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What is weather?
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Weather Factors
• Weather refers to the state of the atmosphere
at a specific time and place.
• Weather describes conditions such as air
pressure, wind, temperature, and the amount
of moisture in the air.
• The Sun’s heat evaporates water into the
atmosphere forming clouds; water returns
to Earth as rain or snow; the Sun also heats
the air.
What is weather?
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Air Temperatures
• Air is made up of molecules that are
always moving randomly, even when there’s
no wind.
• Temperature is a measure of air molecule
movement.
• Energy from the Sun causes air molecules to
move rapidly; the temperature is high, and it
feels warm.
• When less of the Sun’s energy reaches air
molecules, they move less rapidly and it feels
cold. The temperature is lower.
What is weather?
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Wind
• Air moving in a specific direction is
called wind.
• As the Sun heats the air, it expands,
becomes less dense, rises, and has low
atmospheric pressure.
• Cooler air is denser and tends to sink,
causing high atmospheric pressure.
What is weather?
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Wind
Wind results because air moves from regions
of high pressure to regions of low pressure.
What is weather?
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Wind
• Wind direction can be measured using a wind
vane which has an arrow that points in the
direction the wind is blowing from. The name
of the wind tells you the direction the wind is
blowing from. For
example, an east
wind blows from
east to west.
What is weather?
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Wind
• Wind speed can be
measured using an
anemometer.
• Anemometers have
rotating cups that spin
faster when the wind
is strong.
What is weather?
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Humidity
• Heat evaporates water
into the atmosphere.
• Water vapor molecules
fit into spaces between
the air molecules.
• The amount of water vapor present in the air is
called humidity.
What is weather?
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Humidity
• Air doesn‘t always contain the same amount
of water vapor.
• Warm air can hold more water vapor than
cool air, tending to make warm air more
humid.
• Cooler air holds less water vapor than warm
air, tending to make it less humid.
What is weather?
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Humidity
• Air is said to be saturated when it holds the
maximum amount of water vapor possible at
that temperature.
What is weather?
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Relative Humidity
• Relative humidity is a measure of the
amount of water vapor present in the air
compared to what it can hold at a specific
temperature.
• If you hear a weather forecaster say that the
relative humidity is 50 percent, it means that
the air contains 50 percent of the water vapor
needed for the air to be saturated.
What is weather?
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Relative Humidity
• Air at 25°C is
saturated when it
contains 22 g of
water vapor per
cubic meter of
air.
• The relative
humidity is
100 percent.
What is weather?
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Relative Humidity
• If air at 25°C
contains 11 g of
water vapor per
cubic meter, what
is the relative
humidity?
(50 %)
What is weather?
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Dew Point
• When the temperature drops, less water
vapor can be present in air.
• The water vapor in air will condense to a
liquid or form ice crystals.
• The temperature at which air is saturated
and condensation forms is the dew point.
What is weather?
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Dew Point
• When air near the ground cools to its dew
point, water vapor condenses and forms dew.
• If temperatures are near or below 0°C,
frost will form instead of dew.
What is weather?
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Forming Clouds
• Clouds form as warm, moist air is forced
upward and cools.
• At a certain height, the air cools to the dew
point and condensation begins. Water vapor
condenses on tiny particles in the air (dust,
smoke, salt) to form droplets of liquid water.
These droplets remain suspended in the air.
• Billions of these droplets form a cloud.
What is weather?
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Forming Clouds
What is weather?
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Classifying Clouds
• Clouds are classified mainly by shape
and height.
• The shape and height of clouds vary with
temperature, pressure, and the amount of
water vapor in the atmosphere.
• The three main cloud types are stratus,
cumulus, and cirrus.
What is weather?
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Shape - Stratus
• Clouds that form smooth, even sheets or
layers in the sky are called stratus clouds.
• These clouds form low in the sky and often
cover the entire sky.
• When air is cooled
to its dew point
near the ground, it
forms a stratus
cloud called fog.
What is weather?
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Shape - Cumulus
• Cumulus clouds are
masses of puffy, white
clouds, often with flat
bases.
• They sometimes
tower to great heights
and can be associated
with fair weather or
thunderstorms.
What is weather?
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Shape - Cirrus
• Cirrus clouds
are high, thin,
white, feathery
clouds made of
ice crystals.
• Cirrus clouds are associated with fair weather,
but they can indicate approaching storms.
What is weather?
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Height
• Some prefixes describe the height of the
cloud base.
• The prefix cirro- describes high clouds, altodescribes middle-elevation clouds, and
strato- refers to clouds at low elevations.
What is weather?
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Rain- or Snow-Producing Clouds
• Clouds associated with rain or snow often
have the suffix or prefix nimbus attached to
their name. The term nimbus is Latin for
“dark rain cloud.”
• The water
content of these
clouds is so high
that little
sunlight can pass
through them.
What is weather?
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Rain- or Snow-Producing Clouds
When a cumulus cloud
grows into a
thunderstorm, it is called
a cumulonimbus cloud.
