ONLINE WEATHER SUPPLEMENTAL INFORMATION

To complement the Daily Summary for Wednesday, 10 February 1999

Explanation of Upper Air Observations (In Tabular Form)

When meteorologists mention the "upper air" or "conditions aloft", often they are referring to observations that have been made by an instrument package carried aloft by a weather balloon. This instrument package, which contains an FM radio transmitter, is called a "radiosonde", named for the on-board radio transmitter and for "sonde", meaning messenger. This instrument package - roughly the size of a pint milk carton - permits a nearly continuous observation of the air temperature, the humidity and the air pressure as the radiosonde ascends to a height of approximately 20 kilometers, before the balloon bursts. The design is such that the stream of temperature and humidity information is radioed back to the ground station at specific values of air pressure. Ground based radio direction finders track the motion of the radiosonde so that the wind speed and wind direction at various levels can be determined. The complete information system is a rawinsonde.

Current upper air weather observations are made at approximately 70 stations throughout the country twice daily, at 00Z and 12Z. A list of these upper air stations appear in Appendix IX, D of the User Guide. Thirty two radiosonde stations can be accessed from the Online Weather Homepage under the heading Upper Air Data-Text by selecting the desired upper air station. Once selected, a tabulation of the radiosonde observations for that sounding will appear. Remember that these files are updated on a twice daily basis.

INFORMATION CONTENTS OF THE TABLE

Station Information Block
The lead block includes information for the particular radiosonde launch (time and date) and the station (the 3-letter Federal Aviation Administration identifier, the 5-digit WMO identifier of the World Meteorological Organization, the latitude in degrees, the longitude in degrees and the elevation above mean sea level in meters).

• LEVL = Level Index
  A number is used for reference purposes to identify each reported level. The index starts with zero at a reference level that has a pressure of 1000 millibars (mb) and identifies each "significant level" where a significant change in the temperature and/or dewpoint profile occurs. "Mandatory levels" are also included in this sequence, which must be reported for use for various upper air charts and in numerical prediction models. These mandatory levels include the 1000, 925, 850, 700, 500, 400, 300, 250, 100 mb levels.
  The level identified as SFC represents conditions taken locally at the station to represent conditions at the earth's surface. The level identified as "TRP" represents the base of the tropopause on the sounding and "WND" represents the data where a wind speed maximum occurs. Note that some mandatory levels will be underground, especially at locations such as the "Mile High City" of Denver, CO. The total number of levels will depend upon the complexity of the vertical structure of the atmosphere as observed by the radiosonde.
• PRES = Pressure in millibars (mb)
  The radiosonde contains a calibrated aneroid barometer that measures the air pressure as the radiosonde ascends through the atmosphere.
• HGHT = Height of level in meters (m) above mean sea level
  The altitude of the radiosonde at the reported level above the surface (SFC) is computed at the ground station using an equation that incorporates the average air temperature, the atmospheric humidity and the pressure in the atmospheric layer through which the radiosonde has just traveled. "Heights" of mandatory levels that are locally underground are also calculated.
• TMPC = Air temperature in degrees Celsius (C)
  The air temperature at each reported level has been measured by an electronic thermometer carried on the outside of the radiosonde.
• DEWP = Dewpoint in degrees Fahrenheit
  The dewpoint at the reported level, a measure of the water vapor content of the air at that level, is measured by an electronic device called a hygristor carried in the radiosonde. The dewpoint will never exceed the air temperature.
• RELH = Relative Humidity as a percentage (%)
  The relative humidity at a given level as a percentage is used to describe how close the measured water vapor in the atmosphere is to saturation. When RELH = 100%, the air would be saturated with respect to water vapor, and the air temperature and the dewpoint are equal.
• DIR = Wind direction in degrees (deg) measured clockwise from true north (360 degrees)
  The wind direction at a given level above the surface is determined from the tracking of the motion of the radiosonde by the ground-based radio receiver/antenna system The following notation is used:
  • 090 degrees = east wind, 180 = south wind, 270 = west wind and 00 = calm
• SPD = Wind speed in knots (knt)
  The wind speed at each level is based upon the tracking of the radiosonde by the ground based radio receiver.

GRAPHICAL DISPLAYS

The upper air weather data appearing in the text format can be displayed graphically using a special diagram called a Stüve diagram. A more detailed discussion of the display of the plotted sounding on a Stüve diagram will be described in the Thursday Supplemental Summary.

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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 1999, The American Meteorological Society.