INFORMATION OBTAINED FROM RAOB SOUNDINGS
The radiosonde transmits temperature and relative humidity data at each pressure level. Winds aloft are determined from the precision radar tracking of the instrument package. The altitudes of these levels are calculated using an equation (the hypsometric equation) that relates the vertical height of a layer from the mean layer temperature, the humidity of the layer and the air pressure at top and bottom of the layer. Significant levels where the vertical profiles of the temperature or the dewpoint undergo a change are determined from the sounding. The height of the troposphere and stability indices is calculated. These data are encoded into standardized RAOB messages and transmitted worldwide over standard communications network.
A plot of the vertical variations of observed weather elements made above a station is called a sounding. The plots of the air temperature, dewpoint and wind information as functions of pressure are generally made on a specially prepared thermodynamic diagram. The altitude can be determined from the pressure by evaluating the hydrostatic equation. Mandatory and significant levels are determined. The data are encoded into the standard RAOB messages and transmitted by conventional communications networks to the National Meteorological Center. Currently, 70 RAOB stations are distributed across the continental United States. Upper level observations are taken every 12 hours (0000 and 1200 UTC).
DROPSONDE (or DROPWINDSONDE)
The dropsonde is an instrument package that represents a modification of a radiosonde. The dropsonde (or dropwindsonde) is used to collect low level weather data especially over ocean areas. These instruments have been used frequently by aircraft penetrating hurricanes. The dropsonde is literally dropped from an aircraft at flight level. The data collected by the dropsonde are radioed back to the aircraft. A parachute is deployed after the dropsonde leaves the aircraft to slow the flight of the dropsonde.
RADIOSONDE OBSERVATIONS (RAOB)
Meteorologists measure atmospheric conditions from the earth's surface to an altitude of approximately 30 km above sea level through twice daily radiosonde ascents. These balloon-borne instruments are sent aloft just prior to 0000 UTC and 1200 UTC on each day. During their ascent, they radio back to the ground-based receiving station a nearly continuous stream of information until the balloon bursts at approximately 10 mb. Radiosonde observations (also called RAOB) include the observed air temperature, pressure, moisture and wind information at various levels in the atmosphere. Within two hours after the radiosonde has been launched, the RAOB data have been encoded and transmitted over a communications network to the National Meteorological Center. At this center, the data can be processed for analysis onupper air charts and for use in numerical weather prediction models. To accomplish this task, all upper air stations are to report RAOB data for certain mandatory pressure levels. To speed the transmission process, the RAOB operator encodes only the temperature and dew point data for significant pressure levels along with the mandatory pressure levels. The significant pressure levels are those points ascertained from the plotted sounding where a significant change in the temperature and or dew point profile is detected.