Download NC Final Exam Review - Weather


Earth’s atmosphere is made up of nitrogen,
oxygen, carbon dioxide, water vapor, and many
other gases, as well as particles of liquids and
solids.

Nitrogen – Nitrogen is the most abundant gas in the
atmosphere. It makes up more than ¾ of the
atmosphere.
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Oxygen – Oxygen makes up less than ¼ of the
atmosphere, but is the most essential for animal and
human life. Oxygen is taken from the atmosphere
and used to release energy from our food.
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Oxygen is also used in other processes like burning
materials and causing rust on metals.
Ozone is a form of oxygen that has three oxygen
molecules instead of two. Ozone forms when lightning
occurs and is what causes the smell after a storm.
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Carbon Dioxide – Carbon dioxide is essential to
life. Plants must have carbon dioxide to produce
food. When humans and animals break down
food to produce energy, they release carbon
dioxide into the air.
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Carbon dioxide is also created when fuels such as
coal and gasoline are burned. This increases the
amount of carbon dioxide in the air.
Other Gases – Oxygen and nitrogen together
make up 99% of dry air. Argon and Carbon
dioxide make up most of the remaining 1%
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Water Vapor – In reality, air is not dry and contains water
vapor. Water vapor is a gas form of water. It is invisible
and cannot be felt. It is not the same thing as steam.
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The amount of water vapor in the air varies greatly from
place to place and time to time. We sometimes call water
vapor humidity. Water vapor is what eventually forms
clouds and precipitation.
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Particles – Precipitation cannot form without particles in
the air. Our air contains tiny particles of dust, smoke, salt,
and other chemicals. Most of these particles are too tiny
to see.
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1.
2.
3.
Earth’s atmosphere makes conditions on
Earth suitable for living things. It does this
in three ways:
It traps energy from the sun to keep Earth’s
surface warm and water in liquid form.
The atmosphere protects living things from
dangerous radiation from the sun
It also prevents Earth’s surface from being
hit by most meteoroids and rocks from outer
space
United Streaming:
Earth's Atmosphere: The Properties of the
Atmosphere [02:21]
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1.
What is the atmosphere?
–
2.
What are the four most common gases in dry air?
–
3.
Nitrogen, Oxygen, Carbon dioxide, and argon
Why are the amounts of gases in the atmosphere
usually shown as percentages of dry air?
–
4.
A thin layer of gases that surround Earth
The percentage of water vapor in the air varies and
affects the percent composition.
What are three ways in which the atmosphere is
important to life on Earth?
–
It contains oxygen need by many organisms. It
provides warmth by trapping energy from the sun. It
protects us from dangerous radiation and objects from
outer space.
•
•
•
•
Air is made up of atoms and molecules, which have
mass. This means that air also has mass.
Because air has mass, it also has other properties
including density and pressure.
Density – The amount of mass in a given volume of
air is its density.
If there are more molecules in a volume, the density
is greater. If there are less molecules in a volume,
the density is less
Density = Mass
Volume
Air pressure can change from day to day
just like the amount of water vapor in air.
Air pressure is related to density and mass:
 The more dense a substance, the more
mass and pressure it has.

There is a column of
air above you all the
time.

The weight of the air in
the atmosphere
causes air pressure.
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Barometer – A barometer is an instrument that is
used to measure air pressure. There are two
common kinds: mercury and aneroid
Air pressure is measured using several different units.
We will learn about 2 of those.
• Inches of mercury – This is the most common form
used in everyday weather forecasts. The
meteorologist might say – “The pressure reading
today is at 30.75 inches.”
• Millibars – The National Weather Service maps use
millibars to measure air pressure. 1 inch of mercury
= 33.87 millibars, so 30 inches of mercury = 1,016
millibars

Air pressure pushes down on the surface of the
mercury in the dish, causing the mercury in the
tube to rise. The air pressure is greater on the
barometer on the right, so the mercury is higher in
the tube.

