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Chapter 7
th
6
Grade
Section 1
 Weather is the condition of the atmosphere at a
certain time and place.
 This condition is affected by the amount of water in
the air.
 Water cycle: states are liquid, solid, and gas. It is
continuous. Know condensation, evaporation,
precipitation, runoff, transpiration.
 Humidity: The amount of water vapor in the air. It is
invisible. It enters the air because of evaporation. The
higher the temperature the more water the air can
hold.
Section 1
 Relative humidity: the ratio of the amount of water
vapor in the air to the maximum amount of water
vapor the air can hold at a set temperature. It is given
as a percentage.
 When air is holding all it can at a temperature it is said
to be saturated. So its relative humidity is 100%. To
determine relative humidity you simply:
 Actual water vapor content (g/m3)
___________________________ x 100 = relative humidity
saturation water vapor content (g/m3)
Section 1
 Two factors affect relative humidity. Amount of water
vapor and temperature.
 More water vapor = higher relative humidity.
However, if the temperature changes then the relative
humidity will change.
 Relative Humidity will decrease as temperature rises
and increase as it decreases. Why does this occur?
Section 1
 Psychrometer: an instrument that is used to measure
relative humidity. It is basically two thermometers, one
is a wet bulb (just covered with a wet cloth) while the
other is dry.
 The differences in temperature readings indicates how
much water is in the air. The larger the difference
between the two readings the less amount of water
vapor that is present in the air. Therefore, the lower
the humidity.
Section 1
 Condensation: someone explain it to me.
 Dew point: the temperature at which a gas
condenses into a liquid. Air is saturated at its dew
point.
 Clouds: a collection of millions of tiny water droplets or
ice crystals. They form as warm air rises and cools. As
the air rises it becomes saturated. The water vapor
changes to a liquid or a solid. Temperatures above
freezing water vapor forms a liquid. Below freezing it
forms a solid. Clouds are classified by form and
altitude.
Clouds
 Cumulus clouds: puffy white clouds that tend to have
flat bottoms. Form when warm air rises. Tend to
indicate fair weather. When they get large they can
produce thunder storms. The thunderstorm cloud is
called a cumulonimbus cloud. Clouds that have
nimbus or nimbo likely produce precipitation.
 Stratus clouds: clouds that form in layers. Often cover
larger areas of the sky and block the sun. Caused by
a gentle lifting of a large body of air. Nimbostratus
clouds are dark clouds that produce light to heavy
continuous rain. Fog is a stratus cloud that formed
near the ground.
Clouds
 Cirrus clouds: are thin, feathery, white clouds at high
altitudes. They form when the wind is strong. They
tend to indicate a change in the weather.
 Altitudes of clouds: cirro indicates clouds at high
altitudes.
 Alto: clouds that form at middle altitudes.
 Low altitude clouds do not have prefixes.
 Be familiar with the altitudes.
Precipitation
 Rain: when water reaches a certain size. Water in a cloud
begins very small, like a dot or smaller. Before it can fall it
needs to be 100X bigger.
 Sleet-when rain falls through a layer of freezing air.
 Snow: temperatures are so cold that water vapor changes
directly into a solid.
 Hail: balls or lumps of ice that form in a cloud. Forms in
cumulonimbus clouds. Updrafts keep sending the hail up in
the air where it collects more water or layers of ice.
Eventually to heavy to collect more ice and falls.
Section 2
 Air mass: a large body of air where temperature and
moisture content are similar throughout. Causes
changes in weather.
 The moisture content and temperature of an air mass
are determined by the area over which the air mass
forms. These areas are called source regions. Ex. gulf
of Mexico: warm and wet. Pg. 192 details the types of
air masses.
Section 2
 Cold air mass: three polar air masses influence most of
the cold winter in the U.S.
 Continental polar (cP) forms over northern Canada
and brings really cold weather. In summer it tends to
bring cool, dry weather.
 Maritime Polar (mP) forms over the North Pacific
Ocean and brings cool, wet weather. Cool, foggy
weather in the summer.
 (mP) over the North Atlantic brings cool, cloudy
weather and precipitation to New England in the
winter and in the summer cool, foggy weather.
Section 2
 Warm air masses: Four warm masses.
 Maritime Tropical (mT)—develops over warm areas in
the pacific Ocean.
