The ground based study of clouds

• Main Author: Collin, H. L.
• Published: Durham University 1969
• Subjects: 551.57
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.578156

Man has long observed the forms and movement of clouds and attempted to account for them. Aristophenes and Lucretius among others have reported contemporary ideas, but a more detailed knowledge of cloud structure was required before much progress could be made towards understanding them. A certain amount can indeed be learned by visual observations The ice or water phases can be recognised and the lifetime and development, in particular of cumulus clouds, can be observed. Time lapse photography and theodolite measurements of cloud tops (Scorer and Ludlam, 1953) can be used to determine the updraughts inside them. Radar can be used to follow the development of clouds over a large area. Such studies have shown that cumulonimbus exhibits a cellular structure. These cells have a characteristic life history (Byers and Braham, 19^9) and orographic features are often found to influence their initiation . The influence of orography can also be studied by means of a fine network of rai n gauges(Bergeron, I96O). Ice and water have different dielectric, constants so the same intensity of precipitation gives rise to a much stronger radar echo if it is in liquid form. The melting zone gives an especially strong echo since the just melted drops have not yet reached their terminal velocity and spread out. Thus examination of the radar characteristics of clouds can reveal their gross structure. However, such important factors as temperature and humidity cannot be found and the detailed structure of the precipitation and a i r movements only with some ambiguity and in limited circumstances. Since the radar echo exhibits a doppler shift if the target is moving a vertical pointing radar can be used to determine the fall speeds of precipitation elements. In a convective storm the terminal velocities of the particles are less than the updraughts, and Atlas (1965) has made use of t h i s to determine the pattern of convection i n a storm. Such an investigation requires assumptions to be made about the s i z e distributions of the precipitation p a r t i c l e s , but radar can be used to determine these i f the updraughts can be neglected as i n nimbostratus conditions (Caton, 1966). Since radar gives the s i z e distributions at a l l heights i t becomes possible to study the growth of the particles as they fall through the cloud. Similar studies can be made at the ground if a number of observing stations are situated at various heights, for example on a mountain side (Magono, 1960). While this kind of investigation i s limited to only a few levels and these below or only i n the lower part of the cloud, it does not suffer from the uncertainties that are attendant on similar radar work and there is also the advantage of being able to determine the local meteorological conditions. Some studies can be made at ground stations that are impossible for radar, for example the size distributions of cloud droplets (Okita, 1962) and studies in the region where ice crystals are melting and their velocities varying (Ohtake, 1965).