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: Relative Humidity at 700 hPa Description: This chart shows the relative humidity at 700 hPa. In the forefield of a trough line as well as at and near fronts (Jets), warmer less dense air is forced to ascend. As the ascending air cooles, the relative humidity increases, eventually resulting in condensation and the formation of clouds. This process is known as frontal lifting. High relative humidity at 700 hPa - equivalent to ca. 10,000 ft a.s.l. - indicates the areas of frontal lifting and thus the active zones of the current weather.