Clouds and cloud_formation

Unit 4 Lesson 2 Clouds and Cloud Formation
Copyright © Houghton Mifflin Harcourt Publishing Company

Head in the Clouds
What are clouds?
• A cloud is a collection of small water droplets or
ice crystals that are suspended in the air.
• Clouds are visible because water droplets and ice
crystals reflect light.
• Clouds are usually associated with precipitation,
but most cloud types do not produce precipitation.

How do clouds affect climate?
• The pattern of precipitation from clouds in an area
will determine the climate of that area.
• Thick, low-altitude clouds reflect more sunlight
back into space and help to cool Earth.
• Thin, high-altitude clouds absorb some energy
that radiates from Earth and reradiate some of it
back to Earth’s surface. This warms Earth.

How do clouds form?
• Clouds form when water vapor condenses, or
changes from a gas to a liquid.
• For water vapor to condense, moist air must be
cooled to a certain temperature, and the air must
be holding the maximum amount of water vapor
possible.
• If air rises high enough into the atmosphere, it
cools to its dew point.

How do clouds form?
• Dew point is defined as the temperature at which
the rate of condensation equals the rate of
saturation.
• Saturation means that the air is holding the
maximum quantity of water vapor possible at a
given temperature or pressure.
• Dew point is the temperature at which water
vapor in saturated air can condense and form
water droplets or ice crystals.

How do clouds form?
• In clouds, tiny solid particles called cloud
condensation nuclei are the surfaces on which
water droplets condense.
• Clouds are most commonly made of numerous
small water droplets.
• However, at high altitudes, where temperatures
are very cold, clouds are composed of ice crystals.

What is the role of solar energy in
cloud formation?
• Solar energy drives the water cycle and thus
provides the energy for cloud formation.
• Water on the surface of the land and the oceans
absorbs solar energy and becomes water vapor, in
a process called evaporation.
• Cloud formation takes place when water droplets
or ice crystals condense on solid particles in the
air.

What processes cool air enough to form
clouds?
• Air can be cooled to its dew point in several ways,
including frontal and orographic lifting.
• Frontal lifting can occur when a warm air mass
rises over a cold air mass, or when a mass of cold
air slides under a mass of warm air.
• Orographic lifting occurs when an obstacle, such
as a mountain range, forces a mass of air upward.

What processes cool air enough to form
clouds?
• In what ways are the processes of frontal lifting
and orographic lifting similar? In what ways are
they different?

What are three cloud shapes?
• The three classes of clouds based on shape are
stratus, cumulus, and cirrus clouds.
• Stratus clouds are thin and flat, and their edges
are not clearly defined.
• Stratus is a Latin word that means “layer.” Stratus
clouds often merge into one another and may look
like a single layer that covers the entire sky.

• Cumulus is a Latin word that means “heap.”
• Cumulus clouds are thick and puffy on top and
generally flat on the bottom.
• These clouds have well-defined edges and can
change shape rapidly. They can be bright or dark,
and they can produce severe weather.

• Cirrus is a Latin word that means “curl.”
• Cirrus clouds look white and feathery, and their
ends curl.
• They are made of ice crystals rather than liquid
water droplets. They do not produce precipitation
that reaches Earth’s surface.

I’ve Looked at Clouds from Both
Sides Now
What are the types of clouds based on
altitude?
• The four classes of clouds based on altitude are
low clouds, middle clouds, high clouds, and clouds
of vertical development.
• These four classes are made up of 10 cloud types.
• Prefixes are used to name the clouds that belong
to some of these classes.

altitude?
• Low clouds form between Earth’s surface and
2,000 m altitude. They are commonly made up of
water droplets.
• The three types of low clouds are stratus,
stratocumulus, and nimbostratus.
• There is no special prefix for naming low clouds.
However, nimbus means “rain,” so nimbostratus
clouds are rain clouds.

altitude?
• Middle clouds form between 2,000 m and 6,000 m
altitude. They are commonly made up of water
droplets, but may be made up of ice crystals.
• The prefix alto- is used to name middle clouds.
• The two types of middle clouds are altocumulus
and altostratus.

altitude?
• High clouds form above 6,000 m altitude, where
air temperature is below freezing. Therefore, high
clouds are made up of ice crystals.
• The prefix cirro- is used to name high clouds.
• Cirrus, cirrocumulus, and cirrostratus are the
types of high clouds.

altitude?
• A cloud of vertical development can have its base
at low altitude, but its top can reach higher than
12,000 m.
• The two types of clouds of vertical development
are cumulus and cumulonimbus.
• Cumulonimbus clouds are linked to severe
weather and can produce rain, hail, lightning,
tornadoes, and rapidly sinking columns of air.

altitude?
• Identify the various types of clouds shown here.

How does fog form?
• Water vapor that condenses very near Earth’s
surface is called fog. It forms when moist air near
Earth’s surface or moving across cold water cools
to its dew point.
• Ground fog, or radiation fog, generally forms in
low-lying areas on clear, calm nights. Sea fog, or
advection fog, occurs at all times of day.
• Steam fog forms when evaporation takes place
into cold air that is lying over warmer water.

Clouds on Other Worlds
• Like Earth, other bodies in the solar system have
clouds in their atmosphere.
• There are clouds on Venus, Mars, Jupiter, and
Saturn. The clouds of Jupiter and Saturn are
arranged in bands that circle the planets.
• Saturn’s moon Titan has clouds in a thick, planet-
like atmosphere.

Clouds and cloud_formation

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