1. 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?

2. Chapter 20.1

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

4. 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.

5. 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

8. Chapter 20.2

9. 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.

10. 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.

11. 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.

12. 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.

13. 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

14. 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.

15. Chapter 20.3

16. Thunderstorms and Tornadoes

17. 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

18. 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.

19. 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.

20. Storm and Tornado watches and warnings
Watch – conditions are right
Warning – one has been spotted

21. Chapter 20.4

22. Hurricanes and Winterstorms

23. 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

24. 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

25. 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

26. Chapter 20.5

27. Forecasting Weather

28. 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

29. 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

30. 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.