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. hopkins@meteor.wisc.eduMaster Links Page / Current Weather Page /ATM OCN 100 Home Page /AOS Dept. Home Page