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

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

One complete west-to-east spin on the axis

Takes 24 hours

While rotations take place, the earth is traveling in an orbital path around the sun

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

We call March 21 & September 23 “equinoxes”

Marks the beginning of spring/fall

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

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 20-September 23

At South Pole, sun never sets from September 23-March 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

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

There are four major factors that affect climate LATITUDE AIR PRESSURE MOUNTAINS ELEVATION CONTINENTAL POSITIONS OCEAN CURRENTS WIND PATTERNS STORMS

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

Equator

Tropic of Cancer

Tropic of Capricorn

Antarctic Circle

Arctic Circle HIGH

LOW

HIGH

MIDDLE

MIDDLE

90

90

0

23 1/2

23 1/2

66 1/2

66 1/2

Graphic created by EJoyce

What causes the temperature differences?

Low Pressure - cold, wet conditions. Example = hurricane.

High Pressure - hot, dry conditions. Example = TX in summer

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

For example, Mt. Kilimanjaro, near the equator in Africa, has snow year round

Coastal – stable weather conditions because oceans heat and cool slowly

Interior (land) – unstable conditions because land heats and cools quickly

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

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

Caused by LOW pressure systems and HIGH pressure systems colliding (interacting with one another)

Cannot predict the location of tornadoes or hurricanes

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

Near the equator

High year-round temperatures

Can be wet (rain forest) or arid (dry, desert)

Between equator and poles

Experiences more seasonal changes in climate than tropical or high-latitude regions

Near the poles

Low year-round temperatures

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.

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

Mid-latitudes are regions most prone to violent storms

Why? Wind patterns tend to cross over in the mid-latitude 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

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

High pressure system

Contains a region of sinking air

Usually creates clear skies and fair weather

Low pressure system

Contains a region of rising air

Usually creates clouds, rain and strong winds

Hurricanes are extremely low pressure systems