Model: Local weather forecast model from the UK MetOffice Update: 4 times per day, from 00:00, 05:00, 11:00 und 17:00 GMT Greenwich Mean Time: 12:00 GMT = 13:00 BST Variable: Geopotential Height and Temperature at 500 hPa Description: The maps show the predominant tropospheric waves (trough or ridge). They virtually control the "weather" (wet, cold / dry, warm) and the long waves drive the smaller synoptic waves. Thus, this upper-level chart illustrates the dynamics of our atmosphere.