upper-air chart

In meteorological jargon, the name given to any map showing meteorological information at a given atmospheric level, above sea level. It is also common to refer to it as a contour (isohypse) map, since these isolines are plotted on it. Together they make up the topography of a given pressure surface. Each contour (isohypse) is a level curve of the aforementioned isobaric surface. Upper-air charts are routinely produced for the standard levels of 850, 700, 500, 300, 200 and 100 hPa. Strictly speaking, they should be referred to as altitude charts, since they show us the altitudes (expressed in geopotential metres) at which those pressure levels are located. Both on these charts and on surface charts, the synoptic situation is clearly reflected, with all the players of the meteorological scene involved: anticyclones, depressions, cut-off lows, troughs, ridges… The upper-air charts properly speaking are what are known as 'relative topographies'. On these, taking as a base reference level the isobaric surface closest to sea level (usually 1000 hPa), contours (isohypses) are drawn showing the thicknesses reached by certain standard pressure levels. For example, on a 500 hPa relative topography, the variation in those thicknesses represents the variation in height experienced by that standard level relative to the 1000 hPa surface.