The Stüve diagram (a blank Stüve diagram can be obtained from the Online Weather Homepage) is designed to permit evaluation of various atmospheric process problems using simple graphical techniques. The fundamental coordinates on this diagram include temperature (horizontal axis) and pressure variations in the atmosphere (vertical axis). These coordinates appear also on a simplified diagram.
The primary vertical coordinate on this diagram is plotted in pressure units, decreasing from 1000 mb to 100 mb, instead of height because the desired temperature calculations actually involve pressure changes. This scale conversion between height and pressure has been performed for you. If you refer to the simplified diagram provided on page 2B-2, you will note the additional scale along the right margin that contains the approximate geometric altitude of various pressure levels, using a typical, reference atmosphere.
The dry adiabatic lapse rates are constructed on the Stüve diagram as straight lines that slant from lower right toward upper left. These lines are often simply called "dry adiabats" to identify this process lapse rate. If you would lift a dry (unsaturated) air parcel from a known initial point defined by its temperature and pressure, to a final point, you could trace the amount of cooling on the nearest dry adiabat.
On a blank Stüve diagram locate an air parcel as a point at sea level that has a temperature of 20 degrees Celsius and air pressure of 1000 mb. Suppose that you lifted this air parcel to an altitude of 1000 meters; using the approximation that air pressure decreases by 1 mb for every 10 meters ascent, the air pressure at 1000 meters would be approximately 900 mb. Now trace along the dry adiabat from the horizontal line marked 1000 mb to that marked 900 mb to determine how the parcel cools dry adiabatically. The final temperature is 10.5 degrees C; because of the approximations used, this graphically determined value is not significantly different from the value of 10 degrees C that you would calculate from a 10 Celsius degree cooling over 1 kilometer. Continue along this adiabat and prove to yourself that at a pressure of 700 mb (an altitude of approximately 3000 meters) the temperature of your air parcel would have cooled to roughly -10 degrees C. Finally, move back along the same adiabat to simulate sinking, compression and heating. Verify that the parcel would return to 20 degrees C by the time you reached the surface.
The saturation adiabatic lapse rates appear on a Stüve diagram as a set of dashed curves (also called "moist adiabats") with slopes that only become tangent to the dry adiabats at low pressures and cold temperatures when most of the vapor has been removed from the parcel and little latent heat release occurs. This second process lapse rate varies between 2 and 9 Celsius degrees per kilometer; the value depends upon the amount of vapor, as well as the temperature and pressure of the air parcel. Warm moist air parcels can contain large quantities of vapor that liberate large amounts of latent heat when lifted; cool parcels have less vapor and less latent heat. A representative estimate of the saturation adiabatic lapse rate is 6 Celsius degrees per kilometer.
Suppose that you assume that your air parcel had been saturated before it was lifted from the surface (1000 mb at 20 degrees C). Now follow the curve passing through that point that portrays the temperature changes that occur upon a saturated air parcel when lifted. If you ascend 1000 meters to 900 mb, notice that the saturated parcel temperature was 16 degrees C, or 6 degrees warmer than when the parcel was dry. The temperature difference results from warming produced by the release of latent heat into the parcel as vapor is condensed. Continuing to 3000 meters or 700 mb results in a final temperature of 6 degrees C.
In situations where a saturated air parcel is forced to sink and no cloud droplets evaporate into the parcel, the parcel will warm at the dry adiabatic lapse rate because no phase change is involved. In other words, the air parcel is "drying out". Sinking causes compression and adiabatic heating, which means that the saturation vapor pressure increases, and if the actual moisture content (i.e., the vapor pressure) were held fixed, the relative humidity decreases. Cases do occur where evaporation of cloud droplets or precipitation causes the sinking air parcel to cool at the saturation adiabatic lapse rate because the parcel remains saturated until all droplets have evaporated.
By using radiosonde observations of initial atmospheric conditions of air temperature, dewpoint and wind data at the various levels, meteorologists can employ the Stüve diagram (or one of its counterparts) to predict conditions in the troposphere, and inside clouds, as weather systems move and evolve.
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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 1999, The American Meteorological Society.
