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Meteorological Computations

The Weather Processor contains several programs to perform often-used meteorological and non-meteorological calculations. These programs take input from the keyboard and display the results in a tabular form on the display.

Sunrise Sunset Computation

The suncalc program is used to compute sunrise, sunset and twilight.  The input to the program is a location that can be a station identifier or latitude and longitude.  The second input is the date.   This can either be:

  • day+minute - where a date and a minute change is specified.  The output is a table of solar azimuth, ascension and declination and zenith angle for every minute throughout the day in increments of the minute change.
  • day - where the date is specific. The output is a table of sunrise, sunset, twilight, solar noon, length of day and twilight.
  • month - where the month is specified.  The output is a table of sunrise, sunset, twilight and length of day for each day of the month.
  • year - where the year is specified.  The output is a table of sunrise and sunset times for every day of the year.

Unit Conversions

The unit program is used to convert units.  This uses the WXP unit interface.  The input is a value, its units and the desired units.

unit 25 C F

Domain Computation

The domtran program is used to convert between various domain values.  There are three coordinate systems used:

  • latitude, longitude
  • projection x,y - the X and Y coordinates on the projection plane of the specified latitude and longitude.
  • grid x,y - the relative location of the point to grid specified in the plot_domain resource.  This can also be the pixel coordinate.
domtran -pd=us earth2proj 40,-90
domtran -pd=us earth2grid 40,-90

Wind Chill Computation

The wchill program calculates wind chill factors. The wind chill program first displays a wind chill chart for quick reference and then prompts the user for temperature and wind speed. From this information, the wind chill factor is calculated. The wind chill factor is relatively meaningless for a temperature greater than 50F because the wind cools the skin's surface less for warmer temperatures. Also, the wind chill factor changes little for wind speeds greater than 40 miles per hour. At these speeds, the wind removes nearly all heat produced at the skin's surface.

Heat Index Computation

The heat program first displays a heat index chart for quick reference and then prompts the user for temperature and dewpoint. From this information, the heat index is calculated. The heat index is relatively meaningless for temperatures less than 65F, since below this temperature, the skin loses more heat than is produced. Also, the heat index is meaningless for dewpoints less than 65F, since the drier air leads to evaporative cooling of the skin's surface. A second index called the temperature-humidity index is calculated for comparison and a textual heat stress condition such as normal, low, medium, high or extreme based on the temperature-humidity index is displayed.

Moisture Parameter Computation

The moist program derives various moisture variables given three of the four variables: temperature, dewpoint, and station pressure or station elevation. The information displayed includes wetbulb temperature, vapor pressure, saturation vapor pressure, relative humidity, specific humidity, mixing ratio, saturation mixing ratio and equivalent potential temperature.

Standard Atmosphere Computation

The stdatms program relates pressure, height and temperature. Information is provided for height or pressure in one of the following units:

    Height: meters, feet
    Pressure: millibars, Pascals, mm of mercury, inches of mercury, lbs/square inch and atmospheres

The program then calculates the height, pressure and temperature (in various units) at that specific level for a standard atmosphere.

For further information about WXP, email technical-support@weather.unisys.com
Last updated by Dan Vietor on June 7, 1998