This diagram shows an aneroid barometer.
Changes in air pressure cause the walls of the
airtight metal chamber to flex in and out. The
needle on the dial indicates the air pressure.

Air pressure decreases as altitude increases. Altitude
is your elevation, or distance from sea level. The
higher up you go, the lower the pressure. The lower
you go, the higher the pressure.
United Streaming Video:
Matter and Energy: Chapter 2: Pressure [09:18]
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Atmosphere website
Exosphere
Ionosphere
Thermosphere
Mesosphere
Troposphere
Stratosphere
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There are four layers of the atmosphere:
-Troposphere
-Stratosphere
-Mesosphere
-Thermosphere
- Ionosphere
- Exosphere
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Lowest layer
Contains most of the mass
of the atmosphere
Weather occurs here
9-16 kilometers in depth
Temperatures decreases as
altitude increases
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Begins where troposphere ends and goes about 50 km
above Earth
Divided into lower, middle and, upper sections
Lower section is cold, -60 degrees Celsius
Middle section is the ozone layer
Upper section is warm due to ozone absorbing energy
from the sun and converts it into heat.
This is where planes fly because it is above the weather.
Pollutants that reach the lower area may linger for
lengthy periods. Ex: Volcanic eruptions can cause
changes in the weather and climate
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Middle layer – (meso means middle)
Located between 50-80km above the Earth’s
surface
Outer mesosphere is the coldest part of the
atmosphere and has a temperature of -90°C
The Mesosphere is where meteoroids burn up
When you see a shooting star, you
are seeing the trail of a burning up
meteoroid.
Located 80+km above the Earth’s surface
 No definite outer limit and extends out to space
 Very hot (1,800 degrees Celsius) because sunlight
hits the thermosphere first.
 It is divided into to parts:

•
•
Ionosphere (lower layer)
Exosphere (upper layer)
The lower layer of the thermosphere
 Located 80-400km above Earth
 Radio waves bounce off ions in the ionosphere
back to Earth’s surface.
 The aurora borealis is found here.
This is caused by particles
from the sun that strike
atoms in the ionosphere
and causes them to glow.

