Chapter 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 20.1

Air Masses and Weather Meteorology – the study of processes that govern Earth’s Atmosphere

Origin 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 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 20.2

Fronts 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 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.

Stationary front – front is not moving forward 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 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 20.3

Thunderstorms and Tornadoes

Thunderstorms – storms with lightning and thunder formed in 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 supercell 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 and warnings Watch – conditions are right Warning – one has been spotted

Chapter 20.4

Hurricanes and Winterstorms

Hurricanes – huge rotating storm of tropical origin that has sustained winds 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 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 20.5

Forecasting Weather

Gathering 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.