Chpt 16 Section 2 Weather Patterns
I. Weather Changes-Because air and moisture in the atmosphere, weather
constantly changes
A. Air Masses
1. A large body of air that has properties similar to the part of
Earth’s surface over which it developed.
2. Air mass over water has more water vapor
3. Air mass of dry land has less water vapor
4. Can cover 1,000s of miles
B. Highs and Lows
1. Atmospheric pressure varies over the surface of Earth
2. Wind blows from high pressure to low pressure
3. Earth’s rotation causes winds to swirl in a counter-clockwise
direction
4. Large, swirling areas of low pressure are called cyclones and
associated with stormy weather.
II. Fronts-a boundary between two air masses of different density, moisture,
or temperature.
A. Cold and Warm Fronts
1. Blue lines with triangles
2. Occurs when colder air advances toward warm air. The cold
air wedges under the warm like a plow.
3. As the warm air cools, it condenses, forming clouds.
Thunderstorms and even tornadoes may form when the
temperature difference between the cold and warm air is large
4. Warm fronts form when lighter, warmer air advances over
heavier, colder air. A warm front is drawn on maps as a red line
with semicircles.
B. Occluded and Stationary Fronts
1. Occluded involve three air masses-colder air, cool air, and
warm air.
a) Forms when a cold air mass moves toward cool air
with warm air between them.
b) The colder air forces the warm air up and closing off
the warm air from the surface.
c) Are shown as a purple line with triangles and
semicircles.
2. Stationary Fronts
a) Occurs when a boundary between air masses stops
advancing.
b) May remain in same place for several days, producing
light wind and precipitation.
c) Shown on map as a alternating red and blue line the
red semicircles pointing toward the cold air and blue
triangles pointing toward the warm air.
C. Severe Weather
1. Thunderstorms
a) Heavy rains, lightning, thunder and hail.
b) Occur in warm moist air masses and along fronts.
c) Small droplets collide to form larger ones
d) Raindrops cool the air around them
e) Cool, dense air sinks and cause strong updrafts of
warmer air causing high winds
f) Hail may also form as ice crystals alternately fall to
warmer layers and are lifted into colder layers
2. Thunderstorm Damage
a) Flooding-Some thunderstorms can stall over an area
causing rain to fall for a long time
b) Strong Winds
c) Hail
3. Lightning and Thunder
a) Clouds become oppositely charged, current flows
between regions and lightning occurs
b) Thunder-rapid heating and expanding of air (30,000
air cools and contracts causing thunder
4. Tornadoes
a) Violently rotating column of air in contact w/ ground
b) Wind shear-difference in wind speed and direction
parallel to ground
c) An updraft of air tilts the column upward creating a
funnel cloud-contacts the earth called a tornado
5. Hurricanes-most powerful storm
a) Winds must be 119 mph
b) Low pressure of Atlantic ocean-must be 80-C
c) Rotates counter-clockwise
d) Begin off coast of Africa
e) Reaches land-loses power
6. Blizzards-winter storm w/winds of 56mph and low visibility
of 400m-lasting 3 hours or more
7. Sever Weather Safety-issues warnings when conditions are
favorable-take immediate action