Transcript Air Mass - Red Hook Central Schools

Weather
Topic One
Air Masses and Weather
Scientists classify an air mass based on
whether it originates in an arctic, in a
polar, or in a tropical region and whether
it forms over land (continental) or sea
(maritime.) An air mass is a large body
of air with uniform temperature and
humidity. Polar air masses and tropical
air masses influence the weather of
North America.
Topic Two
Fronts and Lows
A front is the band of air between
opposing air masses. Scientists classify a
front based on the temperature of the
advancing air mass. Cold and warm
fronts are associated with characteristic
weather conditions. Fronts are usually
connected to mid-latitude, low-pressure
systems. Upper level air flow influences
the convergence or divergence of air into
and out of pressure systems.
Topic Four
Hurricanes and Winter Storms
Hurricanes are large rotating storms
originating over tropical oceans. They are
classified based on wind speed. Winter
storms are middle-latitude, low-pressure
systems that occur over land in the
winter.
Topic Three
Thunderstorms and Tornadoes
Thunderstorms form in warm, moist,
unstable air. They produce lightening, a
discharge of electricity. Tornadoes can
develop in thunderstorms containing
rotating updrafts.
Topic Five
Forecasting Weather
Weather forecasters must gather huge
amounts of data in order to make their
predictions. They rely on sensing
instruments and computer models to
provide the information they need.
Meteorologists make both daily and longterm forecasts of the weather.
Key Terms
air mass
cold front
hurricane
occluded front
squall line
tornado
station model
storm surge
blizzard
front
lightening
Saffir-Simpson scale
supercell
thunderstorm
stationary front
warm front
Topic One
Air Masses and Weather
Differences in air pressure at different locations on
earth create wind patterns. The equator receives
more solar energy than the poles, heating the air,
causing it to rise. Cold, polar air sinks. As air
moves from high to low pressure, a general,
worldwide movement of surface air from the poles
toward the equator occurs.
Air Mass
 A VERY large mass of air
that sits over an area for
days or weeks and picks up
the temperature and
humidity characteristics of
that area.
 They can be thousands of kilometers in diameter.
Continental = Land
Maritime = Water
Polar = Cold
Tropical = Warm
Maritime Polar (mP)
 An air mass that forms over
a cold, wet area.
Maritime Tropical (mT)
 An air mass that forms over
a warm, wet area.
Continental Polar (cP)
 An air mass that forms over
a cold, dry area.
Continental Tropical (cT)
 An air mass that forms over
a warm, dry area.
Air masses strongly affect the
weather on North America. As an
air mass moves away from the area
over which it formed, it can change.
Cold, dry air can become warmer
and more moist as it moves from
land over the warmer ocean, for
example.
cP air masses move southeast from Canada
into the United States. It brings very cold,
dry air. mP Pacific air masses are moist and
cool, but not cold. They bring cool, foggy
weather to the Pacific northwest and drop
their moisture as the head over the
mountain ranges of the western U.S. mP
Atlantic air masses move east toward
Europe, passing over New England and
maritime Canada, bringing cool, cloudy, wet
weather.
Topic Two
Fronts and Lows
When two unlike air masses meet,
density differences keep the
masses separate. Cool air is dense
and does not mix with warm, less
dense air. In order for a front to
form, one air mass must collide
with another.
Front
 A place where two different
air masses meet.
Cold Front
The boundary between an
advancing cold air mass and
the warm air mass it is
displacing.
The moving cold air pushes up the warm
air. If the air is moist, large cumulus and
cumulonimbus clouds form. Short-lived,
sometimes violent storms result. A squall
line, a line of heavy thunderstorms,
may occur just ahead of a fast-moving
cold front. Slow-moving cold fronts produce
little rain and less cloudiness.
Warm Front
The boundary between an
advancing warm air mass
and the cold air mass it is
displacing.
The slope of a warm front is
gradual. This allows clouds to
appear a long time before the base
of the front. Precipitation is
produced over a large area.
