Thursday Weather Topic in Greater Depth

From: Subject: Thursday Weather Topic in Greater Depth Date: Fri, 30 Oct 2009 14:15:44 -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/f09w08r_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, 29 = October=20 2009

THE SUITE OF OPERATIONAL UPPER AIR WEATHER = CHARTS

The set of current upper air charts that can be accessed from the = AMS=20 Weather Studies Homepage includes constant pressure charts for 850 = mb, 700=20 mb, 500 mb and 300 mb. Each chart contains plotted data obtained for = that=20 particular pressure level from the radiosonde network. These plotted = data are=20 arranged around a given station location using the standard upper air = station=20 model appearing on page 8B-1 of Activity 8B in the Weather Studies=20 Investigations Manual or you can click on the highlighted "AMS = Weather=20 Studies Map Symbols" entry on the AMS Weather Studies website. = The=20 atmospheric variables typically plotted on these isobaric maps include = (1) the=20 height of the pressure surface above sea level; (2) the air temperature; = (3) the=20 wind speed and direction; and (4) when applicable, the dewpoint, an = indicator of=20 atmospheric humidity.

FEATURES OF THE ISOBARIC SURFACES

Essentially all these charts can be produced with analyses that = include=20 height contours (lines connecting all points on the surface having the = same=20 altitude) and isotherms (lines of equal temperature). Some charts, = primarily the=20 300-mb chart, may have "isotachs", which are lines of equal wind speed. =

The altitude of the isobaric surface above sea level depends upon the = density, and hence the temperature of the intervening air column. In = regions=20 where the air in that column is cold and dense, the altitude of that = isobaric=20 surface will be lower than over a region where the air is warmer and = less dense.=20

Since isobaric surfaces are three dimensional surfaces, "height = contours" (or=20 simply, "contours") drawn upon an isobaric chart represent the = topography of=20 that pressure surface in the same way as contours are drawn by = cartographers=20 upon topographic maps to depict the terrain. Contours on upper air maps = separate=20 regions of greater height for a given map region from lower altitude = regions.=20 Because of the contour patterns, the higher altitude regions = representing=20 poleward intrusions of warm air, are identified as "height ridges" or = simply=20 "ridges". On Northern Hemisphere upper air charts, these ridges can be=20 identified where height contours deviate to the north. Strong ridges are = usually=20 associated with warm and dry surface weather. On the other hand, the = lower=20 altitude portions of the pressure surface are "height troughs", or = "troughs",=20 with equatorward intrusions of cold polar air. Troughs can be identified = where=20 height contours are deflected to the south. Stormy weather and cold = temperatures=20 at the surface are often found under upper level troughs.

The isotherms and the resultant analyzed temperature field on many of = the=20 upper air charts often support the above relationships. Typically, the = best=20 agreement occurs in the lower to mid troposphere. Some displacement of = the=20 isotherms away from the ridges and troughs may occur especially in the = upper=20 troposphere.

On the upper tropospheric charts, isotachs are often drawn to = identify the=20 jet stream. Typically, winds are considered part of the jet if the = speeds were=20 at least 70 knots. These regions as highlighted by the isotachs may be = elongated=20 and frequently found near the southern excursion of a trough.

THE SUITE OF UPPER LEVEL CHARTS

Why do meteorologists want to look at more than one level?

The 850-mb chart, representing weather conditions in = the=20 lower troposphere, is at a level that is above approximately 15 = percent of the=20 atmosphere in terms of mass. At an altitude of approximately 1500 = meters (5000=20 feet), this level is above most of the influences of surface friction = in the=20 many sections of the country. Unfortunately, the 850-mb surface = intersects and=20 goes below the terrain in the Rocky Mountains. For example, the "Mile = High=20 City" of Denver, CO usually has a surface pressure - a measured value = not=20 corrected to sea level - of approximately 830 mb. Consequently, ground = level=20 at Denver is at a higher altitude than the 850-mb surface. = Meteorologists=20 often look at the analyzed temperature field because over the = non-mountainous=20 regions, the diurnal temperature cycle is much less than at the = surface. They=20 can frequently tell correctly that precipitation falling in regions = with an=20 850-mb temperature of 0 degrees Celsius or less will probably fall as = snow,=20 while rain would more than likely fall at warmer temperatures.=20
The 700-mb chart at an altitude of approximately = 3000 meters=20 (10,000 ft) depicts the weather in a region of the troposphere that is = above=20 all but the highest peaks in the Rocky Mountains. Consequently, some=20 meteorologists prefer this level to the 850-mb level.=20
The 500-mb chart represents weather conditions in = the mid-=20 troposphere, at a level where approximately half the mass of the = atmosphere=20 lies below this level. This level is at an altitude of approximately = 5,500=20 meters (18,000 ft). This level is often used to represent upper level = flow=20 conditions because the level is well above the effects of topography = and=20 friction and the level is below the region in the upper troposphere = where the=20 air flow may experience strong acceleration and deceleration when in = the=20 vicinity of the upper jet streams. Since many weather systems tend to = follow=20 the wind flow at this level, this level is often considered to = symbolize the=20 steering level of these systems.=20
The 300-mb chart is near the tropopause, at the top = of the=20 troposphere. Only 30 percent of the mass of the atmosphere lies above = this=20 level. The altitude of the 300-mb surface is near 9000 meters (30,000 = ft) --=20 at a level where many long-distance commercial jet aircraft fly. This = level=20 also approximates the level of the upper tropospheric jet stream, a = region of=20 very fast winds that move across the country. Inspection of the = isotach=20 patterns at this level not only reveals the location of the jet = streams, but=20 aids the meteorologist in locating the regions of largest = acceleration,=20 deceleration and wind shear (rapid changes in wind speed and/or = direction over=20 distance); these regions contribute to the upper level horizontal = divergence=20 and convergence patterns that influence surface weather systems. =

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. =