Not all weather occurs at the earth's surface. The troposphere (weather "layer" of the atmosphere) and those embedded weather systems depicted on surface weather maps as Highs and Lows may extend to heights of several kilometers above the Earth's surface. At upper levels, far above the effects of surface friction on wind flow, distinct Highs and Lows usually give way to wave-like air currents that stretch around the globe. Meteorologists have long known that the atmosphere is three-dimensional and the understanding of weather requires knowledge about the atmosphere throughout its breadth and depth.
Meteorologists routinely monitor the atmosphere using essentially horizontal upper-air charts drawn at several different levels. These charts are drawn twice daily based on data collected at 00Z and 12Z by radiosondes launched from a network of stations. Each chart is drawn at a particular pressure level. Constant pressure surfaces are used as radiosondes report the data in terms of pressure, pilots often fly aircraft using pressure altimeters, and meteorological calculations are somewhat easier when expressed in terms of pressure values. A surface of constant pressure undulates slightly in altitude from place to place primarily due to the temperatures of the underlying atmosphere.
The set of current upper-air charts that can be accessed from the RealTime Weather Portal website 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 in Activity 8B of the Weather Studies Investigations Manual or you can click on the highlighted "DataStreme Weather Map Symbols" entry on the RealTime Weather Portal. 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.
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.
Why do meteorologists want to look at more than one level?