Fall Semester Regents Earth Science

John Dewey High School

Mr. Klimetz

Coding and Decoding Ground Atmospheric Pressure Readings

An Exercise in the Coding and Decoding of Ground-Based Atmosopheric Pressure Data

Atmospheric [barometric] pressure readings appear as measurements, in millibars [mb], with four or five digits with a decimal point between the last and next-to-last digit. Such lengthy numbers occupy considerable space when plotted on a synoptic air pressure data map. Hence, such numbers can be difficult to read, distinguish, and decipher, particularly if there is much data acquired from a limited geographic area. Meterologists have developed a coding system for reducing the quantity of numbers in a pressure reading in order conserve space when plotted on a map and provide for more accurate reading and interpretation.

Coding. The following explains how barometric pressures are coded on a weather map.

Example 1: 1013.7  mb

  a. Drop the decimal point, ex.  10137, then

  b. Report the last 3 digits  ex.  137

Example 2:  989.6  mb

  a.  9896

                     b.  896

Code the following:

  1022.7 = _____________

  998.0 = _____________

Meteorologists code the pressures that they report on their weather maps and on their weather station models to save space and improve the ease of reading.

Decoding. The following explains how barometric pressures are decoded when reading a weather map:

Example 1: 146

                    a.  Replace the decimal point between the last two digits, ex. 14.6

  b.  Place a “9”, then a “10”, in front of the coded number,

     ex. 914.6 and 1014.6

  c. Determine which of the 2 decoded pressures falls within the normal range of pressures

      at the earth’s surface, that is, 960 - 1040 mb.

ex. 914.6 is below the normal range, so 146 is the code for atmospheric pressure of

1014.6 mb.

Example 2: 972 equals which of the following true pressure values?

                    a. 97.2     b. 997.2     c. 1097.2

                    Ans: 997.2

I. The map on the following page shows coded pressure data reported from weather stations in the eastern half of the United States whose locations are indicated by the open and closed circles. Based upon the information presented above, decode the pressure readings from the weather stations. You should print the decoded reading below each circle in pencil. Once you have completed fully decoding the pressure data, sketch in isobars with a 4 mb interval beginning with 998.0 mb

II. Uncoded pressure data reported from weather stations is listed below. Based upon the information presented above, code each of the pressure readings and write on the line adjacent to the uncoded readings.

1022.5 mb _____________

997.4 mb ______________

1003.9 mb _____________

977.2 mb ______________

1021.4 mb _____________

969.8 mb ______________

981.4 mb ______________

1039.1 mb _____________

1023.0 mb _____________

1001.1 mb _____________