 (mT) Form in the gulf of Mexico and in the Atlantic
Ocean. They bring a lot of hot and humid weather in
the summer, as well as hurricanes and thunderstorms.
In the winter they bring mild, cloudy weather.
 Continental Tropic (cT) forms over the deserts of
northern Mexico and southwestern U.S. It brings clear,
dry, and hot weather in the summer
Section 2
 Fronts: Air masses that form from different areas that
do not mix. They have different densities. Warm air is
less dense than cold air. A front is basically the
boundary between air masses of different densities
and usually different temperatures. Four types of
fronts. Pg. 194!
 Cold front: Forms when cold air moves under warm air,
which is less dense, and pushes the warm air up.
These often bring thunderstorms, heavy rain, or snow.
 Warm front: when warm air moves over cold air. The
warm air eventually replaces the cold air. They can
bring drizzly rain and clear, warm weather.
Section 2
 Occluded Front: When a warm air mass is caught
between two colder air masses. The coldest air mass
moves under and pushes the warm air mass up. It
continues to move forward until it meets the other
cold air mass. This can produce large amounts of
rain/snow.
 Stationary Front: when a cold air mass meets a warm
air mass. Neither air mass moves the other and the
two remain separated---likely because there is not
enough wind. Can bring cloudy, wet weather.
Section 3
 Severe Weather:
 Thunderstorms: small, intense weather systems that
produce strong winds, heavy rains, lightning and
thunder. Two atmospheric conditions required to
produce a thunderstorm: warm and moist air near
Earth’s surface. Atmosphere is unstable when the
surrounding air is colder than the rising air mass.
Remember cold air is denser.
 When warm air reaches its dew point, the water vapor
condenses and forms a cumulus cloud(s). The warm
air can continue to rise and the cloud will grow dark—
cumulonimbus cloud.
Section 3
 Lightning: is an electric discharge that takes place
between a positively charged area and a negatively
charged area. It can occur between clouds, earth
and cloud, or even in the same cloud. Same as static
electricity—rubbing your feet on carpet and touching
someone. The lightning strike releases energy. The
energy is transferred to the air and sends out sound
waves called thunder.
Section 3
 Tornadoes: happen in only 1% of all thunderstorms.
Starts out as a funnel cloud and protrudes though the
bottom of a cumulonimbus cloud. It becomes a
tornado when it touches the ground.
 75% of the worlds tornadoes occur in the U.S. Most
happen in the spring and early summer when cold,
dry air from Canada meets warm moist air from the
tropics. Most only last a few minutes. Tornadoes can
uproot trees---wind between 120 to 500 km/h.
Section 3
 Hurricanes: wind speeds of at least 120km/h. They are the
most powerful storms on Earth. Also called typhoons or
cyclones. Most form in areas between 5 degrees and 20
degrees north latitude and 5-20 degrees south latitude.
 It begins as a group of thunderstorms over tropical water.
Wind that was traveling in different directions meet and
causes a spinning effect. Because of the Coriolis effect the
storm turns counterclockwise in the Northern Hemisphere
and clockwise in the Southern Hemisphere.
 It gets its energy from water condensation. Fueled by going
over warm water. As the warm, moist air rises the water
vapor condenses and releases large amounts of energy.
When it moves over colder areas it begins to lose energy.
Section 3
 Hurricanes can have wind speeds up to 300 km/h.
Often cause storm surges----high walls of water that
crash onto the shore.
 Know the parts of a hurricane on page 202.
 Storm safety!!!!!
Section 4
 Meteorologist: predicts weather. To predict weather a
meteorologist needs to measure various atmospheric
conditions, such as air pressure, humidity,
precipitation, temperature, wind speed, and wind
direction.
 Weather balloons carry electronic equipment that
measures conditions up to 30 km above the earth’s
surface. Can measure temperature, air pressure, and
relative humidity.
Section 4
 Thermometer: what does it do?
 Barometer: measures air pressure. It is a glass tube
that is sealed at one end and placed in a container
full of mercury. As pressure increases the mercury
moves up the glass tube. The more the air pressure the
higher the mercury will rise.
 Windsock or wind vane: measures wind direction.
 Anemometer: is three or four cups connected to a
pole. It measures wind speed.
Section 4
 Radar is used to find the location, movement, and
amount of precipitation---Doppler radar.
 Weather maps: isobars = air pressure.