Download Cloud Types

Science 10
Weather Dynamics
Investigation 5 – Clouds and Precipitation
Outcomes
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Cloud Formation
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From your own observations, you know that there are many types of clouds. If
all clouds are formed from water, why are there so many different shapes?
Wind, temperature, and humidity are all factors that affect cloud formation.
They also affect the size, shape, and appearance.
How are these conditions (water vapour in the air, temperature drop, and
condensation nuclei) created in nature?
o Water vapour is added to the air when the solar energy evaporates water.
o The most common way that air cools is by moving to a higher altitude.
This is accomplished in three ways:
 As the air is warmed, it expands and becomes less dense and
therefore rises.
 Warm air is pushed up when it encounters a cooler air mass.
 A moving air mass meets a mountain range. It rises over the
mountains, expands, and cools.
o Air may also be cooled by contact with very cool ground. This results in
a cloud forming on the ground (fog).
o Condensation nuclei take the form of dust, smoke, pollen, and sea salt.
The average diameter of one of these particles is about 0.01 mm.
Cloud Shapes
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Clouds are classified according to three general shapes:
o Cumulus – puffy clouds that often have flat bases.
o Stratus – flat clouds that form in seemingly endless layers. Fog is really
a stratus cloud that is sitting on the ground.
o Cirrus – high level clouds made of ice crystals. At these altitudes, the
water vapour condenses directly to a solid by a process called
deposition.
Either stratus or cumulus clouds may cause precipitation. The name of a rain
or snow cloud is made by combining the shape name with a form of the term
nimbus. For example, nimbostratus or cumulonimbus.
Precipitation (rain, snow, sleet, freezing rain, hail)
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Why do some clouds bring rain or snow while others do not?
o The water droplets of most clouds are not heavy enough to fall to Earth.
The slightest whiff of air can send the droplet flying.
o Water droplets of 0.5 mm will fall to the earth but will only create a fine
mist. The average raindrop that makes it to the ground has a diameter
of 2 mm.
So how do water droplets grow to 2 mm?
o Cloud droplets are almost constantly being tossed up, down, and around
by air currents. As a result, they often coalesce, that is, they collide and
blend together.
o In temperate climates such as Canada’s, water vapour in the cold upper
air condenses onto ice crystals. Like droplets, the crystals collide and
stick together. They become snowflakes. As the snowflakes fall through
warmer air, they melt and become raindrops.
So what causes snow?
o If the air beneath the clouds is below the freezing point all the way to the
ground, the crystals in the clouds stay in the form of snow.
o If the air is extremely cold, the crystals tend to remain small, but if the
air is warmer but still below freezing, the snowflakes stick together as
they fall. They form larger flakes.
So what causes sleet?
o Sometimes a layer of air below a cloud is warm enough to melt the snow
and turn it into rain. Then, closer to the ground, the air is below the
freezing point. If the air is very near the ground is cold enough, the
raindrops freeze again. They do not freeze as crystals, however. They
freeze into ice pellets that we usually call sleet.
So what causes freezing rain?
o When the rain hits the cold objects, it freezes instantly. This creates a
solid coating of ice and we call this type of precipitation freezing rain.
So what causes hail?
o Hail starts out as frozen raindrops. They do not fall directly to the Earth,
however. In large cumulonimbus clouds, winds swirl up and down. The
wind carries the small hailstones back up into the clouds. There they
collide with droplets in air that is near or below the freezing point. The
droplets freeze immediately and coat the hailstones. The size of the
hailstone depends on the amount of time it spends being buffeted by the
wind inside the cloud.
Questions
Answer the following questions on a separate sheet of paper in complete sentences.
Attach your answer sheet to this paper.
1. Name two ways that cirrus clouds differ from stratus clouds.
2. Describe one way that cloud droplets can become raindrops.
3. How does sleet differ from hail?
4. Would you expect hail to fall from nimbostratus clouds? Explain the reasoning
behind your answer.