nuclei, or an aircraft. Freezing
nuclei are mainly natural clay mineral particles, bacteria and
volcanic dust, perhaps 0.1 microns in diameter. There are rarely
more than one million freezing particles per cubic metre thus there
are only sufficient to act as a freezing catalyst for a small
fraction of the cloud droplets. Most freezing occurs at temperatures
between –10 °C and –15 °C.
The balance of the
droplets above freezing level remains in a supercooled liquid
state, possibly down to temperatures colder than –20 °C, but
eventually, at some temperature warmer than –40 °C, all droplets
will freeze by self-nucleation into ice crystals, forming the
high level cirrus clouds. In some cases fractured or splintered ice
crystals will act as freezing nuclei. The ice crystals are usually
shaped as columnar hexagons or flat plate hexagons, refer 3.5.2
below and 12.2.2.
atmospheric moisture occurs when:
the volume of air remains constant but
temperature is reduced to dewpoint, e.g. contact cooling, mixing
of different layers
the volume of an air parcel is increased
through adiabatic expansion
evaporation increases the vapour partial
pressure beyond the saturation point
a change of both temperature and volume
reduces the saturation vapour partial
Rain [RA] and drizzle
Cloud droplets tend to
fall but their terminal velocity is so low, about 0.01 metres/sec,
that they are kept aloft by the vertical currents associated with
the cloud construction process, but will evaporate when coming into
contact with the drier air outside the cloud. Some of the droplets
are larger than others and consequently their falling speed is
greater. Larger droplets catch up with smaller and merge or coalesce
with them eventually forming raindrops. Raindrops grow with the
coalescence process reaching maximum diameters, in tropical
conditions, of 4 – 7 mm before air resistance disintegrates them
into smaller raindrops, which start a self perpetuating process. It
takes about one million cloud droplets to form one
The terminal velocity
of a 4 mm raindrop is about 9 metres/sec. Only clouds with extensive
depth, 3000 feet plus, will produce rain (rather than drizzle) but
very small high clouds, generating heads, may produce trails of snow
crystals which evaporate at lower levels – fall streaks or
Drizzle forms by coalescence in stratiform clouds
with depths possibly less than 1000 feet and with only weak vertical
motion, otherwise the small ( 0.2 – 0.5 mm) drops would be unable to
fall. It also requires only a short distance or a high relative
humidity between the cloud base and the surface, otherwise the drops
will evaporate before reaching the surface. Terminal velocity
approximates 1 – 2 metres/sec.
[–DZ]: visibility greater than 1000 metres
drizzle [DZ]: 500 to 1000 metres
[+DZ]: less than 500 metres
Light rain showers:
precipitation rate under 2.0 mm/hour
showers: 2.0 to 10 mm/hour
Heavy rain showers: more
than 10 mm/hour
Light rain [–RA]: under 0.5
mm/hour, individual drops easily seen
[RA]: 0.5 to 4 mm/hour, drops not easily seen
rain [+RA]: more than 4 mm/hour, rain falls in
Weather radar reports precipitation
into six reflectivity levels:
light to moderate rain
moderate to heavy rain
very heavy rain, hail possible
very heavy rain and hail, large
Scotch mist is a mixture of thick cloud and heavy
drizzle on rising ground, formed in conditions of weak uplift of
almost saturated stable air.
At cloud temperatures colder than
–10 °C where both ice and supercooled liquid water exist, the
saturation vapour pressure over water is greater than that over ice.
Air that is just saturated with respect to the supercooled water
droplets will be supersaturated with respect to the ice crystals,
resulting in vapour being deposited onto the crystal. The reduction
in the amount of water vapour means that the air is no longer
saturated with respect to the water droplets and, to achieve
saturation equilibrium again, the water droplets begin to evaporate.
Thus ice crystals grow by sublimation and water droplets lessen,
i.e. in mixed cloud the ice crystals grow more rapidly than the
water droplets. Snow is frozen precipitation resulting from ice
crystal growth and falls in any form between small crystals and
large flakes. This is known as the Bergeron-Findeison theory and
probably accounts for most precipitation outside the tropics. Snow
may fall to the surface or, more often, melt below the freezing
level and fall as rain.
