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 a 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. Wind speed and direction at various levels in the atmosphere are also obtained by tracking the motion of the radiosonde in flight using GPS. 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 map of the available upper air stations appears in the User Guide. Thirty-two radiosonde stations can be accessed from the RealTime Weather Portal 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.
The during its ascent, the radiosonde sends a nearly continuous stream of observed temperature and humidity data keyed to the pressure down to the ground receiving station. To reduce the amount of compiled radiosonde data, a data set for a limited number of pressure levels is transmitted along the traditional weather communications network. This data sequence must include "mandatory levels", 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. A selected number of "significant levels" are also reported where a significant change in the temperature and/or dewpoint (a measure of atmospheric humidity) profile occurs. No information is lost since one could graphically interpolate to obtain the temperature or dewpoint at any altitude between successive significant levels. Wind data (speed and direction) are included at each level. The total number of levels appearing in the radiosonde report will depend upon the complexity of the vertical structure of the atmosphere as observed by the radiosonde.
The first level that appears in each report is the surface data that were reported at the station at the time of launch. Some mandatory levels will be underground, especially at locations such as the "Mile High City" of Denver, CO, and the only information reported will be a computed value of the altitude of the pressure level based upon certain assumptions.
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…In Greater Depth.