From: Subject: Thursday Weather Topic in Greater Depth Date: Thu, 22 Oct 2009 10:29:20 -0700 MIME-Version: 1.0 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.ametsoc.org/amsedu/online/archive/course/09_fall/f09w06r_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, 15 = October=20 2009

THE ADIABATS ON A ST=DCVE DIAGRAM

The St=FCve diagram (a blank St=FCve diagram can be obtained from the = AMS=20 Weather Studies website) is designed to permit evaluation of various = atmospheric process problems using simple graphical techniques. The = fundamental=20 coordinates on this diagram include temperature (horizontal axis) and = pressure=20 variations in the atmosphere (vertical axis). These coordinates appear = also on a=20 simplified diagram.

The primary vertical coordinate on this diagram is plotted in = pressure units,=20 decreasing from 1000 mb to 100 mb, instead of height because the desired = temperature calculations actually involve pressure changes. This scale=20 conversion between height and pressure has been performed for you. If = you refer=20 to the simplified diagram provided on page 2B-3 (Weather Studies=20 Investigation 2B), you will note the additional scale along the = right margin=20 that contains the approximate geometric altitude of various pressure = levels,=20 using a typical, reference atmosphere.

DRY ADIABATS

The dry adiabatic lapse rates are constructed on the St=FCve diagram = as=20 straight lines that slant from lower right toward upper left. These = lines are=20 often simply called "dry adiabats" to identify this process lapse rate. = If you=20 would lift a dry (unsaturated) air parcel from a known initial point = defined by=20 its temperature and pressure, to a final point, you could trace the = amount of=20 cooling on the nearest dry adiabat.

On a blank St=FCve diagram, locate an air parcel as a point at sea = level that=20 has a temperature of 17=BAC and air pressure of 1000 mb. Suppose that = you lifted=20 this air parcel to an altitude of 1000 meters; using the approximation = that air=20 pressure decreases by 1 mb for every 10 meters ascent, the air pressure = at 1000=20 meters would be approximately 900 mb. Now trace along the dry adiabat = from the=20 horizontal line marked 1000 mb to that marked 900 mb to determine how = the parcel=20 cools dry adiabatically. The final temperature is 8=BAC; because of the=20 approximations used, this graphically determined value is not = significantly=20 different from the value of 7=BAC that you would calculate from a 10 = Celsius=20 degree cooling over 1 kilometer. Continue along this adiabat and prove = to=20 yourself that at a pressure of 700 mb (an altitude of approximately 3000 = meters)=20 the temperature of your air parcel would have cooled to roughly -11=BAC. = Finally,=20 move back along the same adiabat to simulate sinking, compression and = heating.=20 Verify that the parcel would return to 17 degrees C by the time you = reached the=20 surface pressure of 1000 mb.

Note that you can also determine the response of parcels whose = initial=20 conditions do not lie on a dry adiabat by using interpolation. For = example, try=20 lifting a parcel with a temperature of 20=BAC and air pressure of 1000 = mb first to=20 900 mb and then to 700 mb. By tracing along an imaginary line parallel = to the=20 dry adiabat you should be able to find that at 900 mb (approximately = 1000=20 meters) the temperature would become 10.5=BAC and at a pressure of 700 = mb (an=20 altitude of approximately 3000 meters) the temperature of your air = parcel would=20 have cooled to roughly -10=BAC.

SATURATION ADIABATS

The saturation adiabatic lapse rates appear on a St=FCve diagram as a = set of=20 dashed curves (also called "moist adiabats") with slopes that only = become=20 tangent to the dry adiabats at low pressures and cold temperatures when = most of=20 the vapor has been removed from the parcel and little latent heat = release=20 occurs. This second process lapse rate varies between 2 and 9 Celsius = degrees=20 per kilometer; the value depends upon the amount of vapor, as well as = the=20 temperature and pressure of the air parcel. Warm moist air parcels can = contain=20 large quantities of vapor that liberate large amounts of latent heat = when=20 lifted; cool parcels have less vapor and less latent heat. A = representative=20 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=20 lifted from the surface (1000 mb at 17=BAC). Now follow the curve = passing through=20 that point that portrays the temperature changes that occur upon a = saturated air=20 parcel when lifted. If you ascend 1000 meters to 900 mb, notice that the = saturated parcel temperature was 12=BAC, or 4 degrees war mer than when = the parcel=20 was dry. The temperature difference results from warming produced by the = release=20 of latent heat into the parcel as vapor is condensed. Continuing to 3000 = meters=20 or 700 mb results in a final temperature of 4=BAC.

In situations where a saturated air parcel is forced to sink and no = cloud=20 droplets evaporate into the parcel, the parcel will warm at the dry = adiabatic=20 lapse rate because no phase change is involved. In other words, the air = parcel=20 is "drying out". Sinking causes compression and adiabatic heating, which = means=20 that the saturation vapor pressure increases, and if the actual moisture = content=20 (i.e., the vapor pressure) were held fixed, the relative humidity = decreases.=20 Cases do occur where evaporation of cloud droplets or precipitation = causes the=20 sinking air parcel to warm at the saturation adiabatic lapse rate = because the=20 parcel remains saturated until all droplets have evaporated.

PUTTING IT ALL TOGETHER

By using radiosonde observations of initial atmospheric conditions of = air=20 temperature, dewpoint and wind data at the various levels, = meteorologists can=20 employ the St=FCve diagram (or one of its counterparts) to predict = conditions in=20 the troposphere, and inside clouds, as weather systems move and = evolve.

Return to the Thursday=20 Daily Weather Summary

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