Wednesday Weather Topic in Greater Depth

From: Subject: Wednesday Weather Topic in Greater Depth Date: Fri, 25 Sep 2009 16:40:58 -0700 MIME-Version: 1.0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.ametsoc.org/amsedu/dstreme/last_week/w_sup.html X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 Wednesday Weather Topic in Greater Depth

SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

To complement the Daily Summary for Wednesday, 16 = September=20 2009

Explanation of Upper Air Observations (In Tabular = Form)

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

Current upper air weather observations are made at approximately 70 = stations=20 throughout the country twice daily, at 00Z and 12Z. A list of these = upper air=20 stations appears in the User Guide. Thirty-two radiosonde = stations can be=20 accessed from the DataStreme Atmosphere website under the heading = Upper Air Data- Text by selecting the desired upper air station. = Once=20 selected, a tabulation of the radiosonde observations for that sounding = will=20 appear. Remember that these files are updated on a twice-daily basis. =

INFORMATION CONTENTS OF THE TABLE

The during its ascent, the radiosonde sends a nearly continuous = stream of=20 observed temperature and humidity data keyed to the pressure down to the = ground=20 receiving station. To reduce the amount of compiled radiosonde data, a = data set=20 for a limited number of pressure levels is transmitted along the = traditional=20 weather communications network. This data sequence must include = "mandatory=20 levels", which must be reported for use for various upper air charts and = in=20 numerical prediction models. These mandatory levels include the 1000, = 925, 850,=20 700, 500, 400, 300, 250, 100 mb levels. A selected number of = "significant=20 levels" are also reported where a significant change in the temperature = and/or=20 dewpoint (a measure of atmospheric humidity) profile occurs. No in = formation is=20 lost since one could graphically interpolate to obtain the temperature = or=20 dewpoint at any altitude between successive significant levels. Wind = data (speed=20 and direction) are included at each level. The total number of levels = appearing=20 in the radiosonde report will depend upon the complexity of the vertical = structure of the atmosphere as observed by the radiosonde.

Station Information Block
The lead block includes = information=20 for the particular radiosonde launch (time and date) and the station = (the=20 3-letter Federal Aviation Administration identifier, the 5-digit WMO=20 identifier of the World Meteorological Organization), LAT=3Dthe = latitude in=20 degrees, LON=3Dthe longitude in degrees and ELV=3Dthe elevation above = mean sea=20 level in meters).=20
PRES=3DPressure in millibars (mb)
The radiosonde = contains a=20 calibrated aneroid barometer that measures and reports the air = pressure as the=20 radiosonde ascends through the atmosphere.=20
HGHT=3DHeight of level in meters (m) above mean sea level =
The=20 altitude of the radiosonde at the reported level above the surface = (SFC) is=20 computed at the ground station using an equation that incorporates the = average=20 air temperature, the atmospheric humidity and the pressure in the = atmospheric=20 layer through which the radiosonde has just traveled. "Heights" of = mandatory=20 levels that are locally underground are also calculated.=20
TMPC=3DAir temperature in degrees Celsius (C)
The air = temperature=20 at each reported level has been measured by an electronic thermometer = carried=20 on the outside of the radiosonde.=20
DWPC=3DDewpoint in degrees Celsius (C)
The dewpoint at = the=20 reported level, a measure of the water vapor content of the air at = that level,=20 is measured by an electronic device called a hygristor carried in the=20 radiosonde. The dewpoint will never exceed the air temperature.=20
RELH=3DRelative Humidity as a percentage (%)
The = relative=20 humidity at a given level as a percentage is used to describe how = close the=20 measured water vapor in the atmosphere is to saturation. When = RELH=3D100%, the=20 air would be saturated with respect to water vapor, and the air = temperature=20 and the dewpoint are equal.=20
DRCT=3DWind direction in degrees (deg) measured clockwise from = true north=20 (360 degrees)
The wind direction at a given level above the = surface is=20 determined from the tracking of the motion of the radiosonde by the=20 ground-based radio receiver/antenna system The following notation is = used:=20
- 090 degrees=3Deast wind, 180=3Dsouth wind, 270=3Dwest wind, = 360=3Dnorth wind and=20 00=3Dcalm
SKNT=3DWind speed in knots (knt)
The wind speed at each = level is=20 based upon the tracking of the radiosonde by the ground based radio = receiver.=20
MIXR=3DMixing Ratio in gms of water vapor per kg of dry = air
The=20 mixing ratio, a dimensionless ratio that represents another measure of = the=20 water vapor content of the air at that level, is reported from the = humidity=20 measured by the hygristor.

NOTE:

The first level that appears in each report is the surface data that = were=20 reported at the station at the time of launch. Some mandatory levels = will be=20 underground, especially at locations such as the "Mile High City" of = Denver, CO,=20 and the only information reported will be a computed value of the = altitude of=20 the pressure level based upon certain assumptions.

GRAPHICAL DISPLAYS

The upper air weather data appearing in the text format can be = displayed=20 graphically using a special diagram called a St=FCve diagram. A more = detailed=20 discussion of the display of the plotted sounding on a St=FCve diagram = will be=20 described in the Thursday Supplemental Information.

Return to the Wedne= sday=20 Daily Summary

Return to DataStreme = Atmosphere=20 website

Prepared by Edward J. Hopkins , Ph.D., email hopkins@meteor.wisc.edu =
=A9=20 Copyright, 2009, The American Meteorological Society. =