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Our everyday lives are
affected by the weather. Should we prepare for rain or is it going to
be a bright, sunny day? Nature often gives us clues when change is
coming in our weather. It is up to us to notice them and understand
what they mean. Let’s focus in on one of those clues and how airplanes
can be used to foretell possible upcoming weather.
For us to have a rainy
day the atmosphere must be filled with moisture. If the air is dry
we’ll have a clear day. So, to get an idea of our rain chances we need
to know how much moisture is in the atmosphere. If we see the amount of
moisture increasing, it can be a sign a front is approaching. The
question becomes, how can we figure out how much moisture there is in
the atmosphere?
One way to measure the
moisture in the atmosphere is to look at a hygrometer. A hygrometer is
a device which measures the humidity in the air. The problem is, it
only tells us how much water there is in the lower atmosphere. While
this can be an indicator of a possible front or afternoon thundershowers
it is not a sure sign. For example, when fog is present the humidity is
100%, but there is rarely ever rain.
The best indicator of
major changes is to measure how much moisture there is in the upper
atmosphere. The question becomes, how do we figure out how much
moisture there is in the upper atmosphere since we can’t easily get up
there?
The National Weather
Service uses balloons and satellites. Twice a day each weather service
office releases a large balloon with an attached instrument pack. These
measure, among other things, the moisture content of the various layer
of the atmosphere. They also use the water vapor images from
satellites.
For the rest of us, we
can look to airplanes.
You’ve probably looked
up before and noticed the contrails behind high flying airplanes.
Sometimes the contrails are long and stretch across the sky. At other
times the contrails are small or even non-existent. Contrails form when
hot humid air from a jet engine mixes with the low temperature and
pressure of the upper troposphere and lower stratosphere. The excess
moisture condenses out of the exhaust and creates the visible cloud of
the contrail.
If the upper atmosphere
is very dry, the moisture in the contrail will quickly evaporate. Thus,
the contrail will be short. The drier the atmosphere, the shorter the
trail. If the upper atmosphere is dry, we can expect good weather for
the short future.
Conversely, if there is
a lot of humidity in the upper atmosphere, the contrail’s moisture can’t
evaporate as quickly so the contrail persists and is long. This high
humidity, can be an indication that a front is on the way. So, if you
see long contrails in the sky, it can be a sign foul weather is on the
way.
Take time to watch
airplanes and their contrails. If they are short, it’ll be clear
weather for a day or two. If they are long, it can be a sign rainy
weather is on the way. Try it and see if you can forecast the weather
just by watching airplanes.
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