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Chapter 17
Weather Patterns
Air Masses and Fronts
Air mass- huge body of air that has similar temperature,
humidity, and air pressure
- may be spread up to 10 km deep
Types of Air Masses
Four major types
Maritime tropical
- warm humid air that forms over tropical oceans
-move North and northeast
- influence weather in central and eastern US
Maritime Polar
-Cool Humid
- forms over the icy cold North Pacific and North
Atlantic
- effects west coast brings fog and rain
Continental Tropical
- hot dry
- forms over land
- smaller air mass
- hot dry air to southern Great Plains
Continental Polar
- large air mass formed over northern Canada and
Alaska
- bitterly cold with low humidity
How Air Masses Move
move by Prevailing Westerlies and Jet streams
Prevailing Westerlies
- major wind belts over continental US
- pushes air masses from west to east
How air masses move
(cont)
Cyclones and Anticyclones
Jet streams
- are within the prevailing westerlies
- bands of high speed winds
Fronts
-
boundary where the air masses meet
storms develop along fronts
cold air sinks and warm air rises
EX. The water experiment
Types of Fronts
4 kinds of fronts
1. Cold Front
- cold air is dense so it sinks
- warm air less dense so it rises
A fast moving cold air mass overtakes a warm air mass
2. Warm Front
- clouds and precipitation come with warm fronts
A warm air mass overtakes a slow-moving cold air mass
3. Stationary Front
- Cold and warm air masses meet but neither can move
the other
4. Occluded Front (cut off)
- A warm air mass is caught between two cooler air
masses
Cyclones and
Anticyclones
Cyclone- a swirling center of low pressure (means
wheel)
“Lows”
Cyclones and decreasing air pressure are associated with
clouds, wind and precipitation.
Large part of weather of US
Winds in a cyclone spin counterclockwise
Anticyclones- opposite of cyclone
High pressure centers of dry air
“Highs”
Winds spin clockwise
The descending air in a anticyclone generally causes dry
clear weather.
Section 2
Storms
-
violent disturbance in the atmosphere
causes sudden changes in air pressure (rapid air
movements)
Thunderstorms
Thunderstorm- small storm accompanied by heavy
precipitation and frequent thunder and lightening
Form in large cumulonimbus clouds (thunderheads)
On hot humid afternoons or when warm air is forced
upward along cold front
When warm humid air rises rapidly within a
cumulonimbus cloud
Lightening- sudden electrical discharge between clouds,
or ground
Thunder- rapidly heated air expands suddenly and
explodes making the sound
Thunderstorm Safety
- Avoid places where lightening may strike
- Avoid objects that can conduct electricity (golf
club or metal bat, bodies of water)
Tornadoes
Rapidly whirling funnel shaped cloud that reaches down
from a storm cloud to touch Earth’s surface
- brief but may be deadly
Form
In cumulonimbus clouds (same as thunderstorms)
Tornado Alley
- Iowa, Nebraska, Kansas, Oklahoma, Texas
(Great Plains states)
Tornado Damage
- From strong winds and flying debris
- Rated on the fujita scale
F0 = light damage
F5= extreme damage (1% of tornados)
Tornado Safety
Tornado watch- tornados are possible
Warnings- tornado has been seen or on radar
Safest place is in storm shelter or basement
Hurricanes
Tropical cyclone that has winds of 119 kilometers per
hour or higher
Form in Oceans
In Western pacific called typhoons
Form
Begins over warm ocean water as a low pressure area or
tropical disturbances
Grows in size then to tropical storm to hurricane