ONLINE WEATHER SUPPLEMENTAL INFORMATION

To complement the Daily Summary for Thursday, 9 April 1998

THE SUITE OF OPERATIONAL UPPER AIR WEATHER CHARTS

The set of current upper air charts that can be accessed from the Online Weather Homepage includes constant pressure charts for 850 mb, 700 mb, 500 mb and 300 mb. Each chart contains plotted data obtained for that particular pressure level from the radiosonde network. These plotted data are arranged around a given station location using the standard upper air station model appearing on page 8B-2 of Online Weather Activity 8B, page 19 of the User's Guide or you can click on the highlighted "Online Weather Map Symbols"entry on the Online Weather Homepage. The atmospheric variables typically plotted on these isobaric maps include (1) the height of the pressure surface above sea level; (2) the air temperature; (3) the wind speed and direction; and (4) when applicable, the dewpoint, an indicator of atmospheric humidity.

FEATURES OF THE ISOBARIC SURFACES

Essentially all these charts can be produced with analyses that include height contours (lines connecting all points on the surface having the same altitude) and isotherms (lines of equal temperature). Some charts, primarily the 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 where the air in that column is cold and dense, the altitude of that isobaric surface will be lower than over a region where the air is warmer and less dense.

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

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

On the upper tropospheric charts isotachs are often drawn to identify the jet stream. Typically, winds are considered part of the jet if the speeds were at least 70 knots. These regions as highlighted by the isotachs may be elongated 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 lower troposphere, is at a level that is above approximately 15 percent of the atmosphere in terms of mass. At an altitude of approximately 1500 meters (5000 feet), this level is above most of the influences of surface friction in the many sections of the country. Unfortunately, the 850 mb surface intersects and goes below the terrain in the Rocky Mountains. For example, the "Mile High City" of Denver, CO usually has a surface pressure - a measured value not corrected to sea level - of approximately 830 mb. Consequently, ground level at Denver is at a higher altitude than the 850 mb surface. Meteorologists often look at the analyzed temperature field because over the non-mountainous regions, the diurnal temperature cycle is much less than at the surface. They can frequently tell correctly that precipitation falling in regions with an 850 mb temperature of 0 degrees Celsius or less will probably fall as snow, while rain would more than likely fall at warmer temperatures.
• The 700 mb chart at an altitude of approximately 3000 meters (10,000 ft) depicts the weather in a region of the troposphere that is above all but the highest peaks in the Rocky Mountains. As a result, some meteorologists prefer this level to the 850 mb level.
• The 500 mb chart represents weather conditions in the mid- troposphere, at a level where approximately half the mass of the atmosphere lies below this level. This level is at an altitude of approximately 5,500 meters (18,000 ft). This level is often used to represent upper level flow conditions because the level is well above the effects of topography and friction and the level is below the region in the upper troposphere where the air flow may experience strong acceleration and deceleration when in the vicinity of the upper jet streams. Since many weather systems tend to follow the wind flow at this level, this level is often considered to symbolize the steering level of these systems.
• The 300 mb chart is in the vicinity of the tropopause, at the top of the troposphere. Only 30 percent of the mass of the atmosphere lies above this level. The altitude of the 300 mb surface is near 9000 meters (30,000 ft) -- at a level where many long-distance commercial jet aircraft fly. This level also approximates the level of the upper tropospheric jet stream, a region of very fast winds that move across the country. Inspection of the isotach patterns at this level not only reveals the location of the jet streams, but aids the meteorologist in locating the regions of largest acceleration, deceleration and wind shear (rapid changes in wind speed and/or direction over distance); these regions contribute to the upper level horizontal divergence and convergence patterns that influence surface weather systems.

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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 1999, The American Meteorological Society.