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Weather
Student Study Guide
DEFINITION OF WEATHER:
The changing condition in the atmosphere (the layer of air that surrounds the earth) is
called weather. Weather is caused by uneven heating of the atmosphere. (Ex. Direct
sunlight, sunlight striking the earth at a slant/diagonal, water temperature, land
temperature).
CLOUD FORMATION/TYPES:
HOW DO CLOUDS FORM?
A cloud occurs when the invisible water vapor in the air becomes visible water droplets
or ice crystals. The water vapor becomes visible by cooling. That’s what happens on a
cold winter day when you “see your breath.” The same is true when you see steam rising
from a pan of boiling water or food.
HOW ARE CLOUDS RELATED TO WEATHER?
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One of the best ways to know what kind of weather is coming is to study the
clouds. Different types of clouds bring different types of weather.
Nimbus/Nimbo means rain, while alto means forming in the middle layer of the
sky.
PRECIPITATION:
HOW DOES PRECIPIATION AFFECT OUR LIVE?
WHAT ARE THE DIFFERENT TYPES OF PRECIPITATION, AND HOW DO THEY
FORM?
Rain, sleet, snow, freezing rain and hail are the five types of precipitation. Most clouds
do not produce precipitation. The amount of rain that falls to the earth is measured with a
rain gauge.
RAIN:
Occurs when water droplets within a cloud grow too large and too heavy
to remain suspended in the cloud.
SNOW:
Ice crystals grow at the expense of water droplets and become heavy
enough to fall to earth. If the ice crystals fall through freezing air
temperatures, crystals hit the earth as snow…through warm air-rain.
SLEET:
Rain that freezes as it falls through a layer of freezing temperatures
on the down to earth.
FREEZING
RAIN:
Rain that freezes after it hits a frozen surface
HAIL:
Droplets that freeze and are bounced up and down through the clouds,
building layers of ice.
HUMIDITY;
WHAT DOES RELATIVE HUMIDITY HAVE TO DO WITH WEATHER?
Humidity is the amount of moisture or water vapor in the air. Kentucky summers are
known for their high humidity.
Relative Humidity compares the amount of water that is in the air to the amount of water
that could be in the air at a certain temperature. A relative humidity of 90% is very
humid and uncomfortable. Relative humidity is measured with a hygrometer,
which is made of two different types of thermometers (dry bulb and wet bulb). If the
humidity is low, the water on the wet bulb thermometer will evaporate and cool the
thermometer. By finding the difference between the temperatures on both thermometers
and using a chart, you can find the relative humidity.
AIR TEMPERATURE:
HOW ARE AIR TEMPERATURE AND THE WATER CYCLE RELATED?
Temperature is the unit of measurement of which we are most familiar. Temperature is
the degree of how hot or cold a substance is. As heat within a substance increases,
molecules within that substance move faster and the temperature increases. If heat is
removed, the molecules move slower and the temperature decreases. A thermometer is
an instrument, usually including mercury or red-colored alcohol, that measures
temperature.
Evaporation occurs when water is heated and the water changes to vapor.
Condensation occurs when water vapor changes to a liquid state. Therefore, temperature
Plays a major role in the operation of a continuous cycle on earth—the water cycle.
WIND:
WHAT KEEPS AIR IN CONSTANT MOTION AROUND THE GLOBE?
Wind is air in motion. Wind speed is measures with an anemometer. The incoming
solar energy from the sun keeps air in motion as it heats the earth’s surface. Land heats
and cools faster than water. This creates air movement. When air moves inland from
water during the day, the wind is called a sea breeze. At night, the land cools quicker
than the water. The reverse cycle of wind blowing out out sea occurs and this creates a
land breeze.
On a global scale, the tropical region receives more solar radiation and heating than the
poles. The difference in the heating of the earth’s surface creates cycles of air movement.
Air rises at the hot equator and sinks at the cold poles. Air moving between the two
regions is curved by the rotation of the earth to create our wind. The Prevailing
Westerlies is the major wind belt across the United States moving weather systems
generally from west to east. The Jet Stream is also created in this way. It is a “tube”
of wind high above the surface of the earth. Winds in the Jet Stream travel at 100 to 250
mph. During the winter, the jet stream is often the boundary separating cold polar air to
the north from the warm subtropical air to the south. Large masses of air moving across
the earth are called air masses. Air masses stretch over thousands of square miles.
