Clouds and Precipitation
A cloud is a visible aggregate of tiny water droplets and/or ice crystals suspended in the
atmosphere and can exist in a variety of shapes and sizes. Some clouds are accompanied
by precipitation; rain, snow, hail, sleet, even freezing rain.
Water is known to exist in three different states; as a solid, liquid or gas.
Clouds, snow, and rain are all made of up of some form of water. A cloud is comprised of
tiny water droplets and/or ice crystals, a snowflake is an aggregate of many ice crystals,
and rain is just liquid water.
Water existing as a gas is called water vapour. When referring to the amount of moisture
in the air, we are actually referring to the amount of water vapour. If the air is described
as "moist", that means the air contains large amounts of water vapour. Common sources
of moisture are the Maritime Air masses.
Luke Howard invented a cloud naming system in 1802. Clouds are classified into a
system that uses Latin words to describe the appearance of clouds as seen by an observer
on the ground. The table below summarizes the four principal components of this
classification system :
Latin Root
1.cumulus
Translation Example
heap
fair weather cumulus
2. stratus
layer
altostratus
3. cirrus
curl of hair
cirrus
4. nimbus
rain
cumulonimbus
Further classification identifies clouds by height of cloud base. For example, cloud names
containing the prefix "cirr-", as in cirrus clouds, are located at high levels above (6000
m). Cloud names with the prefix "alto-", as in altostratus, are found at middle levels
(2000 – 6000m). Low clouds (below 2000 m) are of mostly composed of water droplets.
When temperatures are cold enough, these clouds may contain ice and snow.
High-Level Clouds
Cirrus - high-level clouds form above (6,000
meters) and since the temperatures are so cold at such high elevations, these clouds are
primarily composed of ice crystals. High-level clouds are typically thin and white in
appearance. They move across the sky on bright sunny days. Cirrus clouds mean warm
air is on the way.
Cirrus Clouds - thin and wispy
The most common form of high-level clouds are thin and often wispy cirrus clouds.
Cirrus clouds are composed of ice crystals that originate from the freezing of super
cooled water droplets. Cirrus generally occur in fair weather and point in the direction of
air movement at their elevation.
Mid-Level Clouds
The bases of mid-level clouds typically appear
between 2,000 to 6,000 metres. Alto- clouds, because of their lower altitudes, are
composed primarily of water droplets, they can also be composed of ice crystals when
cold enough.
Low-level Clouds
Low clouds are mostly composed of water
droplets since their bases generally lie below 2,000 meters. When cold
enough, these clouds may also contain ice particles and snow. Nearly all low
clouds are some form of stratus cloud.
Fair Weather Cumulus Clouds
Fair weather
cumulus have the appearance of floating cotton with flat bases have a lifetime of 5-40
minutes. The cloud tops designates the limit of the rising air. Given suitable conditions,
however, harmless fair weather cumulus can develop into cumulonimbus clouds
associated with thunderstorms.
Rain or Snow?
dependent upon temperature •Precipitation typically forms high in
the atmosphere where the temperature is below freezing. As ice crystals form aloft and
fall toward the surface, they collect each other to form large snowflakes. If ground
temperature is above 0C, the freezing level must be located somewhere above the ground.
As the falling snow passes through the freezing level into the warmer air, the flakes melt
puffy cotton balls floating in the sky
and collapse into raindrops. During the summer months, it is not uncommon for the
freezing level to be found at a level above cloud base.
When the air temperature at the ground is less than 0C, the snowflakes do not melt on the
way down and therefore reach the ground as snow.
Rain develops when growing cloud droplets become too heavy to remain in the cloud and
as a result, fall toward the surface as rain. Rain can also begin as ice crystals that collect
each other to form large snowflakes. As the falling snow passes through the freezing
level into warmer air, the flakes melt and collapse into rain drops.
Hail is a large frozen raindrop produced by intense thunderstorms. As the snowflakes fall,
liquid water freezes onto them forming ice pellets that will continue to grow as more and
more droplets are accumulated. Upon reaching the bottom of the cloud, some of the ice
pellets are carried by the updraft back up to the top of the storm where they will repeat
the growth steps. When the stones are too heavy for the updraft, the stones fall.
Freezing Rain
Ice storms can be the most devastating of winter weather
phenomena and are often the cause of automobile accidents, power outages and personal
injury. Ice storms result from the accumulation of freezing rain, which is rain that
becomes supercooled and freezes upon impact with cold surfaces.
Freezing rain develops as falling snow encounters a layer of warm air deep enough for
the snow to completely melt and become rain. As the rain continues to fall, it passes
through a thin layer of cold air just above the surface and cools to a temperature below
freezing. The drops do not freeze, a phenomena called supercooling occurs ("supercooled
drops“ form). When the supercooled drops strike the frozen ground (power lines, or tree
branches), they instantly freeze, forming a thin film of ice, hence freezing rain.
SnowSnowflakes are simply aggregates of ice crystals that collect to each other as they
fall toward the surface. The diagram shows a typical temperature profile for snow with
the red line indicating the atmosphere's temperature at any given altitude. The vertical
line in the centre of the diagram is the freezing line. Temperatures to the left of this line
are below freezing, temperatures to the right are above freezing. Since the snowflakes do
not pass through a layer of air warm enough to cause them to melt, they remain in tact
and reach the ground as snow.