Causes of Weather

Ch. 12.1

What is ‘meteorology’?Meteorology: the study of

atmospheric phenomena Ex: clouds, breezes, warmth of sunlight

The root word “meteoros” is greek for high in the air.Anything that is high in the

sky is an example of a meteor. Ex: rain, snow, sleet, cloud droplets =

hydrometeors Ex: smoke, haze, dust = lithometeors Ex: thunder, lightning = electrometeors

Weather v.s. ClimateWeather: short term variations in

atmospheric phenomena that interact and affect the environment and life on Earth.Variations can take place over

minutes, hours, days, weeks, months, or years.

Climate: long term average of variations in weather for a particular area.Meteorologists use weather-data

that can average over 30 years of weather.

Heating Earth’s SurfaceImbalanced heating

Earth is tilted on its axis affecting the amount of daylight & solar radiation = affects the temperature in different regions.Ex: In January, Miami, FL is much

warmer than Detroit, MI.Miami has more direct solar radiation

than Detroit because of Earth’s tilt.Thermal Redistribution: Air & H2O move

thermal energy around to help maintain average temps.

Imbalanced Heating

Air MassesAir MassSpans over 1000’s of km’sa large volume of air that has the

same characteristics as its source region.Ex: same temp & humidity.

Source region: area over which the air mass forms.Most form over tropical or polar

regions.

Air Masses Around the World

5 Air Masses Affect United States

5 Types of Air Masses in United States

Air Type Mass

Weather Map Symbol

Source Region

Winter Character.

Summer Character.

Artic A Siberia, Arctic Basin

Bitter cold, dry

Cold, dry

Continental polar

cP Interiors of Canada & AK

Very cold, dry

Cool, dry

Continental tropical

cT Southwest US, Mexico

Warm, dry Hot, dry

Maritime polar

mP N. Pacific Ocean

Mild, humid

Mild, humid

Maritime polar

mP N. Atlantic Ocean

Cold, humid

Cool, humid

Maritime tropical

mT G. of Mex., tropical/subtrop. At. & Pa. Ocean

Warm, humid

Hot, humid

Air Mass Modification

Occurs when an air mass moves over land or water that has different characteristics than its source region.

Air mass acquires new characteristics from the land or water.

Weather SystemsCh. 12.2

Coriolis EffectThe direction of Earth’s winds are

influenced by Earth’s rotation.Air & water move in a curved path rather

than a straight line called the Coriolis effect.Northern hemisphere: air curves to the

rightSouthern hemisphere: air curves to the

leftAnimation

Transport colder air to warmer areas near the equator

Transport warmer air to colder areas near the poles.

Coriolis Effect

3 Wind SystemsAlso referred to

a “basic zones”Found at Earth’s

surface in both hemispheres.

They are polar easterlies, prevailing westerlies, & trade winds.

Polar EasterliesFound between 60ºN latitude & the north pole and 60ºS latitude and the south pole.

Begins as dense polar air that sinks.

Flows in a westerly direction away from the poles (comes from the east and moves west)

Typically cold winds.

Prevailing WesterliesLocated between 30 and 60ºN latitude and 30 and 60ºS latitude.

Winds move in a easterly direction toward each pole. Called “westerlies” because the

winds originate from the west and goes east.

Steady winds that move much of the weather across the U.S. and Canada.

Trade WindsFound between latitudes 30 degrees N and 30 degrees S latitude.

Air sinks at 30 N & 30 Smoves toward the equator in a

westerly direction.When air reaches the equator, it warms & rises.

NE Trade Winds & SE Trade Winds

Horse Latitudes

A belt of weak surface winds.Found near 30ºN AND 30ºS

latitudes due to the sinking air of the trade winds.Creates high pressure here.

Earth’s major deserts are under these high-pressure conditions.Ex: Sahara desert.

Intertropical Convergence Zone

Also called ITCZ & Equatorial Doldrums.

Found near the equator where the NE & SE trade winds meet.Air rises here = Creates low pressure & clouds form.Delivers moisture to many of the world’s tropical rain forests.

Jet Streams

Narrow band of fast, high-altitude, westerly wind.Speeds vary with the temperature

differences between the air masses at the wind zone boundaries.

Position varies with the seasons.Represents the strongest core of westerly winds.

Jet Stream

Jet Stream

2 Types of Jet StreamsMajor Jet Stream: Polar jet

streamFound between the polar

easterlies and prevailing westerlies in both hemispheres.

Moves west to east.Minor Jet Stream: Subtropical jet

streamFound between the trade winds

and prevailing westerlies in both hemispheres.

Jet Streams and Weather

Storms form along jet streams and generate large-scale weather systems.

Weather systems generally follow the path of the jet stream.

Also affect the intensity of the weather systems by moving air of different temps from one region to another.

FrontsTransition zone between 2 air

masses of different densities. Extend both vertically and

horizontallyCan cover thousands of

kilometers of Earth’s surface.Types of fronts:

Cold frontWarm frontStationary frontOccluded front

Fronts

Cold FrontsCold, dense air displaces warmer,

less dense air up a steep slope.Air behind front is colder than air

ahead of the front.Creates intense precipitation and

T-storms are common.Warm, moist air is forced up and

condenses.Symbol: Blue line with triangles.

Cold Front

Warm FrontWarm air displaces cooler air

and creates a gradual slope.Air behind front is warmer and

more moist than air ahead of front.

Causes widespread light precipitation & clouds OR no precipitation.

Symbol: red line with semicircles.

Warm Front

Stationary FrontForms when 2 air masses meet

and neither moves into the other’s area = stop moving.Happens when the air masses’

temps are similar to each other. Produces heavy precipitation &

possible flooding because the front stalls/stops.

Symbol: line with alternating blue triangles and red semicircles pointing in opposite directions.

Stationary Front

Occluded FrontForms when a cold air mass

RAPIDLY overtakes a warm air mass moving in the same direction.

Produces very strong winds & heavy rains.

Symbol: line of blue triangles and red semicircles point in the same direction OR a purple line with triangles & semicircles pointing in the same direction.

Occluded Front

Pressure SystemsHigh Pressure

Sinking air Air spirals outwardMoves clockwise in the

northern hemisphere.No Precipitation/Nice

weather.Low Pressure

Rising airAir spirals inwardMove counter-clockwise in

northern hemisphere.Precipitation/Rainy

weather.

Pressure SystemsAir always flows from areas of high pressure to low pressure.

Rotate because of the rising/falling of air and because of the Coriolis Effect.