The upper air data that are collected and transmitted during the flight of a radiosonde include the air pressure, air temperature and humidity measured continuously by the instruments aboard the radiosonde package. The radiosonde observations (RAOB) are directly transmitted by the radio transmitter for various levels in the free atmosphere. At the receiving station, the height of the package is sequentially computed at each reporting level using an equation (hypsometric equation) from the reported pressure, temperature and humidity of each incremental layer. The wind information at various levels is determined The wind speed and wind direction at various levels are determined from the ground-based radio tracking antenna that tracks the instrument package as it is carried by the wind during the ascent of the radiosonde.
These observations are processed, tabulated and encoded for transmission over communication networks. While the radiosonde transmits an essentially continuous stream of temperature and humidity information back to the station, the RAOB information disseminated over the conventional weather communications network is limited by necessity to the above mentioned weather elements at the following atmospheric levels:
SYMBOL VARIABLE and UNITS REMARKS LEV Index number of layer The number of the layer starts with the first significant or mandatory level above the surface, identified as "SFC" in the table. PR Pressure in millibars The pressure measured by the [mb] radiosonde is used to describe its vertical displacement in the atmosphere and identifies both the mandatory and significant levels. H Height in meters [m] This computed height is reported only for the surface and the mandatory levels only. T Air Temperature [deg.C} The measured air temperature and TD Dewpoint [deg. C} dewpoint are reported to tenths of a degree Celsius. DIR Wind Direction The wind direction is from where the [degrees] wind is blowing and is measured as an angle in the clockwise direction from true north (360 deg). Calm conditions are reported as 0 deg. SPD Wind Speed [knt] The wind speed is measured in knots Note: Wind speed and direction are reported in this table at mandatory levels only. THETA Potential Temperature The potential temperature is a [K] computed variable used by meteorologists to identify an air parcel. It is defined as the absolute temperature [in kelvins] that an air parcel would reach if the parcel were brought from its initial conditions to a pressure of 1000 mb by a dry adiabatic process. THETAE Equivalent Potential The equivalent potential is another Temperature [K} computed variable used by meteorologists to identify an air parcel. This variable differs from the potential temperature in that it incorporates the effects of humidity within the air parcel. The value is defined as the temperature that an air parcel would have if all its water vapor were condensed and removed and the parcel brought by a dry adiabatically to a pressure of 1000 mb. W Mixing Ratio [g/kg] The mixing ratio that is computed for each level is defined as the mass of the water vapor in a unit mass of dry air, typically reported in grams of water vapor per kilogram of dry air.
At the bottom of this table two other levels may be included:
TROP Height of the In this tabulation, the height of the Tropopause tropopause is given in pressure (mb) rather than geometric height units. The tropopause represents the boundary between the troposphere and the stratosphere and is usually identified as the region in the upper troposphere where the usual decrease in temperature changes to an isothermal or to an inversion condition. The conventional tropopause is defined as the first tropopause, or the lowest altitude level where the average lapse rate within the next higher 2 km does not exceed 2 Celsius deg per km. WIND Level of Maximum Wind In this tabulation, the height of the tropopause is given in pressure rather than geometric height units. Typically this maximum wind group will only be reported if the maximum wind speed were in excess of 60 knots.
Sounding Variables and indices
Convective or Convection The altitude (expressed as a pressure level condensation level in mb) representing the height of the base of a cumuliform cloud, produced solely from thermal convection caused by surface heating. Defined as that level to which an air parcel, if heated sufficiently from below (with a surface temperature equal to or greater than the convective temperature) will rise dry adiabatically without becoming colder than its environment until the parcel just becomes saturated. Mean mixing ratio The average mixing ratio (see above) in g per kg for the entire layer from the surface to the last reported dewpoint level. Convective temperature The temperature (in degrees Celsius) often used to forecast the onset of convection and cumuliform clouds because if the surface temperature reaches this value, surface air parcels would become sufficiently buoyant to reach the Convective condensation level. Freezing Level The lowest altitude (expressed as a pressure level in mb) where the air temperature falls to 0 degrees Celsius. The height of this level is important for predicting hail size and for forecasting aviation icing hazards. Lifting condensation level The altitude (expressed as a pressure level in mb) equivalent to the height of the base of a stratiform cloud, if condensation were caused by forced accent. This level is the lowest altitude at which a parcel of moist air would just become saturated when lifted dry adiabatically by a mechanical process. 1000-500 mb thickness The vertical distance (in meters) between the 1000 mb and the 500 mb pressure surfaces. The thickness is directly proportional to mean temperature of this 1000 to 500 mb layer. Precipitable water The total amount of water vapor contained in a vertical column of unit cross section area, expressed as the depth (in inches) of the liquid equivalent of the vapor in the column if all that water vapor were condensed and collected. Sfc to 500 mb mean relative A value (expressed as a percent) that humidity indicates on average how close to saturation that the air is in the surface to 500 mb layer. Lifted Index A stability index with the numerical value computed from the sounding, together with the risk for severe thunderstorms Showalter Index Ditto Vertical Totals Index Ditto Cross Totals Index Ditto Totals Totals Index Ditto K Index Ditto Sweat Index Ditto Energy Index Ditto
Last revision 23 June 1996© Copyright, 1996 Edward J. Hopkins, Ph.D. firstname.lastname@example.org
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