Begins at about
400km and extends
out into space
 Satellites and the
space shuttle both
orbit in the
exosphere.
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Water never leaves the Earth. It is constantly being
cycled through the atmosphere, ocean, and land.
This process, known as the water cycle, is driven
by energy from the sun. The water cycle is crucial
to the existence of life on our planet.
DURING PART OF THE WATER CYCLE, THE SUN
HEATS UP LIQUID WATER AND CHANGES IT TO A
GAS BY THE PROCESS OF EVAPORATION. WATER
THAT EVAPORATES FROM EARTH’S OCEANS,
LAKES, RIVERS, AND MOIST SOIL RISES UP INTO
THE ATMOSPHERE.
The process of evaporation from plants is
called transpiration. (In other words, it’s like
plants sweating.)
AS WATER (IN THE FORM OF GAS) RISES HIGHER IN THE
ATMOSPHERE, IT STARTS TO COOL AND BECOME A LIQUID
AGAIN. THIS PROCESS IS CALLED CONDENSATION.
WHEN A LARGE AMOUNT OF WATER VAPOR CONDENSES,
IT RESULTS IN THE FORMATION OF CLOUDS.
WHEN THE WATER IN THE CLOUDS GETS TOO
HEAVY, THE WATER FALLS BACK TO THE EARTH.
THIS IS CALLED PRECIPITATION.
WHEN RAIN FALLS ON THE LAND, SOME OF THE WATER IS
ABSORBED INTO THE GROUND FORMING POCKETS OF
WATER CALLED GROUNDWATER. MOST GROUNDWATER
EVENTUALLY RETURNS TO THE OCEAN. OTHER
PRECIPITATION RUNS DIRECTLY INTO STREAMS OR
RIVERS. WATER THAT COLLECTS IN RIVERS, STREAMS,
AND OCEANS IS CALLED RUNOFF.
http://perso.orange.fr/prof.danglais/animations/watercycle/watercycle.htm
IMPORTANT INFORMATION
• The cycling of water in and out of the
atmosphere is the main cause of weather
patterns on Earth.
• 97% of the Earth’s water is in the oceans
• 3% of the Earth’s water is fresh water.
Fun Facts:
• The length of time an average water molecule
stays in the atmosphere is 9 days.
• The length of time an average water molecule
stays in the ocean is
3200 years.
Credit to:
Burnett Middle School
8th Grade Science
•
is the transfer of heat by the movement of
warmed matter (air or liquid)
The Sun provides the energy that drives
convection within the atmosphere and oceans,
producing winds and ocean currents.
What is most
dense SINKS!
move the air in our
atmosphere!
We have global winds
that blow because of
these large convection
currents
Wind over the
shore
changes
direction
because of
EARTH’S
UNEVEN
WARMING &
COOLING!
Why does hot air rise and cold air sink?
Cool air is more dense than warm air,
so the cool air ‘falls through’ the warm
air.
Why are the heating elements placed
beneath hot water tanks in people’s homes?
Hot water rises.
So when the element heats the water, and the
hot water rises, the water tank is filled with hot
water.
Wind
is the movement of air from an area of high pressure to
an area of low pressure.
This is caused by the unequal heating of the Earth’s surface.
As air is heated, it becomes less dense and rises. The air
pressure is also lower .
Cooler, more dense air, has a higher pressure, and moves
under the warmer air.
This causes a convection current,
creating wind
The greater the pressure
difference, the faster the air
moves, and the stronger the
wind blows.
Wind
direction is measured with a wind vane, which points
in the direction the wind is coming from.
This is how the wind is named.
A western wind is blowing from the west.
This means a northern wind actually blows toward the
south.
Wind speed is measured with an instrument called an
anemometer.
Since the air near the equator is less dense, it
rises forming areas of low pressure.
 The cold air near the poles sinks because it is
more dense, forming areas of high pressure.
 The air moves in large circular patterns called
convection currents.
 The change in density is due to
the unequal heating of Earth’s
surface
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Very Cold Air Sinks
H
Cold Moist Air Rises
L
H
Cool Dry Air Sinks
Warm Moist Air Rises
L
Cool Dry Air Sinks
H
Cold Moist Air Rises
Very Cold Air Sinks
H
L
H
L
H
L
H
L
H
Wind Moves
from HIGH
to LOW
Objects are
deflected to
the
RIGHT in the
Northern
Hemisphere
But why?
As
Earth rotates,
the Coriolis effect
turns winds in the
Northern
Hemisphere
toward the right.
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Polar Easterlies
 90

Prevailing Westerlies
 60

– 60 latitude from the east
– 30 latitude from the west
Trade winds
 30
– 0 latitude from the East
Global Winds
• Polar Easterlies are the wind belts that extend from the poles to
60° latitude in both hemispheres. They form as cold, sinking air
moves away from the poles. They can carry cold arctic air over
the U.S. which can produce snow and freezing weather.
• Westerlies are the wind belts found between 30° and 60°
latitude in both hemispheres. They blow toward the poles from
west to east. These winds can carry moist air of the U.S.
producing rain and snow.
• Trade Winds are the winds that blow from 30° latitude almost to
the equator in both hemispheres. They curve to the West as they
blow toward the equator.
Global Winds, continued
The Doldrums The trade winds of the Northern and Southern
Hemispheres meet in an area around the equator called the
doldrums. there is very little wind because the warm, rising air
creates an area of low pressure. The name doldrums means “dull”
or “sluggish.”
•
Narrow belts of high-speed winds that blow
in the upper troposphere and lower
stratosphere. Knowing the path of a jet
stream is important not only to pilots but
also to meteorologists. Because jet streams
affect the movement of storms,
meteorologists can track a storm if they
know the location of a jet stream.
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Jet Streams form at
latitudes where wind
systems come together
High speed westerly
winds form high above
the surface
Local winds move over a short distance
Caused by the heating of a small area of Earth’s surface
Sea
Breeze
•The
sun warms the land faster than the water, so the air above
the land becomes warmer.
•The cooler air over the water moves inland
Land
•At
Breeze
night, the land cools faster than the water, so the air above
the land becomes cooler.
•The cooler air over the land moves out to the water.
Local Winds
• Local winds generally move short distances and can
blow from any direction.
• Mountain and valley breezes are examples of local
winds caused by an area’s geography.
• Sea and land breezes are affected by temperature.
•
Air Mass:
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An air mass is a huge body of air that has
similar temperature, humidity, and air pressure
at any given height.
Air masses are classified by 2 characteristics
1.
2.
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Temperature
Humidity
The characteristics of an air mass depend on
the temperatures and moisture content of the
region over which the air mass formed.
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•
•
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Tropical: warm, air masses formed in the tropics
Polar: cold, air masses formed north or south of 50º
latitude
Maritime: air masses formed on oceans or seas
Continental: air masses formed over land

