^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