For more than half a century routine upper air observations have been made by instruments carried aloft by a balloon to altitudes of approximately 20 kilometers. Temperature, humidity and air pressure data are transmitted back to earth by an on-board FM radio transmitter from this instrument package called a "radiosonde". Wind speed and wind direction information for various levels are determined by using ground-based radio direction finders that track the motion of the radiosonde. Together the radiosonde data and wind information form what is termed a "rawinsonde". The Wednesday optional Supplemental Information describes the set of current upper air data that are collected from a sounding of the atmosphere provided by a rawinsonde.
We often want to plot the vertical variation in temperature, humidity and wind data obtained from a rawinsonde to display the vertical atmospheric structure. Such a finished plot, known as a vertical profile, reveals certain features of the atmospheric structure that are critical to describing and forecasting the weather above the launch site. A graph, similar to that appearing in Activity 2B, could be used to accomplish this task. You can also get more blank plotting charts by clicking on the "Blank Stüve-T, p lines" entry on the Online Weather Homepage. (This chart is a simplified version of the more complex "Blank Stüve - all lines" that will be described in detail subsequently. The chart is also named the Stüve diagram for its inventor.) This chart differs slightly from the ones routinely delivered by Online Weather since two vertical scales are used. This feature permits us to plot the temperature data either in terms of altitude or in terms of pressure, since the right hand vertical scale is divided into altitude increments, while the left hand scale has vertical pressure increments.
Meteorologists often describe the location of an object (such as the radiosonde or an imaginary parcel of air) above the earth's surface in terms of the pressure, rather than in terms of the actual altitude in feet or meters above the surface. While this choice may not be completely obvious, use of pressure as a vertical scale is advantageous. Until recently, the altitude derived from the observed air pressure was easier to measure than actual vertical distance and the mathematical equations used for weather prediction are simpler when written in pressure units.
The vertical variations in the data can be plotted on the diagram in the same fashion as used for plotting any two- dimensional chart. To locate a data point, you can always interpolate, or draw in all these lines through that point as long as you keep them parallel to the reference lines! Sometimes the temperature (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 developed originally for aircraft design and performance. These data can be plotted using the right-hand height scale. Frequently, the data collected by radiosondes are tabulated in terms of atmospheric pressure. These data are plotted using the left-hand scale. Since pressure always decreases with height, the vertical pressure coordinate can be readily substituted for the vertical height coordinate.
The plot of the vertical temperature data from an instrument sounding of the atmosphere becomes a continuous temperature profile when the data points are connected by straight line segments. In the troposphere, the temperature usually decreases with altitude. Such a cooling in temperature with altitude throughout the troposphere is indicated by the plot of the Standard Atmosphere, an average vertical structure of the atmosphere.
However, on any particular day, the temperature profile obtained from the radiosonde sounding could depart significantly from this reference, especially within the lowest several kilometers of the troposphere. Two other temperature profile patterns can exist. If the environmental temperature measured by the radiosonde increases as the height increases through some distance, then that layer would be called an "inversion". If the temperature remains constant as the height increases, then that layer is called an "isothermal layer".
The tropopause is a seasonally and latitudinally variable boundary between the troposphere and the stratosphere. Its location can be determined from the plotted temperature sounding. Usually this height is defined as the lowest point (well above the surface) where the temperature profile becomes isothermal or develops an inversion over an extended layer. Typically this level occurs somewhere between 6 km in polar regions and 16 km in the tropics. Weather systems are effectively contained within the troposphere and its changing depth directly affects thunderstorm and hurricane development.
Return to the Thursday Daily Weather Summary
Return to Online Weather Homepage
Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 1999, The American Meteorological
Society.