The colder the air the higher the air
pressure subsequently the hotter the air
the lower the air pressure.
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Cold air
Hot air
more dense
less dense
There are 4 major types of air masses
that affect the weather of the U.S.
1.
2.
3.
4.
Maritime tropical
Maritime polar
Continental tropical
Continental polar
1.
Maritime tropical
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Warm, wet air masses
On the east coast they are formed over the Gulf of
Mexico & south Atlantic Ocean.
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Influence weather along the entire east coast.
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Summer: thunderstorms & summer showers
Winter: heavy snow or rain
On the west coast they form over the southern
Pacific Ocean.
Maritime polar
2.
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Cold, wet air masses
On the east coast they are formed over the north Atlantic
Ocean.
On the west coast they are formed over the north Pacific
Ocean.
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Influence the weather of the west coast more so than that of the
east coast.
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Summer/Winter: fog, rain, & cooler temperatures
Continental tropical
3.
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Warm, dry air masses
Typically form over the southwest (New Mexico, Arizona,
Nevada, as well as northern Mexico) during the summer
months.

Influence the weather of the southwestern part of the US &
southern Great Plains (Kansas, Oklahoma, Texas, Iowa).
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Summer: Hot, dry
4.
Continental polar
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Cold, dry air masses
Typically form over central & northern Canada as
well as Alaska.
Influence the weather of the entire United States.
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Winter: Clear, cold, dry
Summer: Potential for storms due to interaction with
Maritime tropical air moving up from the Gulf of Mexico.
•
2 primary methods for air mass
movement
1.
Prevailing Westerlies
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2.
Pushes air masses from west to east.
Jet streams
–
Pushes fast moving air masses from west to east.
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Fronts
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The boundary between two air
masses.
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Air masses do not easily mix with
each other due to the differences
in…
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1.
Density (Air pressure)
Temperature
Moisture content
Storms & different types of weather phenomena occur along
fronts.
Types of fronts
Cold front
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Fast moving cold air mass overtakes a slower moving warm air
mass.
Can cause abrupt weather changes particularly thunderstorms.
Clear skies, a change in wind, & lower temperatures usually follow
2.
Warm front
–
–
–
3.
Stationary front
–
–
4.
A fast moving warm air mass overtakes
a slow moving cold air mass.
Can cause extended periods of rainy or
cloudy weather.
Warm, humid weather usually follows a
warm front.
A cold and warm air mass meet but
neither can move the other.
Can cause extended periods
of precipitation; snow, rain,
fog or clouds.
Occluded front
–
A warm air mass is caught
between 2 cooler air masses.
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Fronts music video
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Cyclones - Low Pressure System
 Greek
for “wheel”
 Formed around centers of low pressure.
 Caused as the boundary between fronts
become distorted by surface features;
mountains or strong winds.
 Represented on weather maps by an L.
 Warm air rises and spins counterclockwise around
the center.
 Storms and precipitation are
associated with areas of low pressure
as the warm air rises & condenses to
form clouds & precipitation.

