Chapter 2 Section 3Chapter 2 Section 3

WindsWinds

What are winds?What are winds? A wind is the horizontal movement of air A wind is the horizontal movement of air

from an area of from an area of high high pressure to an area pressure to an area of of lower lower pressurepressure

WindsWinds are caused by differences in air are caused by differences in air pressurepressure

Convection CurrentsConvection Currents Form when an area of Earth’s surface is Form when an area of Earth’s surface is

heatedheated by the Sun’s rays by the Sun’s rays Air over the heated surface Air over the heated surface expandsexpands and and

becomes becomes lessless dense. As the air becomes dense. As the air becomes less dense, its air pressure less dense, its air pressure decreasesdecreases

Convection CurrentsConvection Currents

CoolCool dense air with a higher pressure flows dense air with a higher pressure flows underneath the underneath the warmwarm, less dense air= warm , less dense air= warm air risingair rising

How do we measure wind speed?How do we measure wind speed?

Winds are described by theirWinds are described by their direction direction andand speedspeed

AnemometersAnemometers are used to measure wind are used to measure wind speedspeed

Wind DirectionWind Direction

The name of a wind tells you where the wind The name of a wind tells you where the wind is coming is coming fromfrom

For example: A west wind comes from the For example: A west wind comes from the westwest and blows toward the and blows toward the easteast

Wind direction is determined with a Wind direction is determined with a wind wind vanevane

The wind swings the wind vane so that one The wind swings the wind vane so that one end points into the windend points into the wind

Wind Direction Represented on a MapWind Direction Represented on a Map

Wind directionWind direction is shown by a line leading is shown by a line leading into a circle into a circle

The example below shows that the wind The example below shows that the wind blew in from the blew in from the easteast

E

Wind Speed Represented on a MapWind Speed Represented on a Map

The wind speed is shown by The wind speed is shown by the “the “feathersfeathers” on the wind ” on the wind direction linedirection line

One “feather” =One “feather” =10 knots10 knots Half of a “feather”=Half of a “feather”=5 knots5 knots A circle within a circle= A circle within a circle=

calm, no windcalm, no wind A A flagflag=50 knots=50 knots (1 knot=1.15mph)(1 knot=1.15mph) (feathers are also (feathers are also

sometimes called “barbs”)sometimes called “barbs”)

Weather Symbol ExamplesWeather Symbol Examples Example #1: What is the wind speed and Example #1: What is the wind speed and

direction?direction?

Wind coming from the Wind coming from the easteast at a speed of at a speed of 85 knots 85 knots

Pressure and WindPressure and Wind IsobarsIsobars are lines joining places are lines joining places

on a weather map that have on a weather map that have the same the same air pressureair pressure

Meteorologists use isobars on Meteorologists use isobars on weather maps to observe weather maps to observe atmospheric atmospheric pressurepressure changes changes over an area and to make over an area and to make predictions about wind predictions about wind speedspeed

Remember-Wind is a result of Remember-Wind is a result of air pressure differences and air pressure differences and wind blows from high to low wind blows from high to low pressurepressure

The closer the isobars are= The closer the isobars are= the the fasterfaster the wind speed the wind speed

The further apart the isobars The further apart the isobars are= the are= the slowerslower the wind the wind speedspeed

Wind Chill FactorWind Chill Factor Wind blowing over your Wind blowing over your skin skin removes body heatremoves body heat The stronger the wind, the more The stronger the wind, the more frictionfriction is is

created from the air molecules hitting your body created from the air molecules hitting your body This creates heat energy which is absorbed by This creates heat energy which is absorbed by

the water near the surface of your skin, causing the water near the surface of your skin, causing it to it to evaporateevaporate

This evaporation removes heat from your body This evaporation removes heat from your body and makes you feel and makes you feel coldercolder

Local WindsLocal Winds Local winds are winds that blow over Local winds are winds that blow over shortshort distances distances Local winds are caused by the Local winds are caused by the unequal heatingunequal heating of of

Earth’s surface within a small areaEarth’s surface within a small area Unequal heating often occurs along the shore of a Unequal heating often occurs along the shore of a

large body of waterlarge body of water It takes It takes moremore energy to warm up a body of water energy to warm up a body of water

than it does to warm up an equal area of land (water than it does to warm up an equal area of land (water has a high specific heat)has a high specific heat)

During the day the During the day the landland warms up faster than the warms up faster than the waterwater (unequal heating) (unequal heating)

At night, the At night, the landland cools faster than the cools faster than the waterwater (unequal heating)(unequal heating)

