Sources of Storms
winds begin moving great sea of cold or warn air from the regions
where they form, air masses come into conflict. The arrival of a
new mass means the old one has to be shoved out of the way. There
is a war on.
where air masses clash is called a 'front'. Fronts are often the
scene of the most dramatic weather changes can stay in place or
move. When they move, one air mass is advancing, another retreating.
When the warmer air is advancing, the boundary is a warm front.
When the colder air is taking over, it is a cold front. When neither
side is wining it is a stationary front.
normally are parts of larger weather systems centered on areas of
low atmospheric pressure. They are parts of weather systems often
Sources of Storms
Sources of Storms
draw much of their energy from temperature contrasts created by
the sun's unequal heating of the Earth.
conditions are right, gravity pulls cold, heavier air under warmer,
lighter air. Contrasts between warm and cold air have potential
energy, just as a skier at the top of a slope has potential energy.
When the skier pushes off and starts down the slope, her potential
energy becomes kinetic energy. When something happens to start warm
and cold air masses moving over and under each other, their potential
energy becomes a storm's kinetic energy.
is added to storms when air rises and the water vapour in it begins
condensing into clouds and precipitation. When water vapour condenses
or turns into ice, it releases heat called 'latent heat'. This heat
adds to a storm's energy. Also, storms draw some energy from the
high-speed winds of the upper atmosphere. After a storm starts,
the winds can pull in more cod air and warm air, enhancing the temperature
contrasts and keeping the storm going.
wind speeds depend on the differences between air pressures around
the storm and in its low-pressure center. The greater the difference,
the stronger the winds. Another factor also comes into play.
how a spinning ice-skater can speed up or slow down by pulling in
her arms or holding them out. When the skater pulls in her arms,
she spins faster. When she holds them out, she spins slower. Scientists
call this the 'conservation of angular momentum' and it works for
storms too. As winds spiral into a low-pressure area they make a
smaller and smaller circle. Like the ice-skater who pulls her arms
in, the winds spin faster.
air is displacing warm air. The heavier, cold air is shoving under
the warm air, pushing it upward. Unless the air is extremely dry,
clouds form. Often the clouds grow into thunderstorms. The slope
of the front is fairly steep, especially if it is moving fast, around
25 mph for examples. About 30 miles back from the front the warm-cold
boundary would be at around 3,000 feet above the ground.
air is replacing colder air. The lighter warm air slides over the
heavier cold air, creating a boundary with a gentle slope. You could
be around 100 miles ahead of the warm frontówstill in the cold
airówand the warm-cold boundary could be only 3,000 feet above
you. If you are 600 or 700 miles ahead of your front, in the cold
air, wispy clouds at about 30,000 feet overhead could be the first
sign of the approaching warm air. As the front moves toward youówor
if you move toward the frontówthe clouds overhead grow thicker
and lower and eventually rain, snow, sleet, or freezing rain begin
the cold nor the warm air is advancing; it is a standoff. Widespread
clouds can form on both sides of the frontal boundary.
fronts, or occlusions, are created when cold, warm and cool air
come in conflict, forming boundaries above the ground as well as
at the surface. They are often described as being caused by a cold
front catching up with a warm front, but that seldom happens. Their
clouds and precipitation are a mix of typical cold-front and warm-front
clouds. Cold occlusion Cold air is shoving under cool air at the
Earth's surface. The cold-warm air boundary aloft is often west
of the surface front. Warm occlusion Cool air rises over cold air
at the surface. The warm-cold air boundary aloft is often east of
the surface front.
are often parts of larger storms.
zones where warm and cold air are close together, sometimes along
of a storm can be described in four stages:
the warm and cold masses of air are high-pressure areas with clockwise
winds. The boundary does not have to be sharp enough to be called
low-pressure are forms on the boundary and counterclockwise winds
around it begin moving the air. Warm air begins advancing on the
east sideówcreating a warm front. Cold air begins advancing on
the west sideówcreating a cold front. The fronts and low-pressure
area begin stirring up clouds and precipitation.
low-pressure area grows stronger. Its pressure decreases. Winds
increase in speed, and clouds and precipitation spread.
the cold front catches up with the warm front, forming an occluded
front, but this is rare. Scientists are still working out the details
of the structure of occluded fronts and how they form. Often, the
formation of an occluded front is the beginning of the end of the
hundred miles east of an advancing warm front, high, thin cirrostratus
clouds will create a haloówa circle of lightówaround the sun
or moon. As the front comes closer, the clouds grow lower and thicker
and turn the sky gray. A few hours later even lower clouds arrive
with rain or snow.
happens depends on whether you're north or south of the storm's
center as it passes by. When a storm's center passes by to the north,
clouds and precipitation arrive ahead of the warm front. The weather
turns warmer and may clear up. More clouds, perhaps thunderstorms,
arrive. Finally the weather turns colder and begins to clear.
storm's center is south of you, clouds will become lower and thinker
with rain or snow lasting around 12 to 24 hours. The precipitation
will taper off to showers or snow flurries, the air will remain
cold and the sky will clear. You'll never be in the storm's warm
some winters, storms seem to follow the same paths. They are being
guided by jet-stream patterns that become established for long periods.
Jet streams are the cores of the general west-to-east flow of winds
in the upper atmosphere. These winds not only flow west to east,
but also often make excursions to the north and south.
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