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HIGH & LOW PRESSURE
The atmosphere is relatively
thin compared to the size of the Earth. It is similar to the peel of an orange,
but the atmosphere is not a solid. In fact, the air surrounding the Earth is
fluid and constantly flows and spins due to the forces of the Earth and the
heat of the Sun. Though we can't see it, air is real and has weight. Air pressure
is the measure of the weight of the air.
A High consists of an area
of relatively high air pressure where air piles up, sinks and spreads over a
region. Typically, this sinking motion warms the atmosphere and has a cloud-dissipating
effect leading to fine weather especially near its center. Unfortunately, there
are many exceptions to this typical pattern especially in coastal areas where
clouds can be abundant. The wind flow in an area of high pressure is clockwise
or anticyclonic. Areas of high pressure are stable bodies of air which make
up many of the characteristic air masses around the world.
A Low consists of an area
of relatively low air pressure where air collides, rises and evacuates a region.
This rising motion mixes and cools the atmosphere and usually increases the
relative humidity leading to cloudiness and precipitation. The wind flow in
an area of low pressure is counter-clockwise or cyclonic. Low pressure centers
develop on the boundaries of converging air masses.
1. If low pressure means less pressure, why does smoke fall when there's a front
approaching?
It is true that low pressure
and fronts are associated with rising motion, but an approaching front is accompanied
with changing wind directions. As a frontal boundary moves through an area,
it acts like a shovel lifting up the air. This, in turn, causes the air to mix
and overturn with some air parcels falling to the ground. It may be difficult
to actually see these motions in clear air. But with a nearby smoke plume, one
may see a dancing trail of smoke following the wind flow.
2. Does high pressure drive
out low pressure?
No, it is just the opposite.
Areas of high pressure make up the airmasses which dominate the weather in a
region. Low pressure areas are disturbances which form on the fringes of conflicting
airmasses. In the life cycle of a low pressure center, it will bring wind, clouds
and precipitation to a particular region before exiting or dissipating. Once
the low pressure is gone, high pressure will regain control over an area.
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