The wind can also affect make the temperature seem different to us. If there is a breeze
in the summer, it does not feel nearly as hot as it is at times. In the winter the bitter cold
winds, wind chill, can make a cold winter day actually be even colder.
AIR PRESSURE:
HOW DOES AIR PRESSURE AFFECT OUR WEATHER?
Air pressure is the force created by the weight of the air above. The air above you create
A force on you much like the weight or force of water on a diver at the bottom of an
ocean. Imagine the weight of approximately 400 miles of air. On the average, the
atmosphere exerts a force of 14.7 pounds per square inch of your body. Fortunately, air
inside your body is pushing outward which counteracts the atmospheric pressure pushing
inward.
A barometer is an instrument that measures changes in atmospheric pressure. The type
of barometer used in most homes is an aneroid barometer, which contains no fluid. It
operates with a small cell inside. As atmospheric pressure changes, so does the size of
the sensitive cell inside the barometer.
When your ears pop when riding in a car or airplane, your ears have sensed a change in
air pressure.
When air is heated, the particles of air move farther apart, and the air becomes less dense.
This lowers the air pressure because there are fewer air particles over a certain part of the
earth. The air begins to rise (just like air rising from a boiling pot). This is called a low
pressure area. It is called low pressure because as the air rises from the surface of the
earth, the force or pressure of the atmosphere is lower. Low pressure areas usually
produce cloudy, stormy weather.
Description
Smoke rises straight
Up
Smoke
Drifts
Wind felt of face,
Leaves rustle
Flag blows
straight out
Loose papers
Blow
Small trees
Sway
Hats blow off,
Branches move
Difficult to walk
against the wind
Small branches
break off trees
Damage to
Buildings
Trees
Uprooted
Widespread
Damage
Violent
damage
Type of Wind
Wind Speed
calm
Light
air
Light
breeze
Gentle
breeze
Moderate
breeze
Fresh
breeze
Strong
breeze
Moderate
gale
Fresh
gale
Strong
gale
Whole
gale
0-1 m/h
1-3 m/h
4-7 m/hr
8-12 m/hr
13-18 m/hr
19-24 m/hr
25-31 m/hr
32-38 m/hr
39-46 m/hr
47-54 m/hr
55-63 m/hr
Storm
64-73 m/hr
Hurricane
74-74+ m/hr
Cool air is more dense than warm air. It forms a high pressure area. This means that the
air in the atmosphere is sinking on you (and the barometer) and the atmospheric pressure
is higher. Since air is not rising and cooling, clouds generally do not form. High
pressure, therefore, usually produces fair weather.
FRONTS:
WHAT TYPES OF WEATHER CAN FRONTS PRODUCE?
In the northern hemisphere, a low pressure area always creates a wind that blows
COUNTECLOCKWISE around the low. The high pressure area creates a wind that
blows CLOCKWISE around the high. This wind around highs and lows plays a major
role in transporting warm air masses from the south and cold air masses from the north.
The transportation of air masses creates what are called fronts. We will learn about two
types of fronts. FRONTS are lines marking the leading edge of a change in air mass or
temperature.
Similar to a front line of soldiers moving into an area first, the leading edge of an air
mass moves forward along a weather front. When the weather man shows us an
approaching front, we know that the air and temperatures behind the front can be much
different from the present air mass in our area.
Fronts are usually connected to rotating low pressure systems that produce rain or snow.
Fronts also help to lift and cool the air, creating clouds and precipitation. Air rotation
counterclockwise around the low brings warm air from the south ahead of the low and
cooler air from the north behind the center of the low.
WARM FRONTS are represented by a red line with rounded “bumps” on the weather
map. Warm fronts force warm air over the top of cool air. Warm fronts form high, thin
clouds that often bring light, steady rain that may last for a day or more. COLD
FRONTS are represented by blue lines with pointed “teeth” on the weather map showing
the leading edge of a clod air mass. Cold fronts are formed when a cold air mass moves
into a warm air mass. A cold front pushes the air up and forms dark, towering clouds.
These clouds form thunderstorms that often bring brief, heavy showers.