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Barometer Rise Or Fall
What does it
mean when a barometer is rising or falling?
A
barometer measures air pressure: A
"rising" barometer indicates increasing air pressure; a
"falling" barometer indicates decreasing air pressure.
In
space, there is a nearly complete vacuum so the air pressure is zero.
On
Earth, because there are many miles of air molecules stacked up and exerting
pressure due to the force of gravity, the pressure is about 14.7 pounds per
square inch at sea level.
The
interesting thing about air pressure is that it is different at different
points on the planet and it changes over time.
Why
might that be? If you've read How Hot Air Baloons Work, you know that hot
air is less dense (lighter) than cooler air.
Therefore,
on any given day you would expect the air over a desert to have a lower
pressure than the air over an ice cap.
And
that would be true. These same sort of pressure differences occur all over the
planet for various reasons.
These
pressure differences have a big effect on the weather, so if you know the
current air pressure at your house, as well as the pressure trend, you are able
to predict certain things about the weather.
As
a very loose rule, a high-pressure area will be clear, and a low-pressure area
will be cloudy and rainy.
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