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WTHRCONV.TXT
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1986-11-14
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^C^1Weather Conversion Routines
^CBy Val J. Golding
Meteorologists often use unfamiliar units of measurement when they describe
the weather. How many miles per hour is a 25 knot wind? How many inches of
mercury are equal to an atmospheric pressure of 1042 millibars? How many
degrees Fahrenheit are equal to a temperature of 16 degrees Celsius?
WEATHER CONVERSION ROUTINES can convert between three major temperature scales
and between the three units used to measure wind speed as well as calculate
wind chill and the heat index. In addition, the program converts between the
two units used to describe atmospheric pressure.
If you are already a dedicated weather watcher, you should find WEATHER
CONVERSION ROUTINES to be an invaluable tool. But even if you know nothing
about meteorology, read the explanations of weather measurements below. Play
around with the program. We hope we pique your interest in a fascinating
hobby.
Temperature
-----------
There are three widely used temperature scales: Fahrenheit, Celsius (or
Centigrade), and Kelvin.
Americans are most familiar with the Fahrenheit scale, named for Gabriel
Fahrenheit, the 18th Century German physicist who developed it. The scale
originally used 32 degrees, the freezing point of water, and 96 degrees, human
body temperature, as its two fixed points. (Note how close Fahrenheit came to
correctly determining human body temperature, which is measured as 98.6 degrees
F today.) Later, the boiling point of water, 212 degrees F, replaced body
temperature as a fixed point.
In countries other than the United States and in all scientific work, the
Celsius scale is used. Invented in 1742 by the Swedish astronomer Anders
Celsius, the scale uses zero degrees, the freezing point of water, and 100
degrees, the boiling point of water, as its base points. Because of the 100
degree interval between the two base points, this scale is also known as the
centigrade scale. The equation used to convert Fahrenheit (F) to Celsius (C)
is: C=5/9(F-32). When Celsius is converted to Fahrenheit, the equation used
is: F=1.8(C+32).
The third scale, the Kelvin scale, is used more by chemists and physicists than
by meteorologists. Why was it developed? As we've seen, both the Fahrenheit
and Celsius scales use the properties of water to fix their base points;
scientists soon found that they needed a fundamental scale independent from
the properties of any one substance. In 1848, the British physicist William
Thomson (Lord Kelvin) based his temperature scale on the theoretical "absolute
zero" (-273.15 Celsius). (A substance at zero degrees Kelvin would have no
thermal energy; its molecules would not move.) The equation used to convert
degrees Celsius (C) to degrees Kelvin (K) is: K = C + 273.15.
The Heat Index
--------------
As anyone who has ever endured a sweltering summer in the South knows, a
thermometer reading doesn't tell all of the story. The 95 degree day with 85%
humidity you might experience in New Orleans will take a much greater physical
toll than a 95 degree day with the low humidity that you might find in Phoenix.
Recently, the Weather Service has developed a "heat index" which uses a
complicated equation to calculate apparent temperature. Only two variables are
required to calculate it: degrees Fahrenheit and percent relative humidity.
The Weather Service rates hot weather in four categories, depending on the heat
index for that day:
CATEGORY I -- Apparent temperatures 130 degrees or higher. Heatstroke highly
likely for high risk groups with continued exposure.
CATEGORY II -- 105-130 degrees. Heat cramps and heat exhaustion likely,
heatstroke possible with prolonged physical exertion.
CATEGORY III -- 90-105 degrees. Heat cramps and heat exhaustion possible with
prolonged physical activity.
CATEGORY IV -- 80-90 degrees. Fatigue possible with prolonged exposure and/or
physical activity.
The Wind Chill Factor
---------------------
The wind chill factor, like the heat index and unlike most other weather
measurements, takes human discomfort into account. Wind chill tells how cold
it ^1feels^0 outside. Wind increases the rate at which heat escapes from the
body. As a result, if the temperature is 20 degrees Fahrenheit and a 40 MPH
wind is blowing, the cold a person experiences is equivalent to the cold
experienced on a -21 degree day with no wind. The higher the wind, the lower
the wind chill factor... up to a point. Above 40 MPH, increases in wind speed
have little additional chilling effect.
Equations used to determine the wind chill factor take wind speed, air and body
temperature, and other constants into account. Because they use sometimes
elaborate "fudge factors," some formulas can give surprising results when you
calculate wind chill. According to Val Golding's program, for example, on a 30
degree day, in a dead calm (wind: 0 MPH), a person would experience the
equivalent of a 27.5 degree air temperature.
Wind Speed
----------
Wind speed is described in three ways: in miles per hour (MPH), kilometers per
hour (KPH), and knots. (One kilometer equals approximately .62 miles.)
A knot is one nautical mile per hour. A nautical mile is defined as one
minute of a "great circle." (A great circle can be thought of as the
circumference of the earth.) For all you landlubbers out there, think of a
nautical mile as about 1.15 statute miles or 6080 feet. Note that it's
redundant to say "knots per hour."
Atmospheric Pressure
--------------------
The mercury barometer, invented in 1643, revolutionized meteorology. The first
barometers were simple: all that was needed to construct one was a bowl, a long
glass tube with one sealed end, and a lot of mercury. The first step was to
fill both the bowl and tube with mercury. The tube was then placed upright in
the bowl, with its opening below the surface of the mercury. (The builder had
to be careful not to let any air seep into the tube as it was placed in
position.)
When the experimenter stepped back from his creation, he would notice that not
all of the mercury in the tube flowed back into the bowl; a column of mercury
remained in the tube. What kept it there? Air pressure, pushing on the
mercury in the bowl.
When scientists measured the height of the mercury column, they noticed that
its height (and thus the air pressure) varied day to day. These fluctuations
in turn were tied to changes in the weather.
Today, while meteorologists have replaced mercury barometers with more
sensitive instruments, air pressure readings are often given in inches of
mercury, i.e. the height of a column of mercury balanced by the air pressure.
Meteorologists also describe barometric pressure in millibars. One thousand
millibars make one bar, the metric measurement for atmospheric pressure. A bar
is equal to 29.53 inches, or 750.062 millimeters of mercury.
One atmosphere, the standard atmospheric pressure, is given as 760 millimeters,
or 29.92 inches, of mercury. One atmosphere equals 1013 millibars.
For More Information...
-----------------------
Your local public library should have several books that introduce the basics
of weather and meteorology. For a comprehensive, well-illustrated text, we
recommend ^1The Weather Book^0, by Ralph Hardy, et al, from Little, Brown (1982).
DISK FILES THIS PROGRAM USES:
^FWTHRCONV.EXE
^FBRUN20.EXE
^FRETURN.EXE