Anticyclones – High Pressure Systems
 Formed
around centers of high pressure.
 Represented on weather maps by an H.
 Cold air sinks and spins clockwise around the
center.
 Dry weather and clear skies are associated with
areas of high pressure as the cooler air falls &
becomes warmer causing a drop in relative
humidity.

Storms:
 Violent
disturbances within the atmosphere.
 Caused by sudden changes in air pressure
which cause rapid air movement in an area.
 Similar conditions often produce different types
of storms.

Types of storms
 Thunderstorms
 Fast
moving storms that are often accompanied by
heavy precipitation, frequent thunder and visible
lightning.

Lightning: sudden spark or electrical discharge typically
caused by the build up of positive charges on Earth with
negative charges within the air.
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Cloud to cloud
Cloud to ground
Ground to cloud (rare)
Thunder is caused as air is superheated (30,000ºC),
expands, and explodes.
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Thunder is the sound wave created from the explosion.
Because sound travels slower than light, thunder always
comes after lightning not the other way around.
•
•
•
•
Formed within cumulonimbus clouds or thunderheads.
Typically form on hot, humid afternoons or when a fast
moving warm front over takes a slower cold front.
Within the cloud fast moving updrafts & downdraft.
Because thunderstorms have the potential to dump a lot of
water in a small amount of time, flooding is a potential
problem.
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•
Flash floods: flooding of low lying areas within a short time period;
less than 6 hours.
Thunderstorm safety
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Safest place is indoors away from objects that can conduct
electricity.
The metal cage of a car will provide protection if trapped inside a car
however try to avoid touching any part of the metal frame.
If outside find a low lying area & lay down.
 Tornadoes
 Tornadoes
can form in any situation that produces
severe weather.
 Typically form during the Spring & Summer under the
same conditions as those of a thunderstorm.
 Tornado formation
Warm, moist air flows in at the bottom of a cumulonimbus
cloud & rapidly moves upward generating a low pressure
area inside the cloud.
 The warm air begins to rotate due to winds within the cloud
blowing in different directions: The result is the cloud
begins to spin like a top.
 As part of the cloud descends to touch the ground, a
tornado or funnel cloud is generated with winds up to 340
mph.

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The Fujita Scale
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
Used to determine the severity of a tornado.
Based on the amount of damage created as well as the
wind speed.
F-0: Gale tornado, 40-72 mph winds
 F-1: Moderate tornado, 73-112 mph winds
 F-2: Significant tornado, 113-157 mph winds
 F-3: Severe tornado, 158-206 mph winds
 F-4: Devastating tornado, 207-260 mph winds
 F-5: Incredible tornado, 261-300+ mph winds

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Tornado alley
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
Located in the Midwest region of the U.S. & is known for
the development of tornadoes.
Includes the states of S. Dakota, Iowa, Nebraska,
Kansas, Oklahoma, and Texas.
 Hurricanes
 Tropical
cyclone (low pressure) that
typically measures 300-500 miles across
with winds from 70-200 mph.
 Comes from the West Indian word
Huracan or “big wind.”
 Called Typhoons when formed
in the Pacific Ocean;

Chinese word, Táifēng or “great wind.”
 Hurricanes
are named by the World Meteorological
Organization.
 Guided or directed by the Trade winds.
 Can only form over water that is at least 80ºF.
 Typically forms during the months of late July to early
October.

Stages of Hurricane Development
1.
2.
3.
4.

Stage 1: Tropical disturbance; 10-23 mph
Stage 2: Tropical depression; 23-39 mph
Stage 3: Tropical storm; 40-73 mph
Stage 4: Hurricane; 74 mph
The Saffir-Simpson scale

Scale used to determine the severity of a hurricane.