Snowflakes are built by snow crystals colliding and
sticking together in clusters of several hundred – aggregation. Most
aggregation occurs at temperatures just below freezing, the snow
crystals tending to remain separate at colder
Hail and other ice
The growing snow
crystals acquire a fall velocity relative to the supercooled
droplets and growth also continues by collision and coalescence with
supercooled droplets forming ice pellets [PE], the process
being termed accretion,or opaque riming if the
freezing is instantaneous incorporating trapped air, glazing
if the supercooled water freezes more slowly as a clear layer. The
ice pellets in turn grow by coalescence with other pellets and
further accretion and are termed hail [GR] when the diameter
exceeds 5 mm. The size reached by hailstones before falling out of
the cloud depends on the velocity and frequency of updraughts within
the cloud. Hail is of course an hazard to aviation, particularly
when it is unexpected, for example hail falling from a Cb anvil can
appear to fall from a clear sky. Snow grains [SG] are very
small, flattened, opaque ice grains, less than 1 mm and equivalent
to drizzle. Snowflakes that, due to accretion, become opaque,
rounded and brittle pellets, 2 – 5 mm diameter, are called snow
pellets or graupel [GS]. Sleet is transparent ice
pellets less than 5 mm diameter that bounce on impact with the
ground. Sleet starts as snow, partially melting into rain on descent
through a warm layer, then refreezing in a cold near-surface layer.
The term is sometimes applied to a snow/rain mixture or just wet
snow. Diamond dust [IC] is minute airborne ice crystals that
only occur under very cold (Antarctic) conditions.
When raindrops form in cloud top temperatures warmer than
–10 °C the rain falls as supercooled drops. Such freezing
rain or drizzle striking a frozen surface, or an aircraft flying
in OAT at or below zero, will rapidly freeze into glaze ice.
Freezing rain is responsible for the ice storms of North America and
northern Europe, but the formative conditions differ from the
The seeder – feeder
Any large scale air flow over
mountain areas produces, by orographic effect, ice crystals in cold
cloud tops. By themselves the falling crystals would cause only
light drizzle at the ground. However as the crystals fall through
the low level mountain top clouds they act as seed particles for
raindrops formed by cloud droplet coalescence with the falling
crystals, producing substantial orographic rainfall in mountain
Aerial cloud seeding involves
introducing freezing nuclei (silver-oxide crystals with a similar
structure to ice crystals) into parts of the cloud where few
naturally exist, in order to initiate the Bergeron-Findeison
Fog is defined as an
obscurity in the surface layers of the atmosphere which is caused by
a suspension of water droplets, with or without smoke particles, and
which is defined by international agreement as being associated with
visibility less than 1000 metres. If the visibility exceeds 1000
metres then the obscurity is mist – met. code
fogs are the prevalent
fogs in Australia, with occurrence peaking in winter; caused by
lowering of ground temperature by re-radiation into space of
absorbed solar radiation from the earth’s surface. Radiation fogs
mainly occur in moist air on cloudless nights within a high
pressure system, particularly after rainfall. The moist air
closest to the colder surface will quickly cool to dewpoint with
condensation occurring. As air is a poor conductor a light wind, 2
– 6 knots, will best facilitate the mixing of the cold air
throughout the surface layer, creating fog. The fog itself becomes
the radiating surface in turn, encouraging further cooling and
deepening of the fog. An increase in atmospheric pollution
products supplies extra condensation nuclei to enhance the
formation of fog or smog.
A low level
inversion forms containing the fog which may vary from scattered
pools in surface depressions to a general layer 1000 feet in
depth. Calm conditions will result in a very shallow fog layer or
just dew or frost. Surface winds greater than 10 knots may prevent
formation of the inversion, the cooled air is mixed with the
warmer air above and not cooling to dewpoint. If the forecast wind
at 3000 feet is 25 knots plus the low level inversion may not form
and fog is unlikely. In winter radiation fog may start to form in
the evening and persist to mid-day, or later if the sun is cut off
by higher level cloud and/or the wind does not pick up
sufficiently to break up the low level inversion.
fog may occur when warm,
moist air is carried over a surface which is cooler than the
dewpoint of the air. Cooling and some turbulence in the lower
layer lowers temperature to dewpoint and fog forms. Sea
fogs drifting into coastal areas are advection fogs forming
when the sea surface temperature is lower than the dewpoint but
with a steady breeze to promote air mixing. Dewpoint can be
reached both by temperature reduction and by increased water
vapour content through evaporation. Advection fogs will form in
valleys open to the sea when temperature falls in the evening
combined with a sea breeze of 5 to 15 knots to force the air
upslope. Thick advection fogs may be persistent in winter,
particularly under a mid-level cloud layer.
evaporation fogs or steaming fogs result from the
immediate condensation of water vapour that has just evaporated
from the surface into near saturated air. Steaming from a sun
warmed road surface after a rain shower demonstrates the process.
Sea smoke or frost smoke is an evaporation fog
occurring in frigid Antarctic air moving over relatively warm
waters and prompting evaporation into the cold air which, in turn,
quickly produces saturation.
fog is a fog composed of
supercooled water droplets which freeze on contact with solid
objects, e.g. parked aircraft. When near saturated air is very
cold, below –24 °C at sea level to –45 °C at 50 000 feet, the
addition of only a little moisture will produce saturation.
