Chapter 20 – Weather
• Page 435
• How do you think the waves caused
damage to this house?
• If this was your house, would your rebuild
in the same place?
• What can people do to avoid such
damage?
• How are you able to predict the weather?
Chapter 20.1
Air Masses and Weather
• Meteorology – the study of processes that
govern Earth’s Atmosphere
Origin of an Air Mass
• An Air Mass is a large body of air with the
same humidity and temperature
– The humidity and temperature of the air mass
is determined by where they form
• Over a southern ocean – moist and warm
– When the air mass travels, it takes with it
the temperature and humidity of its place
of origin.
Origins of air masses
• Classification due to where they form
– Continental Arctic (cA) – extremely cold and
dry
• Forms over land (continental) and cold temperature
causes it to be dry.
– Continental Polar (cP) – cold and dry
– Maritime Polar (mP) maritime – moist; plus
wet and cold
– Maritime Tropical (mT) – warm and wet
– Continental Tropical (cT) – hot and dry
Chapter 20.2
Fronts and Lows
• What is a front?
– Front – Boundary that separates opposing
air masses
• Can range from 200 meters, to 200 kilometers
• Can be as high as 5 kilometers
• Can be as long as 2000 kilometers
– Air masses on either side differ in
humidity, temperature and wind direction.
Kinds of fronts
• Cold front – boundary between an
advancing cold air mass and warmer air
mass it is displacing.
– Cold air denser therefore it slides under the
warm air in front of it, forming a steep slope
– The precipitation along the cold front is
usually heavy and fast (thunderstorms)
– However, the passing front may cause no
greater change than a shift in wind
direction.
• Warm front – boundary between an
advancing warm air mass displacing a
cold air mass
– Warm air is less dense therefore, it rises up
over the cold air forming a gentle slope.
– The first signs of an approaching warm
front are high cirrus clouds, which are
followed by cirrostratus then lower
stratiform clouds.
– Eventually nimbostratus clouds which give
steady rain or snow.
– Occluded front – occurs when the faster
moving cold front catches up to a warm
front.
• The warm air gets pushed up between the two cold air
masses causing cloudiness and precipitation.
– Stationary front – front is not moving
forward
• May give many days of steady rain causing flooding.
Life Cycle of a Mid-Latitude Low
• Warm air mass meets a cold air mass in
the mid latitudes
• Circulation begins due to the warm air
moving northward and the cold southward
(p. 442)
• The circulation around the Low is
counterclockwise in the Northern
Hemisphere
• This counterclockwise motion of a Low
sucks air off the earth’s surface. Air is
constantly spiraling into a low-pressure
system.
• Troughs and Highs (p. 443)
• Low pressure is associated with
unfavorable weather.
• High pressure is associated with clear
conditions.
Chapter 20.3
Thunderstorms and Tornadoes
Thunderstorms – storms with
lightning and thunder formed in
cumulonimbus clouds.
• The cloud can be as tall as 20km
• Formed in convection cells – warm air
being lifted up while cool air descends (p.
445)
• Often form along fronts there may be
many cells
• Squall line – many thunderstorms along a
front
• Supercells – very large singlecell
thunderstorm that can produce tornadoes
• Lightning – a discharge of electricity –
cloud to cloud, cloud to ground. Can
occur in thunderstorms, snowstorms, dust
storms or volcanic eruptions.
Tornadoes – byproducts of
supercell thunderstorms
• Violently rotating column of air
• Tornado formation
• Form from between the wall clouds of a
mesocyclone
(p. 447)
• 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.
Storm and Tornado watches and
warnings
• Watch – conditions are right
• Warning – one has been spotted
Chapter 20.4
Hurricanes and Winterstorms
Hurricanes – huge rotating storm of
tropical origin that has sustained
winds of at least 119 km/h
• Winds and rain are strongest at the eye
wall
• 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.
• Steered by global wind patterns
• Storm surge results, in part, from strong
winds of the eye wall which blows water
into a broad dome.
• If storm surge strikes land the same time
as high tide, hurricane disaster worsens.
• Hurricanes are ranked according to the
Saffir-Simpson Hurricane Scale on p. 452
Winter Storms
• Blizzard – must have winds higher than
56 km/hr, temperature –7°C or lower,
and reduced visibility due to falling or
blowing snow
Chapter 20.5
Forecasting Weather
Gathering data
• Satellites – visible images and infrared
– Visible – the whiter the clouds the thicker –
meteorologists can track the clouds to get
speed and direction.
– Are not available at night
– Infrared satellite – use temperature to plot
colors
– The cooler the cloud tops the higher it is in the
atmosphere
– Can be used at night
• Rawinsondes – measure temperature,
pressure and humidity of air at different
altitude.
– Attached to a large balloon and tracked by
radar
– Identifies the shape of the jet stream
• Surface Observations
– Most are at airports
– Information can help to locate fronts, highs
and lows
– Provide – temperature, dew point, barometric
pressure, wind speed and direction, visibility,
precipitation, height of clouds and the amount.