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Category 1: wind speed 74-95 mph; storm surge 4-5 feet.
Category 2: wind speed 96-110 mph; storm surge 6-8 feet.
Category 3: wind speed 111-130 mph; storm surge 9-12 feet.
Category 4: wind speed 131-155 mph; storm surge 13-18
feet.
Category 5: wind speed 155+ mph; storm surge 18+ feet

Hurricane Katrina was a category 3 hurricane when it made
landfall near New Orleans, La on August 29, 2008.

Costliest natural disaster

6th strongest to form, 3rd strongest to make landfall

1 of the 5 deadliest
•
Humidity – a measure of the amount of water vapor
in the air. Warm air can hold more water vapor than
cold air.
•
Relative humidity – the percentage of water vapor
that is actually in the air compared to the maximum
amount of water vapor the air can hold at a particular
temperature.
•
Relative humidity is the most useful of the two
measurements. This is what you see in the daily
weather forecasts.
Psychrometer – an instrument used to measure relative
humidity. This instrument has two thermometers – a wet blub
and a dry bulb thermometer.
Prefix
Meaning
Example
Alto-
Mid altitude
Altostratus
Cirro-
High altitude
Cirrostratus
Cumulo-
Heaped
Cumulonimbus
Nimbo-
Rain
Nimbostratus
Suffix
Meaning
Example
-nimbus
Rain
Cumulonimbus
http://www.instructables.com/id/Predicting-Weather-withClouds/?ALLSTEPS

Clouds can easily be broken into four
categories. These categories are high
clouds, middle clouds, low clouds and clouds
with vertical growth.
Clouds are also identified by shape.
Cumulus refers to a "heap" of clouds. Stratus
refers to clouds that are long and streaky.
And nimbus refers to the shape of "rain"
because we all know what rain looks like

High clouds form at 16,000 - 43,000 feet. Basically,
these are the clouds that you only encounter on the top
of really high mountains or at the cruising altitude of a
jet airplane. Due to the extreme conditions at which they
form, they tend to be comprised primarily of ice crystals.
High clouds do not block
sunlight.
High clouds include:
Cirrus
Cirrostratus
Cirrocumulus

Cirrus clouds are white wispy clouds that
stretch across the sky. By all accounts,
cirrus clouds indicate fair weather in the
immediate future. However, they can also
be an indication of a change in weather
patterns within the next 24 hours (most
likely a change of pressure fronts).
By watching their movement and the
direction in which the streaks are
pointed, you can get a sense of which
direction the weather front is moving.

Cirrostratus tend to be sheetlike and cover the whole sky.
You can usually tend to see
the sun or moon through
them. Their presence usually
indicates moist weather
within the next 12 - 24 hours.

Cirrocumulus clouds tend to be
large groupings of white streaks
that are sometimes seemingly
neatly aligned. In most climates
these mean fair weather for the
near future.
However, in the tropics, these
clouds may indicate an
approaching tropical storm or
hurricane (depending on the
season).

Middle clouds form at 6,500 to 23,000 feet.
They are comprised of water, and, if cold
enough, ice.
Middle clouds often block sunlight, but not
always.
Middle clouds consist of:
Altostratus
Altocumulus

Altostratus are grey and/or
blue clouds that cover the
whole sky. They tend to
indicate a storm some time in
the very near future since
they usually precede
inclimate weather.

Altocumulus are grayish-white
clouds blanketing the entire
sky. The tend to look like large
fluffy sheets in which there is
a lot of contrast between light
and dark. Sun does not pass
through them. If you see them
in the morning, prepare for a
thunderstorm in the
afternoon.

Low clouds form below 6,500 feet. These clouds are the
ones that like to hang-around just above tall buildings.
These clouds tend to contain water, but can also be
comprised of snow if the weather gets cold enough.
Low clouds block sunlight and can bring precipitation
and wind.
Low clouds include:
Stratus
Stratocumulus
Nimbostratus

Stratus are low-lying
solid clouds that are
often formed when fog
lifts off the ground.
They obviously look like
an elevated fog. Often
they bring drizzle or
light snow.

