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Weather
 Atmospheric condition in one place during a
limited period of time
Climate
 Weather patterns that an area typically
experiences over a long period of time
Many factors influence weather & climate
 No factor is more influential than the earth’s
position in relation to the sun
Let’s review with a quick video
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Axis
 An imaginary line running from pole to pole at
an angle of 23.5 degrees
 Because of the angle, not all places receive the
same amount of direct sunlight at same time
Rotation
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One complete west-to-east spin on the axis
Takes 24 hours
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While rotations take place, the earth is traveling in an
orbital path around the sun
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This is called a revolution
Takes 365.25 days
Combination of tilt & revolution creates distinct
seasons for most places in the world
Seasons are reversed for areas north & south of the
Equator
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We call March 21 & September 23 “equinoxes”
 Marks the beginning of spring/fall
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These lines represent the northernmost and
southernmost places where direct rays hit the
earth…this happens on “solstice” days
 June 21: rays hit Tropic of Cancer
 December 22: rays hit Tropic of Capricorn
 These dates represent the start of
summer/winter
Length of daylight for a place varies year-round
based on how much direct & indirect sunlight the
place receives
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Amount of sunlight at the poles varies more
dramatically than in any other place on earth
 At North Pole, the sun never sets from March 20September 23
 At South Pole, sun never sets from September 23March 20
Therefore, if one goes to northern Alaska in the
summertime, the sun rarely sets
 Conversely, in the wintertime the sun rarely
appears
Remember: at the equinox, the sun is directly above the equator
Let’s review with a quick video
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Earth’s atmosphere traps much of the warmth from the sun
Without this “greenhouse effect” the earth would be too
cold for most living things
Levels of carbon dioxide in the atmosphere have risen
rapidly in recent decades as a result of human interaction
with the environment
 Scientists call the coinciding rise in global temperatures
“global warming”
 This is controversial
 Worst case scenario: melting of ice caps & mountain
glaciers that could cause a rise in sea levels that would
submerge coastal lands
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There are four major factors that affect climate
 LATITUDE
 AIR PRESSURE
 MOUNTAINS
 ELEVATION
 CONTINENTAL POSITIONS
 OCEAN CURRENTS
 WIND PATTERNS
 STORMS
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There are three latitude zones
 Within each zone, climates follow patterns
Low Latitudes
 Runs from tropic line to tropic line
 Area is warm or hot almost year-round
 Rainfall varies from place to place
High Latitudes
 Includes areas north of Arctic Circle &
south of Antarctic Circle
 Cool to cold almost year-round
Mid-Latitudes
 Includes everything else
 Has dramatic seasonal weather changes
90
HIGH
Arctic Circle
66 1/2
MIDDLE
Tropic of Cancer
LOW
23 1/2
Equator
0
Tropic of Capricorn
23 1/2
MIDDLE
Antarctic Circle
66 1/2
HIGH
Graphic created by EJoyce
90
What causes the temperature differences?
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Low Pressure - cold, wet conditions.
Example = hurricane.
High Pressure - hot, dry conditions.
Example = TX in summer
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Coastal lands tend to have less dramatic changeable
weather
 Less variance in rainfall & temperatures
Another important landform impact is the “orographic
cycle” also known as the “rain shadow effect”
 Winds that come in from the ocean are pushed upward
when the meet a mountain range
 The coastal or “windward” side is typically cooler and
wet
 As the winds reach the interior or “leeward” side of the
mountain, the air is drier and warmer
 Many times deserts develop in leeward regions
Windward side of
the Mountain
Leeward side of
the Mountain
Rain
Shadow
Graphic created by EJoyce
Air gets thinner as altitude increases
 Thinner air retains less heat
 Despite fact that sunlight is brighter
in higher places
 As elevation increases, temperatures
decrease
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For example, Mt. Kilimanjaro, near the equator in
Africa, has snow year round
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Coastal – stable weather conditions
because oceans heat and cool
slowly
Interior (land) – unstable
conditions because land heats and
cools quickly
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Ocean currents are caused by many of the same factors
that cause winds
Cold water tends to flow from poles to the equator
Warm water tends to flow from equator to the poles
Wind patterns & ocean currents have one major thing in
common
 They either cool or warm up areas they pass or hit
 North Atlantic Drift, a warm-water ocean current,
keeps Europe mild despite its northern latitude
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Wind occurs because the sun heats up the Earth’s atmosphere
& surface unevenly
 Interaction of warm air & cool air as well as pressure
systems
All areas have prevailing wind patterns
 They tend to be horizontal
 They have names based on their locations
 Trade winds are north & south of equator
 Westerlies are in mid-latitude areas
 Polar easterlies are in polar areas
 Area near equator is called the “doldrums”---least
amount of wind
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Caused by LOW pressure systems
and HIGH pressure systems
colliding (interacting with one
another)
Cannot predict the location of
tornadoes or hurricanes
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While the text and other references discuss numerous
categories of climates, we will focus on just four
 Tropical
 Mid-Latitude
 High Latitude
 Highlands
Different climate types exist within each category
Different vegetation types also exist within each
climate category
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Near the equator
High year-round temperatures
Can be wet (rain forest) or arid (dry, desert)
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Between equator and poles
Experiences more seasonal changes in
climate than tropical or high-latitude
regions
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Near the poles
Low year-round temperatures
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High mountainous areas
Usually maintain low year-round
temperatures, regardless of their location
For example, the
Himalaya
Mountains show
cold on this
temperature map
despite the
latitude zone.
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El Nino is a periodic reversal of the pattern of
ocean currents & water temperatures in the
Pacific
 Causes precipitation to increase along the
western coasts of North & South America
 Causes drought in Southeast Asia & Australia
La Nina does the direct opposite
No one really knows why either of these two
climatic events take place
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Mid-latitudes are regions most prone to violent storms
 Why? Wind patterns tend to cross over in the midlatitude region
U.S. experiences more tornadoes than any other country
Most massive storms develop in the tropics, but can end
up being most destructive in the mid-latitudes
 Hurricanes in the Atlantic
 Typhoons in the Pacific
 Cyclones in the Indian
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Definition of a front
 Two air masses of widely different
temperatures or moisture levels meet
Rainfall tends to happen along these fronts
Once again, the mid-latitudes are the
regions that are most prone to frontal
systems
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High pressure system
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Contains a region of sinking air
Usually creates clear skies and fair weather
Low pressure system
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Contains a region of rising air
Usually creates clouds, rain and strong winds
 Hurricanes are extremely low pressure systems