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Chapter 20. Weather. Section 20.1. Meteorology- study of weather. Air mass: - large body of air, in the lower troposphere. -May be several kilometers in diameter and height -has similar temperature and humidity through out the air mass.

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chapter 20

Chapter 20


section 20 1
Section 20.1
  • Meteorology- study of weather
Air mass:

- large body of air, in the lower troposphere.

-May be several kilometers in diameter and height

-has similar temperature and humidity through out the air mass

-the temperature and humidity of the air mass depends on where the air mass originates

-air masses from the poles are cold and air masses from the tropics are warm.

-Air masses formed over land will be dry

- Air masses formed over water will be humid

As an air mass moves, the temperature will begin to change

ex: Air masses moving south from Canada will slowly warm up

5 types of air masses
5 types of air masses
  • Classified by where they originate

-Temperature is determined by where it began:

-Arctic, Polar, or Tropical

-Humidity is determined if air mass began over sea (maritime) or over land (continental)

Continental Arctic cA
    • Originates over arctic land
    • Extremely cold, it may warm up slightly
    • Very dry air
Continental Polar cP
    • Originates over Alaska and Canada
      • A bit warmer than cA
      • Air is dry, but it will pick up moisture as it passes over the great lakes, bringing snow on the other side. This is called lake effects snow
Maritime Polar mP
    • Originates over ocean in high latitudes so it is cold and damp
    • b/c the ocean is warmer than the continents the mP air masses are slightly warmed than the cP air masses
These air masses cause the fog and damp weather on the west coast.
  • mP air masses from the Northern Atlantic may cause Nor’easters on the east coast
Maritime tropical mT
    • Air masses over warm topical oceans
    • Air is warm and humid
    • Can cause summer time thunder storms
Continental Tropical cT
    • Originates over hot and dry deserts
    • May begin as a maritime air mass, but becomes dry as it passes over a mountain range
    • In the summer it creates heat waves w/o thunderstorms
    • Can cause drought conditions
section 20 2
Section 20.2
  • Fronts and lows
Different air masses don’t always mix well together
  • The boundary between two air masses that meet is called a front
  • A front can stretch over 2000 km-so it affects the weather in several states
Polar air masses and tropical air masses will usually meet in mid- latitudes
  • The temperature and humidity may be very difference on either side of the boundary
When 2 air masses meet- the less dense one (warmer) is forced up over the denser air mass (colder)
  • This creates a wedge shape of the air mass
Four kinds of fronts

1. cold

2. warm

3. occluded

4. Stationary

Cold front-boundary between a cold air mass and warm air mass
  • The cold air is denser and it slides under the warmer air.
  • The cold air mass is slower because of the friction of the mass with the ground.
When a cP meets with a mT in the summer- thunderstorms are created in winter a cold front may bring a quick rain/snow storm
If a dry air mass meets another dry air mass- there may not be much change in the weather- maybe change wind direction
Warm front:
  • a warm front pushes out a cold front
Warm air rises over the slower cold front
  • Weather changes are not dramatic
  • Produces high cirrus clouds changing to thicker nimbus clouds
  • Clouds thick enough to block the sun/moon
Occluded front
  • 2 Cold fronts squeeze a warm front
  • The warm front has to rise over the cold air
Stationary front- not moving
  • If it carries rain- it could lead to local flooding
  • Thunderstorms
  • Contain thunder lightning and sometimes hail
  • Usually only a few kilometers across
  • Started in a cumulonimbus clouds
  • Form in warm moist air
Clouds may be 20 kilometers high
  • May occur at any hour- but mote often in the afternoon (because the air is warmer then)
Thunderstorms are formed as convection cells (page 445)
  • The convection cell has a life span of about 1 hour. There are many convection cells in a thunderstorm
  • A frontal thunderstorm is one that occurs ahead of a weather front. Often called a squall line. Contains strong winds and may lead to flooding
Super cell- large single cell thunderstorm with strong rotating updrafts.
  • These may create a tornado
Lightning- found in all thunderstorms
  • -discharge of electricity that may follow a path of :

Cloud to cloud

Cloud to ground

One point in a cloud to a second point in the same cloud

Lightning occurs in snow storms, dust storms, and volcanic eruptions
  • Unsure how clouds become electrically charged, cloud particles may bump against each other creating a charge or even a spark
Lightning heats the air to 25,000 C
  • This causes an expansion in the air which we hear as thunder
  • Light travels faster than sound. So even though thunder and lightning start at the same place, we see the lightning before we hear the thunder
To figure out the distance between you and where the lightning originated:
  • Count the seconds between the time you see the lightning and when you hear the thunder. Divide the seconds by 5. and that will tell you how many miles away the lightning is.
Heat lightning- lightning we see, but don’t hear the thunder.
  • We can see farther than we can hear. The thunder exists- it’s just too far away to hear
  • A byproduct of a thunderstorm
  • It is a rotating column of air
  • Formation is unpredictable
Begins as an updraft of air called a mesocyclone.
    • Winds at lower altitudes blow at a different direction than the wind at higher altitudes
      • 1/3 of mesocyclones become a tornado
Tornadoes appear funnel shape

may be a meter wide to 1 km wide

May occur anywhere and anytime of year.

US has the highest number of tornadoes than anywhere else in the world

Tornado Alley- Texas to South Dakota. Usually from Spring to early summer (March to May)
  • Tornado Watch- conditions are right for a tornado to form
  • Tornado Warning- Tornado has been observed
  • Area of low pressure – wind and rain spiral around. May include tornadoes
  • Can be 300- 600 km across
The eye- is an area of no rain but does include the storm surge
  • The storm surge is a bulge of water that moves very fast. It brings flooding water onto land.
  • The eye can be 15-50 km across
naming hurricanes
Naming hurricanes
  • Before 1953, hurricanes were named for where they came ashore
  • 1953- hurricanes were given women's names in alphabetical order
  • 1970’s- hurricanes are now given men and women's names, alternating and alphabetically.
Names are repeated in 6 year cycles
  • Names are retired if the storm was particularly note worthy
  • Storms are named when they are tropical storms (65 km/hour winds)
Hurricane season is from May to November

this is because the ocean water is warmest

What stops a hurricane?

cold water, sheering winds, land

Station model-
  • Use of symbols on a map to track weather