Download 18.3 Cloud Types and Precipitation

Chapter 18 – Moisture, Precipitation, &
Clouds
• Define all vocabulary for Chapter 18
• Read each night to review material
covered in class and to gain an
understanding of material to be
covered next class meeting
• Good Luck on the test to come 
Chapter
18
Moisture, Clouds, and
Precipitation

Key Concepts

Which gas is most important for understanding atmospheric
processes?

What happens during a change of state?

How do warm and cold air compare in their ability to hold
water vapor?

What is relative humidity?

What can change the relative humidity of air?

Vocabulary

Precipitation, latent heat, evaporation, condensation, sublimation,
deposition, humidity, saturated, relative humidity, dew point,
hygrometer
18.1 Water in the Atmosphere
 Precipitation is any form of water that falls
from a cloud.
 When it comes to understanding
atmospheric processes, water vapor is the
most important gas in the atmosphere.
 Water vapor is the source of all condensation
and precipitation.
 Water vapor is 0-4% of the atmosphere
Water’s Changes of State
3 states of matter are solid, liquid, and gas
 The process of changing state required energy to be
gained or lost. It is transferred in the form of heat.
 Latent heat is energy added to matter that does not
produce a temperature change.

Solid - Liquid

Solid to Liquid
 Melting: when ice gains energy the molecules
move faster until they become a liquid
 Freezing: when liquid water loses energy the
molecules slow down until they become a solid
Liquid - Gas

Liquid to Gas
 Evaporation: when liquid water gains energy
the molecules move even faster until they
become a gas
 Condensing: when water vapor loses energy
the molecules move slower until they condense
into a liquid
BIOLOGY: The Water Cycle
• The water cycle is essential to life on
land. Water evaporates from the
oceans and from the leaves of plants.
Some of the water vapor condenses
and falls as precipitation over the
land back to the oceans, it is drunk
by animals and absorbed by plants
through their roots.
BIOLOGY: The Water Cycle
• What would happen if water
evaporated and condensed at
temperatures not found on Earth’s
surface?
• What effect would this have on life
on Earth?
Solid - Gas

Solid to Gas

Sublimation is the change from a solid directly to a gas
without a liquid state.


Example: Dry ice
Deposition is the change from a gas to a solid without a
liquid state.
18.1 Water in the Atmosphere
 Humidity is a general term for the amount
of water vapor in air.
 Saturation
• Air is saturated when it contains the maximum
quantity of water vapor that it can hold at any
given temperature and pressure.
• When saturated, warm air contains more water
vapor than cold saturated air.
18.1 Water in the Atmosphere
 Relative Humidity
• Relative humidity is a ratio of the air’s actual
water-vapor content compared with the amount
of water vapor air can hold at that temperature
and pressure.
• When the water-vapor content of air remains
constant, lowering air temperature causes an
increase in relative humidity, and raising air
temperature causes a decrease in relative
humidity.
Relative Humidity Varies
with Temperature
Relative Humidity varies with temperature.
18.1 Water in the Atmosphere
 Dew Point
• Dew point is the temperature to which a parcel of air
would need to be cooled to reach saturation.
 Measuring Humidity
• A hygrometer is an instrument to measure relative
humidity.
• A psychrometer is a hygrometer with dry- and wetbulb thermometers. Evaporation of water from the
wet bulb makes air temperature appear lower than
the dry bulb’s measurement. The two temperatures
are compared to determine the relative humidity.
Figure 4
Dew on a Spider Web
Figure 5
Sling Psychrometer
This psychrometer is used to
measure both relative humidity and
dew point.
18.2 Cloud Formation

Key Concepts





What happens to air when it is compressed or allowed
to expand?
List four mechanisms that can cause air to rise.
Contrast movements of stable and unstable air.
What conditions in air favor condensation of water?
Vocabulary

