Download Chapter 20 – Weather

Chapter 20 – Weather
I.
Air Masses and Weather
 Meteorology – the study of processes that govern
Earth’s Atmosphere
A. Origin of an Air Mass
1. An Air Mass is a large body of air with the same humidity
and temperature
2. The humidity and temperature of the air mass is
determined by where they form
i.
Over a southern ocean – moist and warm
3. When the air mass travels, it takes with it the
temperature and humidity of its place of origin.
B. Origins of air masses
1. Classification due to where they form
a. Continental Arctic (cA) – extremely cold and dry
 Forms over land (continental) and cold
temperature causes it to be dry.
b. Continental Polar (cP) – cold and dry
c. Maritime Polar (mP) maritime – moist; plus wet and
cold
d. Maritime Tropical (mT) – warm and wet
e. Continental Tropical (cT) – hot and dry
II.
Fronts and Lows
A. What is a front?
1. Front – Boundary that separates opposing air masses
a. Can range from 200 meters, to 200 kilometers
b. Can be as high as 5 kilometers
c. Can be as long as 2000 kilometers
2. Air masses on either side differ in humidity,
temperature and wind direction.
B. Kinds of fronts
1. Cold front – boundary between an advancing cold air mass
and warmer air mass it is displacing.
a. Cold air denser therefore it slides under the warm air in
front of it, forming a steep slope
b. The precipitation along the cold front is usually heavy
and fast (thunderstorms)
c. However, the passing front may cause no greater
change than a shift in wind direction.
2. Warm front – boundary between an advancing warm air
mass displacing a cold air mass
a. Warm air is less dense therefore, it rises up over the
cold air forming a gentle slope.
b. The first signs of an approaching warm front are
high cirrus clouds, which are followed by
cirrostratus then lower stratiform clouds.
c. Eventually nimbostratus clouds which give steady rain
or snow.
3. Occluded front – occurs when the faster moving cold
front catches up to a warm front.
a. The warm air gets pushed up between the two cold air
masses causing cloudiness and precipitation.
4. Stationary front – front is not moving forward
a. May give many days of steady rain causing flooding.
C. Life Cycle of a Mid-Latitude Low
1. Warm air mass meets a cold air mass in the mid latitudes
2. Circulation begins due to the warm air moving northward
and the cold southward (p. 442)
3. The circulation around the Low is counterclockwise in the
Northern Hemisphere
4. This counterclockwise motion of a Low sucks air off
the earth’s surface. Air is constantly spiraling into a
low-pressure system.
5. Throughs and Highs (p. 443)
6. Low pressure is associated with unfavorable weather.
7. High pressure is associated with clear conditions.
III. Thunderstorms and Tornadoes
A. Thunderstorms – storms with lightning and thunder formed
in cumulonimbus clouds.
1. The cloud can be as tall as 20km
2. Formed in convection cells – warm air being lifted up
while cool air descends (p. 445)
3. Often form along fronts there may be many cells
4. Squall line – many thunderstorms along a front
5. Supercells – very large singlecell thunderstorm that can
produce tornadoes
6. Lightning – discharge of electricity – cloud to cloud, cloud
to ground. Can occur in thunderstorms, snowstorms, dust
storms or volcanic eruptions.
B. Tornadoes – byproducts of supercell thunderstorms
1. Violently rotating column of air
2. Tornado formation
3. Form from between the wall clouds of a mesocyclone
(p. 447)
4. 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.
C. Storm and Tornado watches and warnings
1. Watch – conditions are right
2. Warning – one has been spotted
IV. Hurricanes and Winterstorms
A. Hurricanes – huge rotating storm of tropical origin that has
sustained winds of at least 119 km/h
1. Winds and rain are strongest at the eye wall
2. 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.
3. Steered by global wind patterns
4. Storm surge results, in part, from strong winds of the eye
wall which blows water into a broad dome.
5. If storm surge strikes land the same time as high tide,
hurricane disaster worsens.
6. Hurricanes are ranked according to the Saffir-Simpson
Hurricane Scale on p. 452
B. Winter Storms
1. Blizzard – must have winds higher than 56 km/hr,
temperature –7°C or lower, and reduced visibility due
to falling or blowing snow
V.
Forecasting Weather
A. Gathering data
1. Satellites – visible images and infrared
a. Visible – the whiter the clouds the thicker –
meteorologists can track the clouds to get speed and
direction.
b. Are not available at night
c. Infrared satellite – use temperature to plot colors
d. The cooler the cloud tops the higher it is in the
atmosphere
e. Can be used at night
2. Rawinsondes – measure temperature, pressure and
humidity of air at different altitude.
a. Attached to a large balloon and tracked by radar
b. Identifies the shape of the jet stream
3. Surface Observations
a. Most are at airports
b. Information can help to locate fronts, highs and lows
c. Provide – temperature, dew point, barometric pressure,
wind speed and direction, visibility, precipitation,
height of clouds and the amount.