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How does a
physics formula sort out the complications in any
job processing weather data? This simple example relates elevation changes with
air pressure science. It can be used for air pressure experiments and has
metric metric and imperial unit conversions built in.
More sophisticated pressure conversion physics formulae incorporate:
1 - barometric pressure in recent history,
2 - temperature and
3 - humidity changes
...as formula constituents.
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The simplest physics formula applicable here goes like this:
Pressure = Sea Level Pressure / 2 ^ (Z/Zo)
...where Z is the altitude and Zo is about 18,400 feet.
Meteorologists need corresponding pressures at sea level for each surface measurement. Suppose the barometric pressure today in Seattle, for instance, is 1000
millibars but only 930 at nearby Spokane. The difference in elevation, 2400 feet, would produce the majority of that pressure change.
A quick look at any
standard atmospheric pressure chart combined with some interpolation reveals an expected pressure of around 929 mb for that elevation. That agrees with a sea-level pressure of 1000.
Now we can compare the two properly.Adaptation of pressures to
equivalent sea level values puts all locations on a "uniform field" and highlights pressure differences accountable to weather systems.
Analysts use an algorithm of this sort to
calculate the equivalent pressure as if the ground at each location were lowered to sea level.
In this case, the conversion would yield a corrected pressure for Spokane of around 1000 millibars. The detailed physics formula would take the
temperature and
humidity into account.
Meteorologists calculate a quantity known as potential temperature. They determine the temperature any parcel of air would have if it were
adiabatically pressurized to exactly 1000 mb. Adiabatic means without any air or heat leaking in or out.
Using this pseudo-value simplifies subsequent calculations they must make. Of course, the presence of moisture further complicates this procedure and we then use
Wet-Bulb Potential Temperature, Θw.
When weather systems move and progress through their natural course of development, they produce measurable pressure changes.
Forecasters draw isallobars, lines of equal pressure change, on their maps. These curves look similar to isobars. On this
chart, synoptic patterns appear in these curves, which give more clues about the current systems.
Color-coded 3 hour pressure change map.
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