Dry adiabatic rate, wet adiabatic rate, orographic lifting,
front, temperature inversion, condensation nuclei.
18.2 Cloud Formation
 Adiabatic Temperature Changes
• When air is allowed to expand, it cools, and
when it is compressed, it warms.
 Expansion and Cooling
• Dry adiabatic rate is the rate of cooling or
heating that applies only to unsaturated air.
• Wet adiabatic rate is the rate of adiabatic
temperature change in saturated air.
• Unsaturated air cools as it rises, at the constant rate of
10°C for every 1000 meters of ascent.
• Descending air encounters higher pressures,
compresses, and is heated 10°C for every 1000 meters
it moves downward.
• If a parcel of air rises high enough, it will eventually
cool to its dew point. (condensation begins)
• As the air rises, latent heat of condensation will be
released.
• The released latent heat works against the adiabatic
cooling process.
• The wet adiabatic rate varies from 5–9°C per 1000
meters.
Cloud Formation by Adiabatic Cooling
Figure 7
Rising air cools at
the dry adiabatic
rate until the air
reaches the dew
point and
condensation
(cloud formation)
begins. As air
continues to rise
the latent heat
released by
condensation
reduces the rate
of cooling.
18.2 Cloud Formation
Processes that Lift Air
 Four mechanisms that can cause air to rise
are orographic lifting, frontal wedging,
convergence, and localized convective
lifting.
 Orographic Lifting
• Orographic lifting occurs when mountains act
as barriers to the flow of air, forcing the air to
ascend.
• The air cools adiabatically; clouds and
precipitation may result.
18.2 Cloud Formation
 Frontal Wedging
• A front is the boundary between two adjoining
air masses having contrasting characteristics.
Orographic Lifting and Frontal Wedging
Figure 8
A Orographic Lifting
Frontal Wedging
Why does the warm air mass move upward over the
cold air mass?
18.2 Cloud Formation
 Convergence
• Convergence is when air flows together and
rises.
 Localized Convective Lifting
• Localized convective lifting occurs where
unequal surface heating causes pockets of air to
rise because of their buoyancy.
Con’t Figure 8
Convergence
Convergence and Localized
Convective Lifting
Localized Convective Lifting
18.2 Cloud Formation
Stability
 Density Differences
• Stable air tends to remain in its original position,
while unstable air tends to rise.
 Stability Measurements
• Air stability is determined by measuring the
temperature of the atmosphere at various
heights.
• The rate of change of air temperature with height
is called the environmental lapse rate.
18.2 Cloud Formation
Stability
 Degrees of Stability
• A temperature inversion occurs in a layer of
limited depth in the atmosphere where the
temperature increases rather than decreases with
height. A temperature inversion is the most stable
condition.
 Stability and Daily Weather
• When stable air is forced above the Earth’s
surface, the clouds that form are widespread and
have little vertical thickness compared to their
horizontal dimension.
18.2 Cloud Formation
Condensation
 For any form of condensation to occur, the
air must be saturated.
 Types of Surfaces
• Generally, there must be a surface for water
vapor to condense on.
• Condensation nuclei are tiny bits of particulate
matter that serve as surfaces on which water
vapor condenses when condensation occurs in
the air.
18.3 Cloud Types and Precipitation

Key Concepts





How are clouds classified?
How are clouds and fogs similar and different?
What must happen in order for precipitation to form?
What controls the type of precipitation that reaches
Earth’s surface?
Vocabulary

Cirrus, cumulus, stratus, Bergeron process, supercolled
water, supersaturated air, collision-coalescence process
Cloud Chart
A cloud is
High
classified based on Clouds
________ &
_________.
Altitude
Type, description,
and precipitation
Type, description,
and precipitation
Type, description,
and precipitation
Middle
Clouds
Low Clouds
Multi-level
Clouds
18.3 Cloud Types and Precipitation
Types of Clouds
 Clouds are classified on the basis of their
form and height.
 The three basic forms are:
-cirrus
- cumulus
- stratus

Cirrus
(cirrus = a curl of
hair)
 They are high, white,
and thin.
 They can occur as
patches or as delicate
veil-like sheets or
extended wispy fibers
that often have a
feathery appearance.


Cumulus
(cumulus = a pile)
 clouds consist of
rounded individual
cloud masses.
 They have a flat base
and the appearance of
rising domes or
towers. (cauliflower
structure)


Stratus
(stratum = a layer)
 clouds are best
described as sheets or
layers that cover much
or all of the sky.
 There are no distinct
individual cloud units.

