Baggrund   Dew point is the temperature at which saturated air cools to form water vapour (with relative humidity at 100 percent). Should any further cooling occur, condensation will begin to occur forming clouds.   The figure shows what happens when an air parcel with a temperature of 20 degrees centigrade and a relative humidity of 57% is forced to cross a mountain range with an altitude of 2 500 m. The dew point is reached at 900m and clouds are formed. The air is cooled by approximately 1 degree centigrade per 100 metres until the dew point is reached. Above the condensation level the air is cooled at a lapse rate of 0.5 degrees centigrade per 100m. So the temperature of the air at the top of the range will be 3 degrees centigrade. Jetstrømmen     The jet stream in the upper atmosphere determines the weather situation in Europe.