Occluded Front
The front that is formed
when a cold front overtakes
a warm front and displaces it
upward in an area of low
pressure.
The advancing cold front
comes in contact with the
cool air underneath the lifted
warm air, cutting off the
warm front from the ground
below, holding it high in the
atmosphere.
When two air masses meet and
neither is displaced, a
stationary front results. The
two air masses move parallel to
the front between them. The
weather around a stationary
front is similar to that produced
by a warm front.
An anticyclone is the opposite of a
cyclone, which sinks and flows out from
the center. Because of the Coriolis
Effect, the circulation around an
anticyclone is clockwise in the Northern
Hemisphere. Cyclones bring cloudy,
stormy weather, while anticyclones
bring dry weather. If an anticyclone
stalls over an area, it can cause airpollution problems.
During a thunderstorm, clouds release
electricity in the form of lightening.
The released electricity heats the air,
causing it to expand rapidly. The expansion
and release of the air causes thunder.
Squall Line
 A line of thunderstorms that
occur ahead of a front.
There are often strong winds
before the rain.
 Strong winds often precede squall lines. This
happens because the rain falling within a
thunderstorm causes the air to cool, becoming more
dense, and sinking. This is called a downdraft.
Super Cell
 A very large, single
thunderstorm with very
strong updrafts.
 Strong winds often precede squall lines. This
happens because the rain falling within a
thunderstorm causes the air to cool,
becoming more dense, and sinking. This is
called a downdraft.
Lightning
A discharge of electricity from a thundercloud to
the ground, to another cloud, or to another spot
within the cloud itself.
Tornado
 A violent, rotating column of air
that extends down from dark
clouds and moves overland in a
narrow, destructive path.
 They may reach speeds of 400 km/hr.
Hurricane
 A large, rotating storm of
tropical origin with sustained
winds of at least 119 km/hr.
They are rarely more than 700
km in diameter, and are most
powerful of all storms. They
occur in both the Atlantic and
Pacific.
They occur when:
1. Warm, moist air evaporates off the
surface of the ocean and rises rapidly
2. The moisture condenses, releasing
latent energy in the form of heat
3. The heat increases the force of the
rising air
4. The patterns continues, sustaining the
process
An average
hurricane has the
same amount of
energy as all the
electricity used in
the U.S. in 6 months.
Storm Surge
 A rapid rise in water level
along the coast as a
hurricane or other tropical
storm approaches.
Saffir-Simpson Hurricane Scale
 The 1 to 5 scale used to rate
a hurricane’s intensity and
estimate potential property
damage and flooding.
Winter storms are mid-latitude
low pressure systems that can
bring several types of weather,
including heavy snow, ice, and
rain. When conditions become
severe, a blizzard may occur.
A blizzard is characterized by
high winds, low temperatures
and falling or blowing snow.
To be considered a blizzard,
three conditions must be
met.
1. Winds exceed 56 km/hr.
2. Temperatures are below 7˚C.
3. Falling and/or blowing
snow reduce visibility.
On the east coast, nor’easters
occur over the north Atlantic
where extremely heavy snow
consisting of a maritime polar air
mass is blown from the ocean to
the land.
Topic Five
Forecasting Weather
Weather can be measured at
both the surface of the earth
and above the surface.
As discussed in the last
chapters, at the surface,
psychrometers, and barometers
are used to measure humidity.
Rain gauges measure the
amount of precipitation.
RADAR
 A method of detecting distant
objects and recording their
features and properties by
looking at the electromagnetic
waves that are reflected from
their surface.
Coded weather
information is put on maps
prepared by centers
around the world. Cluster
of symbols are plotted for
each station.
Station Model
A picture representation for all
of the data that make up a
weather report for a particular
location at a particular time.
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Surface weather maps are used to
present a picture representation of
current weather conditions. In locating
fronts, the following guidelines are
used:
1. Wind direction changes behind
fronts
2. Temperature changes sharply
across fronts
3. Dew point changes sharply across
fronts