Stratocumulus are lowlying bumpy and grey
clouds. They do not
bring precipitation.
They also do not cover
the entire sky and tend
to come in rows and
patches.

Nimbostratus is your
standard rain cloud. It
is a large flat sheet of
grey cloud with a little
bit of differentiation. If
you see these, chances
are it's raining outside.

And last, but not least, are clouds with vertical
growth which tend to have a base that hangs
really low (5,000 feet) and a top that climbs
really high (over 50,000 feet).
Clouds in this category include:
Cumulus
Cumulonimbus

Cumulus clouds are your
stereotypical white
"cottonball" clouds. So long
as the clouds remain low
clumps floating across the
sky, there will be fair
weather. However, you
need to keep an eye on
these clouds because any
vertical growth can indicate
the start of a large storm.
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Cumulonimbus are
cumulus clouds that have
grown vertically into an
anvil-like shape. The anvil
tends to point in the
direction the storm is
moving. These clouds
bring most
dangerous weather such
as rain, lightning, hail and
A Cold Front symbol shows the edge of a mass
of cool air.
A Warm Front symbol shows the edge of a
warmer air mass.
A Stationary Front means neither the warm air or
the cool air are moving very much.
An Occluded Front symbol shows a warm front
being trapped by two masses of cold air.
•
•
•
•
If you look at a weather map, you will see areas
marked with an L. The L stands for “low,” and
indicates an area of relatively low air pressure.
Low air pressure usually causes clouds, wind, and
precipitation.
High pressure areas are usually called “highs” and
are marked as an H on a weather map.
High pressure areas usually cause dry, clear
weather
Dr. R. B. Schultz
*
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Air pollution and weather are linked in two ways. One way
concerns the influence that weather conditions have on the
dilution and dispersal of air pollutants.
The second way is the reverse and deals with the effect that air
pollution has on weather and climate.
Air is never perfectly clean.
Examples of “natural” air pollution include:
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Ash,
salt particles,
pollen and spores,
smoke and
windblown dust
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Although some types of air pollution are recent creations,
others, such as London's infamous smoke pollution, have
been around for centuries. One of the most tragic air pollution
episodes ever occurred in London in December 1952 when
more than four- thousand people died.
Air pollutants are airborne particles and gasses that occur in
concentrations that endanger the heath and well-being of
organisms or disrupt the orderly functioning of the
environment.
Pollutants can be grouped into two categories:
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(1) primary pollutants (point source), which are emitted directly from
identifiable sources, and
(2) secondary pollutants (non-point source), which are produced in the
atmosphere when certain chemical reactions take place among
primary pollutants.
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The most obvious factor influencing air pollution is the quantity
of contaminants emitted into the atmosphere.
However, when air pollution episodes take place, they are not
generally the result of a drastic increase in the output of
pollutants; instead, they occur because of changes in certain
atmospheric conditions.
Two of the most important atmospheric conditions affecting the
dispersion of pollutants are:
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(1) the strength of the wind
(2) the stability of the air.
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In most areas within several hundred kilometers of
large centers of human activity, the pH value is much
lower than the usual value found in unpopulated
areas.
This acidic rain or snow, formed when sulfur and
nitrogen oxides produced as by-products of
combustion and industrial activity are converted into
acids during complex atmospheric reactions, is called
acid precipitation.
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Beyond possible impacts on health, the damaging effects of
acid precipitation on the environment include the lowering of
pH in thousands of lakes in Scandinavia and eastern North
America.
Besides producing water that is toxic to fish, acid precipitation
has also detrimentally altered complex ecosystems by many
interactions at many levels of organization.
EPA and local officials use the AQI to provide
simple information about your local air
quality, how unhealthy air may affect you, and
how you can protect your health.
 The AQI focuses on health effects you may
experience within a few hours or days after
breathing polluted air.
 To make it easier to understand, the AQI is
divided into six categories.
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