Transcript No Slide Title

LECTURE 8. AIR MASSES, FRONTS AND STORMS
AIR MASSES
PRESSURE SYSTEMS
WEATHER
Air masses are large bodies of air with uniform moisture,
temperature and stability characteristics. Often, day-to-day
weather in the mid-latitudes is the result of a single air mass or
the interaction of 2 or more air masses
E.g.1: DOMINANCE by 1 air mass -> weather reflecting air
mass characteristics (warm-moist; cold-dry; etc.)
E.g.2: INTERACTION of several air masses -> weather system;
involves
air
masses
brought
together
by
atmospheric
circulations; found especially in the mid-latitudes (35-600),
where warm tropical air meets cold polar air.
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An air masses alone can have a
dominant influence on midlatitude weather; for example,
an outbrust of cP can bring
cold temperatures to the south
central U.S. (Notice that the
air itself is modified
(warmed) as it moves
into warmer latitudes).
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Fronts
A front is the boundary between a warm air mass and a cold air
mass. When these air masses meet, the warm air always rises up
over the cold air, because it is less dense. Although fronts can be
stationary, more often they are moving - either a warm air mass
pushes along and overrides a cold air mass, forming a gently
sloping (1:200) WARM FRONT; or a cold air mass pushes
along and under a warm air mass, forming a steeper (1:100)
COLD FRONT.
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Along the gentle warm front clouds are produced which are
extensive and usually thin/layered; precipitation can occur some
distance ahead of the front and is often light/showery. Above the
steeper cold front, clouds of greater vertical extent are produced
(this is aided by the fact that cold fronts often travel faster than
warm fronts - causing more rapid uplift); rain is often heavier
and occurs immediately behind the front. A well-developed cold
front can form an abrupt squall line.
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Sometimes a cold front overtakes a warm front forming an
OCCLUDED FRONT.
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These various types of front are shown on weather maps by
standard symbols (symbols face direction of movement).
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Midlatitude Cyclones
The mid-latitudes are the principle zone of air mass conflict.
Probably the most important result of this conflict is the
midlatitude cyclone (or called a low or depression in Europe),
which is responsible for much of the day-to-day weather in the
mid-latitudes. It is called a cyclone because it is characterized
by low pressure at the center, which causes cyclonic circulation
(counterclockwise in the northern hemisphere). Because these
cyclones occur in the mid-latitudes they are "embedded" in the
westerlies of the global circulation and usually travel from west
to east at about 20-30 miles per hour, crossing the U.S. in 3-4
days.
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These cyclones usually begin as a small disturbance in the Polar
Front (where cP meets cT air). This small wave develops into
the WARM SECTOR of the cyclone; cold air begins to curve
around in the COLD SECTOR, beginning cyclonic circulation.
As air rises up the fronts it produces low pressure at the
cyclone's center.
Cyclones are more numerous and better developed in winter
because that is when polar air pushes down more frequently
into the mid-latitudes.
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System moving east
5
4
3
2
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1. In advance of warm front: prolonged widespread lowintensity precipitation; steadily falling pressure.
2. At the warm front: abrupt wind change east to south;
temperature increase.
3. Between fronts: showery convective conditions, unstable
warm air; pressure steadily falls as center gets closer, then rises
as center passes.
4. At/immediately after cold front: shorted-lived intense
precipitation; abrupt temperature drop; abrupt wind change
south to west/northwest.
5. After cold front: skies clearing; temperature falling steadily;
pressure rising steadily; wind shifting to more northerly.
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Anticyclones (High)
The other major disturbance to the general westerly circulation
of the mid-latitudes is the anticyclone or high. This highpressure center involves no conflict of air masses and so no
fronts. The air above is subsiding and diverging at the surface so
skies are clear. This gives rise to hot dry weather in the summer
and cold dry weather in the winter.
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Thunderstorms
Thunderstorms are associated with towering cumulus clouds,
undergoing rapid uplift resulting in adiabatic cooling and
pronounced cloud formation. In the mid-latitudes, rapid uplift is
often aided by
1. the ascent of warm air up a cold front of a cyclone.
2. the release of latent heat during condensation, which warms
the air causing more vigorous uplift. After the cumulus stage, ice
crystals grow at the top of the thundercloud and begin to fall
through the cloud creating a downdraft - this is the mature stage
of the thunderstorm, with heavy rain, lightening and hail stones
common. The hail results from ice pellets being carried up and
down within the cloud, gaining a new ice layer each time.
Formation of hail may also be involved in the production of
lightening - shattered ice pellets may carry positive charges
upwards (by updrafts), while hail pellets carry negative charges
downwards - thus the cloud becomes electrically charged.
Eventually, downdrafts dominate the cloud and the
thunderstorm dissipates.
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Thunderstorms are most common along the intertropical
convergence zone, due to the combination of high temperatures,
high humidity and high instability. In the U.S. they are most
common in the southeast, where warm humid air from the Gulf
of Mexico penetrates inland.
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Tornadoes: A moving mass of
air often moves faster aloft
than at the surface – a rotation
or “forward-rolling” effect can
result. If this air encounters
strong uplift e.g. a front or
developing thunderstorm, the
rotating air can be lifted
upright. It becomes a vertical
rotation.
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Strong updrafts can stretch the rotating air vertically, so that it
narrows horizontally and spins faster – a funnel cloud. If the funnel
cloud extends to earth, it is a tornado. Rapid uplift of air within the
tornado lowers the air pressure – often 10% lower than
surroundings. This causes strong winds at the surface (pgf) –
sometimes exceeding 300 mph – these winds cause much of the
destruction.
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Many tornadoes are associated with thunderstorms, so they occur in
thunderstorm season – May and June – and they occur where
thunderstroms are common e.g. Texas, the southeast, the mid-west.
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Tropical Cyclones
Cyclonic circulations that develop in the tropics can be very
powerful because the warm tropical oceans provide an
abundant supply of energy (in the form of latent heat released
by rising tropical air). Unlike mid-latitude cyclones, they are
no contrasting air masses or fronts involved in the growth of
tropical cyclones.
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They probably develop along the lee of easterly waves of low
pressure. Surface winds converging on the wave are deflected
counterclockwise and cause rising air (convergent uplift). The
uplift lowers pressure even more and the system intensifies into
a cyclonic circulation.
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Once developed, these cyclonic systems move generally to the
west. The central low pressure can intensify (e.g. < 900 mb),
causing very large PGF and high wind speeds. If the winds
reach 74 mph (and sometimes exceeding 200 mph), they are
hurricanes (or cyclones or typhoons).
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Whether or not a hurricane is damaging to the U.S. depends on
where it matures – if it matures early (mid-Atlantic 400 west) it will
usually curve to the north and miss the U.S. coast. Hurricanes that
mature further west (after 700 west – Dominican Republic), have a
greater chance of striking the U.S. Like tornadoes, high speed winds
cause much of the damage, although flooding from heavy rain and
storm surges (sea water pushed onland by strong winds) can also be
very destructive and deadly, especially in coastal areas e.g. 300,000
dead in Bangladesh in 1970 (many drowned).
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