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We imagine a beautiful, cloudless day. The chance that the weather will change is slim. There are whiteclouds high up in the sky these ane calle cirrus clouds. They announce there arrival by causing the aor to go warm and damp. The warm air is already being transported because of its height. The aeroplanes leave a white vapour trail that looks like a white flag being pulled along by the plane. After some time the vapour clouds slowly disperse not to dissimilar from a natural cirrus cloud. More cirrus  clouds appear and gradually cover the sky with a white veil.Now the sun is barely visible.The barometer indicates that the pressure is about to fall.the white gradually clouds change into a bright grey , now the sun is very difficult to see.

Note: Since the zirrus clouds rose in the middle of the night, we used an exemplary picture (left).In the lower diagram the described period is coloured (lightgreen) emphasized.
Herald of a depression


© Carlo S. und Marcel S. (10f) Juni 2007/ Translated by: Nicholas Ingram (9bil) Feb 2008