Over the past years many questions which come under the broad heading of cloud physics have been put to the Meteorological Office by aircraft firms and others, questions on the water and ice crystal contents of clouds and the size distribution of raindrops are examples. These questions have invariably been easy to ask and at first sight the answers would appear to be straightforward, but almost equally invariably these simple questions have proved difficult to answer with precision and in many cases estimates only have been possible. This may well appear surprising and it is the purpose of this paper to indicate some of the difficulties encountered.