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Weather
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What is weather? 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
provides the energy that drives earth's weather. Weather is the
result of heat and Earth's air and water.
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Air temperature: The temperature of air influences
your daily
activities. Air is made up of molecules that are always moving
randomly, even when there's no wind. Temperature is a
measure of the average amount of motion of molecules. When the
temperature is high, molecules in the air move rapidly and it feels
warm. When the temperature is low, molecules in the air move less
rapidly, and it feels cold.
HOW
TO MEASURE TEMPERATURE
Low pressure: When
air is heated, it expands and becomes less dense. This creates
lower pressure.
High pressure: Molecules making up air
are closer together in cooler temperatures, creating high pressure.
Wind blows from higher pressure toward lower pressure.
INTERACTIVE
AIR PRESSURE SIMULATOR
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 Wind:
Why
can you fly a kite on some days but not on others? Kites fly
because air is moving. Air moving in a specific direction is
called wind. As the sun warms the air, the air expands,
becomes less dense, and tends to rise. Warm, expanding air has
low atmospheric
pressure. Cooler air is denser and tends to sink, bringing about
high atmospheric pressure. Wind results because air moves from
regions of high pressure to regions of low pressure. You may have
experienced this on a small scale if you've ever spent time along a
beach.
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Heat evaporates water into the
atmosphere. Where does the water go? Water vapor molecules
fit into spaces among the molecules that make up air. The amount
of water vapor present in the air is called humidity.
Air doesn't always
contain the same amount of water vapor. More water vapor can be present when the air is
warm than when it is cool. At warmer temperatures, the molecules of water
vapor in the air move quickly and don't easily come together. At cooler
temperatures, molecules in air move more slowly. The slower movement
allows water vapor molecules to stick together and form droplets of liquid
water. The formation of liquid water from water vapor is called condensation.
When enough water vapor is present
in the air for condensation to take place, the air is saturated.
condensation
on window
 On a hot, sticky afternoon, the weather forecaster
reports that the humidity is 50%. How can the humidity be low
when it feels so humid? Weather forecasters report the amount of
moisture in the air as relative humidity. Relative humidity is a measure of the amount of water
vapor in the air compared to the amount needed for saturation at a
specific temperature. The portable humidity and temperature
reader pictured to the left can be used to accurately measure relative
humidity.
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. The dew point
changes with the amount of water vapor in the air. Some common
examples are when water droplets form on the outside of a glass of cold
water or when the air near the ground cools and dew forms on blades of
grass. Frost may form when temperatures are near 0°C.
Dew
Point
and Relative Humidity Calculator
The Water Cycle:
Hydrosphere is a term that
describes all the water on Earth's surface. Water moves
constantly between the atmosphere and the hydrosphere in the water cycle.
If you watch a puddle in the Sun, you'll
notice that over time the puddle gets smaller and smaller. Energy
from the Sun causes the water in the puddle to change from a liquid to
a gas by a process called evaporation.
Water that evaporates from lakes, streams, and oceans enters Earth's
atmosphere. Some water vapor is also released into the atmosphere
by plants and trees through a process called transpiration.
If water vapor in the atmosphere cools enough, it
changes back into a liquid. This process of water vapor changing
into a liquid is called condensation.
Clouds form when condensation occurs high in the
atmoshpere. Clouds are made up of tiny water droplets that can
collide to form larger drops. As the drops grow, they fall to
earth as precipitation, then
the water collects in bodies
of water such as rivers, oceans, and ground water, which completes the
cycle by returning water to the hydrosphere.
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Forming
clouds: Clouds form
as warm air is forced upward, expands, and cools. As the air cools, the
amount of water vapor needed for  saturation decreases and the relative
humidity increases. When the relative humidity reaches 100%, the
air is saturated. Water vapor soon begins to condense in tiny water droplets, around small
particles such as dust and salt. These droplets of
water are so small that they remain suspended in the air.
Billions of these droplets form a cloud.
Classifying
clouds: Clouds are
classified mainly by shape and height. Some clouds extend high
into the sky, and others are low and flat. Some dense clouds
bring rain or snow, while thin, wispy clouds appear on mostly sunny
days. The shape and height of clouds vary with temperature,
pressure, and the amount of water vapor in the atmosphere.
 Shape:
The three main cloud
types
are stratus, cumulus, and cirrus. Stratus
clouds form layers or even
sheets in the sky. They usually form at low altitudes and may be
associated with fair weather, rain, or snow. When a stratus cloud
forms near the ground, its called fog.
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. Cirrus
clouds appear fibrous or curly. They are high, thin, white,
feathery clouds made of ice crystals. Cirrus clouds are
associated with fair weather, but they can indicate approaching storms.
Height: Some
prefixes of cloud names describe the height of the cloud base.
The prefix cirro-
describes high clouds, alto-
describes middle-elevation clouds, and strato- refers
to clouds at low elevations. Some cloud's names combine the
altitude prefix with the term stratus or cumulus.
Cirrostratus
clouds form at high levels and altostratus
clouds form at middle levels.
Rain- or Snow-Producing
Clouds: Clouds associated
with rain or snow often have the word nimbus attached to them. The
term nimbus is Latin for
"dark rain cloud" and this is a good description because the water
content of these clouds is so high that little sunlight can pass through them. When a cumulus cloud grows
into a thunderstorm, it is called a cumulonimbus
cloud. These clouds can tower to nearly 18,000 km. Nimbostratus clouds are layered
clouds that can bring long, steady rain or snowfall.
Cumulonimbus Cloud
LESSON-Classifying
Clouds
*Lessons are .pdf
files and must be opened using Adobe Acrobat Reader.
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Precipitation:
Precipitation
occurs when cloud droplets combine and grow large enough to fall to
Earth. The cloud droplets form around small particles, such as
salt and dust. These particles are so small that a puff of smoke
can contain millions of them. Not all raindrops are the same
size. The size of raindrops depends on the strength of updrafts
in a cloud & the rate of evaporation as the drop falls to Earth.
Air temperature determines whether water forms rain, snow, sleet, or
hail.
Drops of water falling in temperatures above freezing fall as rain.
Snow forms when
the air temperature is so cold that water vapor changes directly to a
solid.
Sleet forms when
raindrops pass through a layer of freezing air near Earth's surface,
forming ice pellets.
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. Hailstones grow larger as they're
tossed up and down by rising and falling air.
Of all forms of precipitation, hail produces the most damage
immediately, especially if winds blow during a hailstorm. Falling
hailstones can break windows and destroy crops.
CURRENT
DAILY PRECIPITATION PLOT
 Ledovakrasa
Ice
Sculptures-PowerPoint Presentation
*File
for presentation is large and may
take several minutes to load depending on the speed of
your
Internet
connection.
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Even
though wind is invisible, you can see the direction it's blowing by its
effects on such things as trees and flags. A measurement of wind
direction can be learned from a wind vane or
a wind sock. You may have seen wind vanes on top of houses and
barns; they are one of the oldest tools for observing weather.
