z T 500 hPa:
Geopotential height at 500 hPa (solid line)
Temperature at 500 hPa (colored, dashed)

The maps show the predominant tropospheric waves (trough or ridge). They virtually control the "weather" (dry, warm / wet, cold) and the long waves drive the smaller synoptic waves. Thus, this upper-level chart illustrates the dynamics of our atmosphere.

GMT = Greenwich Mean Time