EARTH’S WEATHER
 The atmospheric factors that cause weather are
heat energy, air pressure, winds, and moisture
in the air.
 Air pressure depends on the density of the air.
 Factors affecting air pressure are temperature,
water vapor in the air, and elevation.
 Barometers measures air pressure and are
very important in predicting the weather.
 There are two types of barometers; mercury
and aneroid.
 Mercury barometers were invented in 1643
and contain mercury in a glass tube. As air
pressure decreases, mercury level drops. As
air pressure increases, mercury level
increases. High pressure usually means fair
weather. Low pressure leads to cloudy, rainy
or snowy weather but there are exceptions.
 Aneroid barometers (used in homes) are an
airtight metal box which most air has been
removed. When there is a change in pressure
it causes the needle to move along a dial.
 Local and global winds are caused by
differences in air pressure due to unequal
heating of the air. In the United States jet
stream winds blow from west to east.
 Water vapor, or moisture, in the air is called
humidity.
 There are three main types of clouds,
Cumulus, Stratus, and Cirrus.
 Cumulus clouds look like piles of cotton balls.
They are fluffy and white with flat
bottoms. Cumulus clouds usually mean fair
weather. However, When cumulus clouds get
larger and darker on the bottom
(cumulonimbus), they produce
thunderstorms.
 Stratus clouds are smooth, gray clouds
that cover the whole sky and block
out the sun. Light rain and drizzle are
usually associated with stratus clouds.
 Cirrus clouds are feathery and form at
very high altitudes, You can see cirrus
clouds in fair weather, but they often indicate
that rain or snow will fall within several hours.
 Water vapor that condenses and forms
clouds can fall to the Earth as precipitation in
the form of rain, sleet, snow, or hail.
 The four major air masses that affect weather
in the United States are maritime tropical,
maritime polar, continental tropical, and
continental polar.
 When two air masses meet, a cold front, a
warm front, and occluded front, or a
stationary front may form.
 A storm is a violent disturbance in the
atmosphere. It is marked by sudden changes
in air pressure and rapid air movements.
Storms may cover a huge area and others
cover only a small area.
 Rainstorms and snowstorms -- when two
different fronts collide, rainstorms or
snowstorms form.
 Thunderstorms --when a cold front moves in
and meets a warm front, cumulonimbus
clouds produce thunderstorms. What makes
thunderstorms so dangerous is the lighting.
Lighting is the sudden discharge, or spark, of
electricity between two clouds or between a
cloud and the ground.
 Hurricanes -- a low-pressure area containing
rising warm air that forms over tropical
oceans.
 Tornadoes -- a whirling, funnel-shaped cloud
that develops in low, heavy cumulonimbus
clouds
 Meteorologists use data from local weather
observers. Balloons, satellites, and weather
stations to predict the weather.
 Greenhouse effect - process in which carbon
dioxide and other gasses in the atmosphere
absorb infrared radiation from the sun,
forming a "heat blanket" around the Earth.
Instruments used in measuring and predicting
weather.
 Anemometer - instrument used to measure wind
speed.
 Barometer - instrument used to measure air pressure.
 Wind Vane - instrument used to measure wind
direction.
 Rain Gauge - instrument used to measure the amount
of rainfall.
 Thermometer - instrument used to measure air
temperature.
Cloud formation and
Precipitation
 Clouds form when water vapors, or moisture,
in the air condenses on small particles of dust
in the air. When a cloud gets heavy enough,
precipitation will fall back to the Earth.
Types of precipitation
include:
 Rain - drops of liquid water.
 Sleet - water droplets that freeze into small
ice pellets.
 Snow - water vapor that changes directly into
solid flat six-sided ice crystals.
 Hail - most damaging form of precipitation.
Usually formed in Cumulonimbus clouds. Hail
is small balls or chunks of ice ranging from 5
to 75 mm in diameter. They form when water
droplets hit ice pellets in a cloud and freeze.
Fronts
 Cold Front - A cold front forms when a cold
air mass meets and pushes under a warm air
mass. Violent storms are associated with a
cold front. Fair, cool weather usually follows.
 Warm Front - A warm front forms when a
mass of warm air overtakes a cold air mass
and moves over it. Rain and showers usually
accompany a warm front. Hot, humid
weather usually follows.
Fronts
 Occluded Front - An occluded front occurs when
a cold front travels faster than a warm front.
When a cold front overtakes a warm front, an
occluded front forms. An occluded front
produces less extreme weather than a cold front
or a warm front.
 Stationary Front - A stationary front occurs when
a warm air mass meets a cold air mass and no
movement occurs. Rain may fall in an area for
many days when a stationary front is in place.
Measuring Temperature
 Temperature is measured in units called
degrees. The temperature scale used by
scientists is the Celsius scale. On the Celsius
scale, the freezing point of water is 0o C. The
boiling point of water is 100o C. Normal body
temperature is 37oC.