1. Weather patterns and severe storms
Earth Science Ch. 20
Weather patterns and severe storms

2. 20.1 Air Masses
An air mass is a large body of air that is located in the troposphere and is characterized by similar temperatures and moisture content.

3. An air mass carries the temperature and moisture conditions from the area where it formed.
Air masses are named for the source region.

4. Temperature characteristics
Polar (P): high latitudes, cold
Tropical (T): low latitudes, warm
Moisture characteristics
Continental (c): form over land, dry
Maritime (m): form over water, humid

5. Most of our weather is influenced by cP and mT air masses

6. Continental Polar (cP)
Lake effect snows are caused by cP air masses that move over warm bodies of water.

7. Indian Summers are caused by cT air masses that move over the great lakes during the fall causing mild weather.

8. 20.2 Fronts
A front forms when 2 air masses with different properties meet.
The front is the boundary between the 2 air masses.
Fronts are often associated with some form of precipitation

9. Types of Fronts Warm Fronts
Forms when warm air moves into an area formerly covered with cooler air.
Light to moderate precipitation over a large area for an extended period.

10. Cold Fronts
Forms when cold dense air moves into a region occupied by warmer air.
More violent weather than a warm front (heavy rain and gusty winds).

11. Stationary Front
Forms when 2 air masses come together but neither is moved.
Gentle to moderate precipitation

12. Occluded Front Forms when a cold front overtakes a warm front.
The cold air lifts the warm air off of the ground.
Generally complex weather

14. Middle-Latitude Cyclones
Mid latitude cyclone characteristics:
winds spinning counterclockwise around a low (L) pressure center
the main weather producers in the US
travel from west to east

15. How cyclones form Cyclones develop from stationary fronts.
The front forms as 2 air masses move in opposite directions.
The warm air moves toward the poles and the cold air moves toward the equator.

16. The air moves in a counterclockwise direction.
The cold front moves faster than the warm front and eventually closes in and lifts the warm front.
A cyclone is sustained as long as the air flow is strong enough.

20. 20.3 Severe Storms Thunderstorms
Thunderstorms develop due to vertical movement of warm unstable air.
Thunderstorms are commonly produced by cumulonimbus clouds.

21. 3 stages of development
Cumulus stage
strong updrafts supply moist air and the cloud grows rapidly.
Mature stage
Heavy precipitation forms.
Dissipating stage
Downdrafts form
The cooling air causes the storm to die down.

23. Tornadoes
Tornadoes are violent wind storms that take the form of a rotating column of air called a vortex.
Tornadoes form from cumulonimbus clouds
Tornadoes occur most frequently in Florida and in the south central US
The frequency of tornadoes is greatest from April – June
Most tornadoes form in association with severe thunderstorms.
Tornadoes form from mesocyclones.
A mesocyclone is a vertical cylinder of rotating air that develops in the updraft of a thunderstorm

26. Because the air in a tornado rises due to low pressure, the air near the ground rushes into the tornado from all directions
The air that rushes in spirals upward around the core
The spiraling air merges with the air of the cumulonimbus cloud.

28. A scale commonly used to estimate tornado intensity is the Fujita scale
The ratings of the scale assess the damage caused by the storm
Tornado watches alert people to the possibility
Tornado warnings are issued when a tornado has been sighted

31. Hurricanes Hurricanes are the most violent storms on earth
Hurricanes are tropical cyclones that produce winds of at least 119 Km/hr (74 mph)
Hurricanes are associated with strong winds and extensive flooding

32. Most hurricanes form between 5° and 20° of the equator
Hurricanes form in this region because ocean water is warm
Hurricanes begin in the late summer when water temperatures are warm enough to provide heat and moisture
Hurricanes can last for a week or more

34. The energy for a hurricane comes from the condensing of huge amounts of water vapor
In a well developed hurricane, warm moist surface air rushes inward toward the core of the storm
The air turns upward and rises in a ring of cumulonimbus clouds

36. The area surrounding the center of the hurricane is called the eye wall.
The eye wall is where you find the greatest wind speeds and heaviest rain fall.
The eye wall is surrounded by curved bands of clouds.

38. At the center of the hurricane is the eye
Precipitation stops and winds weaken in the eye
The eye wall has the highest pressure and the eye has the lowest pressure

40. The Saffir-Simpson scale describes the intensity of a hurricane
The most devastating damage from a hurricane is caused by storm surges
A storm surge is a dome of water that sweeps across a coast
A hurricane weakens when it moves over cool ocean water or land, or encounters weaker air flow.

44. El Nino and La Nina
During normal conditions, the Trade winds blow from east to west.
The trade winds push warm surface water towards Asia.
Every 3 to 7 years however, the strength of the trade winds changes.
El Nino
Trade winds weaken and the warm surface water moves eastward toward S. America.
Upwelling decreases
La Nina
Stronger than normal Trade winds
Upwelling once again occurs

45. What are El Nino and La Nina?
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