Nimbostratus clouds
are flat, layered clouds
that cover the sky and
bring long, steady rain
or snowfall.
What is weather?
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Classifying Clouds
What is weather?
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Precipitation
• Precipitation is water falling from clouds.
• Precipitation occurs when water droplets in
clouds combine and grow large enough to fall
to Earth.
• Air temperature determines whether water
forms rain, snow, sleet, or hail—the four
main types of precipitation.
What is weather?
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Precipitation
• Drops of water
falling in
temperatures
above freezing
fall as rain.
• Snow forms
when the air
temperature is
freezing.
What is weather?
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Precipitation
• Sleet forms when
raindrops pass through a
layer of freezing air near
Earth’s surface, forming
ice pellets.
What is weather?
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Precipitation
• Hail is precipitation in the
form of lumps of ice.
• Hail forms in
cumulonimbus clouds of a
thunderstorm when water
freezes in layers around a
small nucleus of ice.
Weather Patterns
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Weather Changes
Because air and moisture move in the
atmosphere, weather constantly changes.
Weather Patterns
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Weather Changes—Air Masses
• An air mass is a large body of air that has
properties similar
to the part of
Earth’s surface
over which it
develops.
• Six major air
masses affect
weather in the
Unites States.
Weather Patterns
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Weather Changes—Air Masses
• Air masses can
be classified
according to
temperature and
humidity.
• They can be
warm or cold, and
moist or dry.
Weather Patterns
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Weather Changes—Air Masses
Type of Air Mass
Characteristic
Maritime Tropical
Humid and Warm
Maritime Polar
Humid and Cool
Continental Tropical
Dry and Warm
Continental Polar
Dry and Cool
Weather Patterns
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Highs and Lows
• Weather has high- and low-pressure systems.
• Stormy weather is associated with low
pressure areas.
• Fair weather is associated with high pressure
areas.
• Air pressure is measured using a barometer.
Weather Patterns
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Fronts
• A boundary between two different air
masses is called a front. This is an area
where two air masses meet and do not mix.
• Clouds, precipitation, and storms sometimes
occur at frontal boundaries.
• There are four types of fronts:
• Cold front
• Warm front
• Occluded front
• Stationary front
Weather Patterns
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Fronts - Cold front
• A cold front occurs when a cold air mass moves under a warm
air mass.
• Cold fronts move quickly, so they can cause abrupt weather
changes, including thunderstorms.
• After a cold front
passes, cool, dry air
moves in, often
bringing clear skies
and cooler
temperatures.
Weather Patterns
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Fronts - Warm front
• A warm front occurs when a warm air mass moves over a cold
air mass.
• Warm fronts move more slowly than cold fronts, so the weather
may be rainy or foggy for several days.
• After a warm front passes through an area, the weather is likely
to be warmer and more humid.
Weather Patterns
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Fronts - Occluded front
• An occluded front involes
three air masses. It occurs
when a warm air mass is
caught between two cooler
air masses and pushed
upward.
• At an occluded front, the
temperature near the
ground becomes cooler.
The rising warm air may
cause the weather to turn
cloudy and rainy or snowy.
Weather Patterns
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Fronts - Stationary front
• A stationary front occurs
when a cold air mass and a
warm air mass meet and
neither has enough force to
move the other. The air
masses and their boundaries
stop advancing.
• Stationary fronts may bring
many days of clouds and
precipitation.
Weather Patterns
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Fronts
What type of
weather is brought
by each front?
Weather Forecasts
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Weather Observations
• A meteorologist is a person who studies and
predicts the weather.
• Meteorologists take measurements of
temperature, air pressure, winds, humidity,
and precipitation.
Weather Forecasts
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Weather Observations
• Computers, weather satellites, Doppler radar,
and instruments attached to balloons are used
to gather data.
Weather balloon launch
NOAA-18 Weather satellite in orbit
Weather Forecasts
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Weather Maps
• Meteorologists at the National Weather Service
use the information provided by weather
instruments to make weather maps.
• Weather maps are used to make predictions
about future weather patterns.
• However, meteorologists cannot always
predict the weather exactly because
conditions can change rapidly.
Weather Forecasts
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Weather Maps - Station Models
• Meteorologists record the weather data on a
map by using
symbols. This
forms a
station model,
which shows
weather
conditions at a
specific
location.
Weather Forecasts
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Weather Maps
• Weather maps have lines called isotherms
that connect locations of equal temperature.
Weather Forecasts
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Weather Maps
• An isobar is a line drawn to connect points
of equal atmospheric pressure.
• On a weather map, pressure areas are drawn
as circles with the word High or Low in the
middle of the circle.
Weather Forecasts
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Weather Maps
• Fronts are drawn as
lines and symbols.
• In general, weather fronts
move from west to east.
Weather Forecasts
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Weather Maps
Fig. 19, p. 472
Weather Forecasts
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Weather Maps
Weather Forecasts
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Weather Maps