Normally little evaporation takes place in very cold conditions
but release of water vapour from engine exhausts, for instance,
can quickly saturate calm air, even though the engine exhaust heat
tends to lower the relative humidity, and will produce ice
fog at the surface or contrails at altitude. If the
temperature is below –40 °C ice crystals form directly on
saturation. Contrails persist if relative humidity is high but
evaporate quickly if low. Distrails occur when the engine
exhaust heat of an aircraft flying through a thin cloud layer
dissipates a trail.
fog or rain-induced
fog occurs when warm rain evaporates at surface level in light
wind conditions and then condenses forming
An airstream reaching
a mountain barrier is forced to rise, both at the surface and the
upper levels, and cools adiabatically. If the lift and the moisture
content are adequate condensation occurs at the lifting condensation
level and cloud is formed on or above the barrier. Stratus is formed
if the air is stable, cumulus if the air is slightly unstable. If
there is instability in depth, coupled with high moisture, CB may
develop. Refer 3.6 below. Solar heating of mountain ridges causes
the adjacent air to be warmer than air at the same level over the
valleys, thus the ridge acts as a high level heat source, increasing
buoyancy and accentuating the mechanical lifting.
Orographic cloud – cap
cloud – in stable conditions tends to form continuously on the
windward side, clearing on the lee side. Lenticular cloud may also
form a high cap above a hill when there is a layer of near saturated
air aloft, orographic lifting causing condensation, descent causing
evaporation. A mountain wave may form, particularly in a sandwiched
stable layer resulting in the formation of a series of lenticular
Warm air rises. Owing to the heating of the
ground by the sun, rising currents of air occur. The upward movement
of air is known as convection. (The downward movement of air is
known as subsidence. ) As currents of air rise due to convection,
they expand. The expansion is accompanied by cooling. The cooling
produces condensation' and a cumuliform cloud forms at the top of
each rising column of air. The cloud will grow in height as long as
the rising air within it remains warmer than the air surrounding it.
The height of the cloud, however, is also dependent on the stability
of the air in the mid levels of the troposphere. Convection also
occurs when air moves over a surface that is warmer than itself. The
air is heated by advection and convective currents develop. Warming
of air by advection does not depend on daytime heating. Convection
will, therefore, continue day or night so long as the airflow
remains the same.
When a mass of warm air is advancing on a colder
mass, the warm air rises over the cold air on a long gradual slope.
This slope is called a warm frontal surface. The ascent of the warm
air causes it to cool, and clouds are formed, ranging from high
cirrus through altostratus down to thick nimbostratus from which
continuous steady rain may fall over a wide area.
When a mass of cold air is advancing on a mass
of warm air. The cold air undercuts the warm air and forces the
latter to rise. The slope of the advancing wedge of cold air is
called a cold frontal surface. The clouds which form are heavy
cumulus or cumulonimbus. Heavy rain, thunderstorms, turbulence and
icing are associated with the latter.
atmospheric vertical motion is found in cyclones and anticyclones,
mainly caused by air mass convergence or divergence from horizontal
motion. Meteorological convergence indicates retardation in air flow
with increase in air mass in a given volume due to net three
dimensional inflow. Meteorological divergence, or negative
convergence, indicates acceleration with decrease in air mass.
Convergence is the contraction and divergence is the spreading of a
field of flow.
If, for example, the front end of moving air
mass layer slows down, the air in the rear will catch up – converge,
and the air must move vertically to avoid local compression. If the
lower boundary of the moving air mass is at surface level all the
vertical movement must be upward. If the moving air mass is just
below the tropopause all the vertical movement will be downward
because the tropopause inhibits vertical motion. Conversely if the
front end of a moving air mass layer speeds up then the flow
diverges. If the air mass is at the surface then downward motion
will occur above it to satisfy mass conservation principles, if the
divergence is aloft then upward motion takes place.
air must diverge before it reaches the tropopause and sinking air
must diverge before it reaches the surface. As the surface pressure
is the weight per unit area of the overlaying column of air, and
even though divergences in one part of the column are largely
balanced by convergences in another, the slight change in mass
content (thickness) of the over-riding air changes the pressure at
The following diagrams illustrate some examples
of convergence and divergence:
Note: referring to the
field of flow diagrams above, the spreading apart (diffluence) and
the closing together (confluence) of streamlines alone do not imply
existence of divergence or convergence as there is no change in air
mass if there is no cross isobar flow or vertical flow. (An
isobar is a curve along which pressure is constant and is usually
drawn on a constant height surface such as mean sea
Divergence or convergence may be induced by a
change in surface drag, for instance when an airstream crosses a
coastline. An airstream being forced up by a front will also induce
convergence. For convergence / divergence in upper level waves. Some
divergence / convergence effects may cancel each other out e.g.
deceleration associated with diverging streamlines.