18.3 Cloud Types and Precipitation
Types of Clouds
 High Clouds (above 6000m)
• Cirrus clouds are high, white, and thin.
• Cirrostratus clouds are flat layers of clouds.
• Cirrocumulus clouds consist of fluffy masses.
All high clouds are thin and white and are often made
up of ice crystals.
 These clouds are not considered precipitation makers.
(they may warn of approaching stormy weather)

18.3 Cloud Types and Precipitation
Types of Clouds
 Middle Clouds (from about 2000 to 6000 meters)
• Altocumulus clouds are composed of rounded
masses that are larger and denser cirrocumulus
clouds.
• Altostratus clouds create a uniform white to gray
sheet covering the sky with the sun or moon
visible as a bright spot.
• Infrequent light snow or drizzle may accompany these
clouds.
18.3 Cloud Types and Precipitation
Types of Clouds
 Low Clouds (below 2000m)
• Stratus clouds are best described as sheets or
layers that cover much or all of the sky. (May
produce light precipitation.
• Stratocumulus clouds have a scalloped bottom
that appears as long parallel rolls or broken
rounded patches.
• Nimbostratus clouds are the main precipitation
makers.
Cloud Classification
Clouds are classified according to form and height.
18.3 Cloud Types and Precipitation
Types of Clouds
 Clouds of Vertical Development
• Some clouds do not fit into any one of the three
height categories mentioned. Such clouds have
their bases in the low height range but often
extend upward into the middle or high altitudes.
• They all are associated with unstable air.
• Once upward movement is triggered, acceleration is
powerful, and clouds with great vertical range form.
• The end result often is a cumulonimbus cloud that
may produce rain showers or a thunderstorm.
18.3 Cloud Types and Precipitation
Fog
 Fog is defined as a cloud with its base at or
very near the ground.
 Fog Caused by Cooling
• As the air cools, it becomes denser and drains
into low areas such as river valleys, where thick
fog accumulations may occur.
 Fog Caused by Evaporation
• When cool air moves over warm water, enough
moisture may evaporate from the water surface
to produce saturation.
18.3 Cloud Types and Precipitation
How Precipitation Forms
 For precipitation to form, cloud droplets
must grow in volume by roughly one million
times.
 Cold Cloud Precipitation
• The Bergeron process is a theory that relates
the formation of precipitation to supercooled
clouds, freezing nuclei, and the different
saturation levels of ice and liquid water.
The Bergeron Process
Ice crystals grow at
the expense of cloud
droplets until they are
large enough to fall.
The size of these
particles has been
greatly exaggerated.
18.3 Cloud Types and Precipitation
How Precipitation Forms
 Cold Cloud Precipitation
• Supercooled water is the condition of water
droplets that remain in the liquid state at
temperatures well below 0oC.
• Supersaturated air is the condition of air that is
more concentrated than is normally possible
under given temperature and pressure
conditions.
18.3 Cloud Types and Precipitation
How Precipitation Forms
 Warm Cloud Precipitation
• The collision-coalescence process is a theory
of raindrop formation in warm clouds (above 0oC)
in which large cloud droplets collide and join
together with smaller droplets to form a raindrop.
18.3 Cloud Types and Precipitation
Forms of Precipitation
 The type of precipitation that reaches
Earth’s surface depends on the temperature
profile in the lower few kilometers of the
atmosphere.
 Rain and Snow
• In meteorology, the term rain means drops of
water that fall from a cloud and have a diameter
of at least 0.5 mm.
• At very low temperatures (when the moisture
content of air is low) light fluffy snow made up
of individual six-sided ice crystals forms.
18.3 Cloud Types and Precipitation
Forms of Precipitation
 Rain and Snow
• Sleet is the fall of clear-to-translucent ice.
• Hail is produced in cumulonimbus clouds.
• Hailstones begin as small ice pellets that grow
by collecting supercooled water droplets as they
fall through a cloud.
Largest Recorded Hailstone
The largest recorded hailstone fell over Kansas in 1970
and weighed 766 grams.
Precipitation Chart
Type
Rain
Snow
Sleet
Glaze
Hail
Description