From: Subject: Thursday Weather Topic in Greater Depth Date: Fri, 25 Sep 2009 16:41:08 -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/r_sup.html X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 Thursday Weather Topic in Greater Depth

SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

To complement the Daily Summary for Thursday, 17 = September=20 2009

THERMODYNAMIC DIAGRAMS

For more than half a century, routine upper-air observations have = been made=20 by instruments carried aloft by a balloon to altitudes of approximately = 20=20 kilometers. Temperature, humidity and air pressure data are transmitted = back to=20 earth by an on-board FM radio transmitter from this instrument package = called a=20 "radiosonde". Wind speed and wind direction information for various = levels are=20 determined by using ground-based radio direction finders that track the = motion=20 of the radiosonde. Together the radiosonde data and wind information = form what=20 is termed a "rawinsonde". The Wednesday optional Suppl= emental=20 Information describes the set of current upper-air data that are = collected=20 from a sounding of the atmosphere provided by a rawinsonde.

INSPECTING THE CHART

We often want to plot the vertical variation in temperature, humidity = and=20 wind data obtained from a rawinsonde to display the vertical atmospheric = structure. Such a finished plot, known as a vertical profile, reveals = certain=20 features of the atmospheric structure that are critical to describing = and=20 forecasting the weather above the launch site. A graph, similar to that=20 appearing in Investigation 2B, could be used to accomplish this task. = You can=20 also get more blank plotting charts by clicking on the "Blank St=FCve-T, = p lines"=20 en try on the DataStreme Atmosphere website. (This chart is a = simplified=20 version of the more complex "Blank St=FCve - all lines" that will be = described in=20 detail subsequently. The chart is also named the St=FCve diagram for its = inventor.) The Investigation 2B chart differs slightly from the ones = routinely=20 delivered by DataStreme Atmosphere since two vertical scales were = used.=20 This feature permits us to plot the temperature data either in terms of = altitude=20 or in terms of pressure, since the right hand vertical scale is divided = into=20 altitude increments, while the left hand scale has vertical pressure = increments.=20

Meteorologists often describe the location of an object (such as the=20 radiosonde or an imaginary parcel of air) above the earth's surface in = terms of=20 the pressure, rather than in terms of the actual altitude in feet or = meters=20 above the surface. While this choice may not be completely obvious, use = of=20 pressure as a vertical scale is advantageous. Until recently, the = altitude=20 derived from the observed air pressure was easier to measure than actual = vertical distance and the mathematical equations used for weather = prediction are=20 simpler when written in pressure units.

DATA PLOTTING

The vertical variations in the data can be plotted on the diagram in = the same=20 fashion as used for plotting any two- dimensional chart. To locate a = data point,=20 you can always interpolate, or draw in all these lines through that = point as=20 long as you keep them parallel to the reference lines! Sometimes the = temperature=20 (or other weather element) data are provided in terms of vertical height = coordinates, as in the tabulation of the U.S. Standard Atmosphere, a = model=20 developed originally for aircraft design and performance. These data can = be=20 plotted using the right-hand height scale. Frequently, the data = collected by=20 radiosondes are tabulated in terms of atmospheric pressure. These data = are=20 plotted using the left-hand scale. Since pressure always decreases with = height,=20 the vertical pressure coordinate can be readily substituted for the = vertical=20 height coordinate.

DATA INTERPRETATION - VERTICAL TEMPERATURE PROFILES

The plot of the vertical temperature data from an instrument sounding = of the=20 atmosphere becomes a continuous temperature profile when the data points = are=20 connected by straight-line segments. In the troposphere, the temperature = usually=20 decreases with altitude. Such a cooling in temperature with altitude = throughout=20 the troposphere is indicated by the plot of the Standard Atmosphere, an = average=20 vertical structure of the atmosphere.

However, on any particular day, the temperature profile obtained from = the=20 radiosonde sounding could depart significantly from this reference, = especially=20 within the lowest several kilometers of the troposphere. Two other = temperature=20 profile patterns can exist. If the environmental temperature measured by = the=20 radiosonde increases as the height increases through some distance, then = that=20 layer would be called an "inversion". If the temperature remains = constant as the=20 height increases, then that layer is called an "isothermal layer".

DETERMINING HEIGHT OF THE TROPOPAUSE

The tropopause is a seasonally and latitudinally variable boundary = between=20 the troposphere and the stratosphere. Its location can be determined = from the=20 plotted temperature sounding. Usually this height is defined as the = lowest point=20 (well above the surface) where the temperature profile becomes = isothermal or=20 develops an inversion over an extended layer. Typically, this level = occurs=20 somewhere between 6 km in polar regions and 16 km in the tropics. = Weather=20 systems are effectively contained within the troposphere and its = changing depth=20 directly affects thunderstorm and hurricane development.

Return to the Thurs= day Daily=20 Summary

Return to DataStreme = Atmosphere=20 website

Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu=20
=A9Copyright, 2009, The American Meteorological Society.=20