Clouds, Air, Condensation
Learning to look skyward provides a constant
reference point for endlessly shifting weather patterns. Rarely does a
particular weather pattern linger for more than a few hours without some change.
Clouds give way to sunshine, and alternatively, a blue sky can rapidly disappear
beneath raging storm clouds. By learning to recognize changes in clouds, wind
and temperature we can also learn how to anticipate the corresponding changes to
Clouds are merely a collection of water
vapour that acts as natures atmospheric cleanser. As particulate matter
accumulates in the atmosphere, the air would quickly become toxic were it not
for the process of cloud formation and rainfall. Large particles usually will
not remain airborne for long periods, but particles 1 micron (1/1000 mm) and
smaller may. The 1 micron particles, like dust, act as a condensation nucleus,
and water vapour will condense upon them to form a droplet. Every rain drop has
a particle of dust at its centre. Smaller particles will become attached to
these droplets, and as more and more water is added to the drops, they
eventually fall as rain. When the rain falls, these particles are returned to
the surface of the earth, and the air is left significantly cleaner.
Historically, clouds were viewed with a
mixture of wonder and ignorance. This all changed in 1802 when Luke Howard, an
amateur English Naturalist, created the first classification system for clouds.
He grouped them into two main categories, heap
clouds and layer clouds. Current
systems remain largely true to his original system, but may refer to them as
Type I and Type II
Field Guide To Clouds
These distinctive clouds are cauliflower-like
in shape, and have clearly visible edges. Generally, the clouds are as high as
they are wide, and may rise as high as 20 km. The clouds begin as fluffy white
pillows, but may darken to become a threatening storm-cloud (see type IV). A
close look at heap clouds show them to be a circulating mass of rising warm air.
This is referred to as a convective cloud, and they rotate in a
These soft fair-weather clouds often begin
forming with the heat of the morning. They are small, rarely rising more than 1
km in height. Their base is flat, indicating the condensation level.
The clouds may begin to swell vertically,
indicating an increasingly energetic and unstable condition. Within the cloud,
as water vapour condenses to liquid water droplets, much heat is released, and
this heat rises within the cloud. Along the outer margins there may be area’s
where colder air is moving downward. The bases are still flat.
As increasing amounts of heat are released,
the cloud becomes more buoyant, and its vertical growth continues. This is
similar to the way in which a balloon rises when hot air is inflated. At the
very highest elevations, the water droplets may be converted to ice. This
indicates the point at which a cumulus becomes a cumulonimbus. The base of the
flat, vertical cloud may begin to darken threateningly.
This group of clouds can be easily
distinguished by its layer-like nature. Usually the clouds show little vertical
thickness, but tend to be spread out into a thin sheet, usually less than 1 km
from top to bottom. On the other hand, the horizontal coverage of the cloud may
be in excess of 1 million sq. km. In addition, there are no convection cells as
in the Heap Clouds.
As air flows over the top of a gently
sloping, but heavy air mass, a slow condensation may occur. This results in the
thin sheets of clouds known as stratus. Occasionally, these may be so low to the
ground that the clouds are known by another name – fog.
If a warm front lies closer to the ground, as
an opposing pattern moves slowly to climb above it, stratus clouds may form at
elevations of between 3 and 6 km. These are generally referred to as
altostratus. The clouds may block the sun.
High elevation cirrostratus clouds may form
at elevations in excess of 6 km. They may form a uniform coverage of the sky, or
may appear as wispy, feathery sheets.
Heap and Layer Clouds
Often, there may be layers of stable air in
the atmosphere. These generally cause the vertical heap clouds to spread out
With low level clouds, the clouds may appear
almost like a stratus cloud with increased thickness caused by some slight
convection. These clouds may cover most of the sky, leaving slight patches of
blue peeking through.
At higher elevations, the smooth altostratus
begins to show evidence of thickening with increased convection. Rather than the
sheet like appearance normally associated with altostratus, these begin to look
like sheets of puffier clouds with some slight rising.
Occurring at very high elevations, above the
freezing point, thin clouds formed of supercooled water mixed with ice,
cirrocumulus clouds form. They lack the clearly defined convective cells of
altocumulus, and may have wispy edges.
The term nimbus is generally used to refer to
clouds that produce moisture. They come in a variety of forms, from towering,
anvil shaped cumulonimbus to flat, gray, nimbostratus.
These flat gray clouds often obscure the sky,
forming flat layers of clouds at medium elevations. The edge of the cloud may be
difficult to discern, and they usually produce a steady stream of rain or snow.
These are the familiar storm clouds that
regularly appear at the tail end of a hot summer day. As moisture and heat rises
during the day, and the rapidly rising convective cell reaches the condensation
point, large cumulonimbus clouds may form. They also hold the potential to
produce violent storms with high amounts of thunder and lightning.
They are easily recognized. They are a large
cumulus cloud, often with a flat top and base, making them resemble a large
anvil. The base is generally quite dark and can appear blue-black in very strong
Rarely, cirrus clouds, which are very high in
elevation, may release ice crystals that form in the supercooled air. These ice
crystals may fall to the earth as precipitation, but most commonly evaporate
before reaching the ground. They do technically produce moisture though and so
must be considered a rain cloud.
These wonderfully unique clouds form as a
result of high winds aloft and are commonly associated with the strong winds of
the Chinook. They are a cumulus type cloud that has been sculpted by the winds
to create a smooth cloud that almost looks like a flying saucer. They are very
common along the eastern slopes where high winds crest the continental divide
and then sweep outwards towards the plains.
The Chinook arch is a sight recognized by all who live in the land of the Chinook. As the winds crest the continental divide, and rapidly drop (see Chinooks) down the eastern slopes, they often push a band of clouds in front of them. From the plains, there is a band of clouds visible on the western horizon. The curvature of the Earth makes this band of clouds appear like an arch and this wonderful arch of clouds often foretells the arrival of a Chinook. When the temperatures plummet in January, the Chinook arch is a welcome sight on the prairies.
All Material © Ward Cameron 2005