Chapter 20 – WeatherChapter 20 – Weather

• Page 435

• How do you think the waves caused damage to this house?

• If this was your house, would your rebuild in the same place?

• What can people do to avoid such damage?

• How are you able to predict the weather?

Chapter 20.1Chapter 20.1

Air Masses and WeatherAir Masses and Weather

• Meteorology – the study of processes that govern Earth’s Atmosphere

Origin of an Air MassOrigin of an Air Mass

• An Air Mass is a large body of air with the same humidity and temperature– The humidity and temperature of the air mass

is determined by where they form• Over a southern ocean – moist and warm

– When the air mass travels, it takes with it the temperature and humidity of its place of origin.

Origins of air massesOrigins of air masses

• Classification due to where they form– Continental Arctic (cA) – extremely cold and dry

• Forms over land (continental) and cold temperature causes it to be dry.

– Continental Polar (cP) – cold and dry– Maritime Polar (mP) maritime – moist; plus wet

and cold– Maritime Tropical (mT) – warm and wet– Continental Tropical (cT) – hot and dry

Chapter 20.2Chapter 20.2

Fronts and LowsFronts and Lows

• What is a front?– Front – Boundary that separates opposing

air masses• Can range from 200 meters, to 200 kilometers

• Can be as high as 5 kilometers

• Can be as long as 2000 kilometers

– Air masses on either side differ in humidity, temperature and wind direction.

Kinds of frontsKinds of fronts

• Cold front – boundary between an advancing cold air mass and warmer air mass it is displacing.– Cold air denser therefore it slides under the

warm air in front of it, forming a steep slope– The precipitation along the cold front is

usually heavy and fast (thunderstorms)– However, the passing front may cause no

greater change than a shift in wind direction.

• Warm front – boundary between an advancing warm air mass displacing a cold air mass– Warm air is less dense therefore, it rises up over

the cold air forming a gentle slope.– The first signs of an approaching warm front

are high cirrus clouds, which are followed by cirrostratus then lower stratiform clouds.

– Eventually nimbostratus clouds which give steady rain or snow.

– Occluded front – occurs when the faster moving cold front catches up to a warm front.

• The warm air gets pushed up between the two cold air masses causing cloudiness and precipitation.

– Stationary front – front is not moving forward

• May give many days of steady rain causing flooding.

Life Cycle of a Mid-Latitude LowLife Cycle of a Mid-Latitude Low

• Warm air mass meets a cold air mass in the mid latitudes

• Circulation begins due to the warm air moving northward and the cold southward (p. 442)

• The circulation around the Low is counterclockwise in the Northern Hemisphere

• This counterclockwise motion of a Low sucks air off the earth’s surface. Air is constantly spiraling into a low-pressure system.

• Troughs and Highs (p. 443)• Low pressure is associated with unfavorable

weather.• High pressure is associated with clear

conditions.

Chapter 20.3Chapter 20.3

Thunderstorms and TornadoesThunderstorms and Tornadoes

Thunderstorms – storms with Thunderstorms – storms with lightning and thunder formed in lightning and thunder formed in

cumulonimbus clouds.cumulonimbus clouds.• The cloud can be as tall as 20km

• Formed in convection cells – warm air being lifted up while cool air descends (p. 445)

• Often form along fronts there may be many cells

• Squall line – many thunderstorms along a front

• Supercells – very large singlecell thunderstorm that can produce tornadoes

• Lightning – a discharge of electricity – cloud to cloud, cloud to ground. Can occur in thunderstorms, snowstorms, dust storms or volcanic eruptions.

Tornadoes – byproducts of Tornadoes – byproducts of supercell thunderstormssupercell thunderstorms

• Violently rotating column of air• Tornado formation• Form from between the wall clouds of a

mesocyclone (p. 447)• A tornado’s funnel cloud results when

the air pressure at its center is very low and air sucked into the funnel expands and cools; water vapor in the air condenses.

Storm and Tornado watches Storm and Tornado watches and warningsand warnings

• Watch – conditions are right

• Warning – one has been spotted

Chapter 20.4Chapter 20.4

Hurricanes and WinterstormsHurricanes and Winterstorms

Hurricanes – huge rotating storm of Hurricanes – huge rotating storm of tropical origin that has sustained tropical origin that has sustained

winds of at least 119 km/hwinds of at least 119 km/h

• Winds and rain are strongest at the eye wall

• Hurricanes rely on the transfer of heat from the ocean, they form only when surface ocean waters are sufficiently warm, and they weaken as soon as they make landfall.

• Steered by global wind patterns

• Storm surge results, in part, from strong winds of the eye wall which blows water into a broad dome.

• If storm surge strikes land the same time as high tide, hurricane disaster worsens.

• Hurricanes are ranked according to the Saffir-Simpson Hurricane Scale on p. 452

Winter StormsWinter Storms

• Blizzard – must have winds higher than 56 km/hr, temperature –7°C or lower, and reduced visibility due to falling or blowing snow

Chapter 20.5Chapter 20.5

Forecasting WeatherForecasting Weather

Gathering dataGathering data

• Satellites – visible images and infrared– Visible – the whiter the clouds the thicker –

meteorologists can track the clouds to get speed and direction.

– Are not available at night– Infrared satellite – use temperature to plot

colors– The cooler the cloud tops the higher it is in the

atmosphere– Can be used at night

• Rawinsondes – measure temperature, pressure and humidity of air at different altitude.– Attached to a large balloon and tracked by

radar– Identifies the shape of the jet stream

• Surface Observations– Most are at airports– Information can help to locate fronts, highs

and lows– Provide – temperature, dew point, barometric

pressure, wind speed and direction, visibility, precipitation, height of clouds and the amount.