Developing anti-cyclones – “highs” and high pressure ridges,
are associated with converging air aloft and consequent wide area
subsidence with diverging air below . This subsidence usually occurs
between 20 000 and 5000 feet typically at the rate of 100 – 200 feet
per hour. The subsiding air is compressed and warmed adiabatically
at the DALR, or an SALR, and there is a net gain of mass within the
developing high. Some of the converging air aloft rises and, if
sufficiently moist, forms the cirrus cloud often associated with
As the pressure lapse
rate is exponential and the DALR is linear the upper section of a
block of subsiding air usually sinks for a greater distance and
hence warms more than the lower section and if the bottom section
also contains layer cloud the sinking air will only warm at a SALR
until the cloud evaporates. Also when the lower section is nearing
the surface it must diverge rather than descend and thus adiabatic
warming stops. With these circumstances it is very common for a
subsidence inversion to consolidate at an altitude between 3000 and
6000 feet. The weather associated with large scale subsidence is
almost always dry, but in winter persistent low cloud and fog can
readily form in the stagnant air due to low thermal activity below
the inversion, producing ‘anti-cyclonic gloom’. In summer there may
be a haze layer at the inversion level which reduces horizontal
visibility at that level although the atmosphere above will be
bright and clear. Aircraft climbing through the inversion layer will
usually experience a wind velocity change.
Developing cyclones, “lows” or
"depressions", and low pressure troughs are associated with
diverging air aloft and uplift of air leading to convergence below.
There is a net loss of mass within an intensifying low as the rate
of vertical outflow is greater than the horizontal inflow, but if
the winds continue to blow into a low for a number of days,
exceeding the vertical outflow, the low will fill and disappear. The
same does not happen with anti-cyclones which are much more
A trough may move with pressure
falling ahead of it and rising behind it giving a system of pressure
tendencies due to the motion but with no overall change in pressure,
i.e. no development, no deepening and no increase in
Like CU, surface
heating, may provide the initial trigger to create isolated CB
within an air mass but the initial lift is more likely to be
provided by orographic ascent or convergence effects.
In the formative
stages of a CB the cloud may have an updraught pulse of 1000 – 2000
feet/min, the rising parcel of air reaches altitudes where it is
much warmer than the surrounding air, by as much as 10 °C, and
buoyancy forces accelerate the parcel aloft possibly reaching speeds
of 3000 – 7000 feet/min. Precipitation particles grow with the cloud
growth, the upper levels of the cloud gaining additional energy from
the latent heat released from the freezing of droplets and the
growth of snow crystals and hailstones. When the growth of the
particles is such that they can no longer be suspended in the
updraught, precipitation, and its associated drag downdraught,
If the updraught path
is tilted, by wind shear or veer, rather than vertical, then the
precipitation and its downdraught will fall away from the updraught
rather than back down through it (consequently weakening, or
stopping, the updraught) and a co-existing updraught/downdraught may
become established. An organised cell system controlling its
environment and lasting several hours may evolve.
Middle level dry air
from outside the cloud is entrained into the downdraught of an
organised cell. The downdraught is further cooled by the dry inflow
air evaporating some of its water and ice crystals and tends to
accelerate downwards in vertical gusts and, at the same time,
maintaining the higher horizontal momentum it gained at upper levels
from the higher forward speed of the storm at that height. When the
cold plunging air nears the surface the downburst spreads out
in all directions as a cold gust front or squall, strongest at the
leading edge of the storm, weakest towards the trailing
take several forms:
Cumuliform: forms when a very strong
updraught spreads rapidly and without restriction.
Incus: a severe storm
attains maximum vertical development when the updraught reaches a
stable layer which it is unable to break through, often the
tropopause, and the cloud top spreads horizontally in all
directions forming an overhanging anvil.
Back-sheared: the cloud top spreads
upwind, against the high level flow and indicating a very strong
Mushroom: a rollover or lip around
the underside of an overhanging anvil indicating rapid expansion.
Overshooting top: a dome-like
protusion through the top of an anvil indicating a very strong
updraught pulse. The overshooting top in large tropical storms has
been known to develop into a 'chimney' towering maybe 10 000 feet
into the stratosphere with an extensive plume cloud extending
downwind from its top. Such clouds transfer moisture to the
Each organised cell
system contains an updraught / downdraught core beneath which
is the outflow region containing the rain shield and
bounded by the downdraught gust front, a flanking line
with a dark flat base underneath which is the inflow region
of warm moist air and a spreading anvil. The CU and TCU
generated by the inflow within the flanking line are the genesis of
new cells. Within the core the condensation of moisture from the
inflow region produces rain, hail and snow and the associated
downdraught to the outflow region. When the cool air outflow exceeds
and finally smothers ,or undercuts and chokes off, the inflow the
High moisture content
in the low level air with dry mid level air plus atmospheric
instability are required to maintain CB development. The amount of
precipitation from a large storm is typically 200 000 tonnes
but severe storms have produced 2 million