Local WindsLocal Winds

Land BreezeLand Breeze –Local –Local wind that blows from wind that blows from the the landland to a body of to a body of water water during the nightduring the night

Sea BreezeSea Breeze -Local -Local wind that blows from an wind that blows from an oceanocean or or lakelake to the to the landland during the day during the day

Global WindsGlobal Winds Global winds are winds that blow steadily Global winds are winds that blow steadily

from specific directions over from specific directions over longlong distancesdistances

Global winds are created by the Global winds are created by the unequalunequal heating of Earth’s surfaceheating of Earth’s surface

WarmWarm air rises at the Equator and air rises at the Equator and coldcold air air sinks at the polessinks at the poles

Jet StreamsJet Streams About 10 km above Earth’s surface are bands of About 10 km above Earth’s surface are bands of

high-speed winds called high-speed winds called jet streamsjet streams Meteorologists use the location of some of the Meteorologists use the location of some of the

jet streams as an aid in jet streams as an aid in weather forecastingweather forecasting Jet streams generally blow Jet streams generally blow westwest to to easteast Jet streams form at the boundaries of adjacent Jet streams form at the boundaries of adjacent

air masses with significant differences inair masses with significant differences in temperature temperature (polar region vs. Equator)(polar region vs. Equator)

Jet streams have aJet streams have a meandering meandering shapeshape

Jet Stream ExampleJet Stream Example

Chapter 3 Section 1 WeatherChapter 3 Section 1 Weather

Air MassesAir Masses

A huge body of air that has similar A huge body of air that has similar temperature temperature (warm or cold),(warm or cold), humidity humidity (dry or wet), and (dry or wet), and air air pressurepressure (high or low) at any given height is called (high or low) at any given height is called an an air massair mass

Four major types of air masses influence the Four major types of air masses influence the weather in North America:weather in North America:– Maritime tropicalMaritime tropical– Continental tropicalContinental tropical– Maritime polarMaritime polar– Continental polarContinental polar

Types of Air MassesTypes of Air Masses

Continental: Continental: land (dry)land (dry) Mar: Mar: sea (wet)sea (wet) Tropical: Tropical: warm/hotwarm/hot Polar: Polar: coldcold

The area that the air mass forms over will determine its The area that the air mass forms over will determine its characteristicscharacteristics

For example:For example:– If the source region is an If the source region is an oceanocean, the air mass will have a , the air mass will have a

lot of lot of moisturemoisture– If the source region is If the source region is landland, the air mass will be , the air mass will be drierdrier

Putting it togetherPutting it together

Maritime Polar…Maritime Polar…– MoistMoist and and coldcold

Continental PolarContinental Polar……– Dry and ColdDry and Cold

Maritime Tropical…Maritime Tropical…– MoistMoist and and warmwarm

Continental TropicalContinental Tropical……– Dry and warmDry and warm

Air Masses and Their OriginAir Masses and Their OriginAir MassesAir Masses OriginOrigin

Maritime TropicalMaritime Tropical Gulf of Mexico and Atlantic Gulf of Mexico and Atlantic and Pacific Oceanand Pacific Ocean

Continental TropicalContinental Tropical Southwest and Northern Southwest and Northern MexicoMexico

Maritime PolarMaritime Polar North Pacific and North North Pacific and North AtlanticAtlantic

Continental PolarContinental Polar Central Central andand Northern Northern Canada Canada and and AlaskaAlaska

North American Air MassesNorth American Air Masses

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How Air Masses MoveHow Air Masses Move Prevailing Westerlies- Prevailing Westerlies- push air masses push air masses

west to eastwest to east Jet streamJet stream

FrontsFronts

The boundary where 2 unlike air masses meet is The boundary where 2 unlike air masses meet is called a called a frontfront

4 types of fronts4 types of fronts– ColdCold– WarmWarm– StationaryStationary– OccludedOccluded

Storms and changeable weather often develop Storms and changeable weather often develop along frontsalong fronts

Symbols for FrontsSymbols for Fronts

Front Symbols and DirectionFront Symbols and Direction

Front symbols point in the Front symbols point in the direction direction of of movementmovement

Example: In the picture below, the warm, Example: In the picture below, the warm, cold, and occluded fronts are moving cold, and occluded fronts are moving northnorth and the stationary front is not moving in any and the stationary front is not moving in any directiondirection

Front Symbols and DirectionFront Symbols and Direction

Draw a cold front moving east:Draw a cold front moving east:

Draw a warm front moving south:Draw a warm front moving south:

Draw an occluded front moving northeast:Draw an occluded front moving northeast:

Cold FrontsCold Fronts

A cold front is when a fast-moving A cold front is when a fast-moving cold cold air air mass takes over a mass takes over a warm warm air massair mass

Possible weather: Clouds with possible Possible weather: Clouds with possible thunderstorms with heavy rains or snow; thunderstorms with heavy rains or snow; cooler cooler weather on the wayweather on the way

Warm FrontsWarm Fronts

A warm front is when a A warm front is when a warm warm air mass air mass overtakes a slow-moving overtakes a slow-moving cold cold air massair mass

Possible weather: Humid, light rain or snow Possible weather: Humid, light rain or snow for several days; warmer weather on the for several days; warmer weather on the wayway

Stationary FrontsStationary Fronts

A stationary front is when cold and warm air A stationary front is when cold and warm air masses masses meetmeet, but neither can , but neither can move move the the otherother

Possible weather: The air masses remain Possible weather: The air masses remain stalledstalled over an area and may bring over an area and may bring manymany days of clouds and precipitationdays of clouds and precipitation

Occluded FrontOccluded Front

An occluded front is when a An occluded front is when a warm warm air mass air mass is caught between twois caught between two cooler cooler air massesair masses

Possible weather: Cloudy andPossible weather: Cloudy and rain rain oror snow snow

Front Symbols on a Weather MapFront Symbols on a Weather Map

Low Pressure and WeatherLow Pressure and Weather

If you look at a weather map, you will see areas marked with a letter If you look at a weather map, you will see areas marked with a letter LLand the L stands for “and the L stands for “low pressurelow pressure””

Low pressure systems lead to Low pressure systems lead to cloudycloudy conditions that often bring conditions that often bring precipitationprecipitation

Here’s how it works:Here’s how it works:– Areas of Areas of lowlow pressure are caused by massive amounts of air pressure are caused by massive amounts of air risingrising from the ground from the ground

into the atmosphere into the atmosphere As the air molecules rise, they take their As the air molecules rise, they take their massmass with them with them

– Air pressure is a Air pressure is a forceforce– Force = Force = massmass x x accelerationacceleration– Less mass of air molecules left at ground level means Less mass of air molecules left at ground level means lowlow air pressure air pressure

– As this air rises it takes whatever water vapor it is holding along with itAs this air rises it takes whatever water vapor it is holding along with it– As altitude increases in the troposphere, the temperature As altitude increases in the troposphere, the temperature decreasesdecreases– Eventually the air mass reaches a temperature at or below the Eventually the air mass reaches a temperature at or below the dew pointdew point and the and the

water vapor water vapor condensescondenses Relative humidityRelative humidity rises because colder air cannot hold as much water vapor rises because colder air cannot hold as much water vapor

– This condensation forms cloudsThis condensation forms clouds If there is enough water vapor in the air, it might also bring If there is enough water vapor in the air, it might also bring precipitationprecipitation

High Pressure and WeatherHigh Pressure and Weather If you look at a weather map, you will see areas marked with a letter If you look at a weather map, you will see areas marked with a letter HH

and the H stands for “and the H stands for “high pressurehigh pressure””

High pressure systems lead to High pressure systems lead to clearclear and and calmcalm weather weather

Here’s how it works:Here’s how it works:– Areas of Areas of highhigh pressure are caused by massive amounts of air pressure are caused by massive amounts of air sinkingsinking from from

the upper troposphere down towards the ground the upper troposphere down towards the ground As the air molecules sink, they take their As the air molecules sink, they take their massmass with them with them

– Air pressure is a Air pressure is a forceforce– Force = Force = massmass x x accelerationacceleration– More mass of air molecules coming to ground level means More mass of air molecules coming to ground level means highhigh air pressure air pressure

– As this air sinks it takes whatever water vapor it is holding along with itAs this air sinks it takes whatever water vapor it is holding along with it– As altitude decreases in the troposphere, the temperature As altitude decreases in the troposphere, the temperature increasesincreases– The higher the temperature is, the farther from the The higher the temperature is, the farther from the dew pointdew point you get you get

Warmer air is less dense so it has more space to hold water vapor so Warmer air is less dense so it has more space to hold water vapor so relative relative humidityhumidity drops drops

– This means that condensation does not occur so no clouds formThis means that condensation does not occur so no clouds form Because there are no clouds, there will be no Because there are no clouds, there will be no precipitationprecipitation

Weather Map ExampleWeather Map Example

Weather Map ExampleWeather Map Example