Before Luke Howard invented his system for classifying clouds, they had simply been described by their shape and color as each person saw them: they were too changeable and moved too quickly for anyone to think they could be classified in any useful way. Howard had been interested in clouds - and meteorology in general - ever since he was a small boy, and for thirty years kept a record of his meteorological observations. In 1802-1803, he produced a paper in which he named the clouds, or, to be more precise, classified them, claiming that it was possible to identify several simple categories within the various and complex cloud forms. As was standard practice for the classification of plant and animal species, they were given Latin names, which meant that the system could be understood throughout Europe.
Howard believed that all clouds belonged to three distinct groups; cumulus, stratus and cirrus. He added a fourth category, nimbus, to describe a cloud "in the act of condensation into rain, hail or snow". It is by observing how clouds change color and shape that weather can be predicted, and as long as the first three types of cloud keep their normal shape there won't be any rain.
This system came to be used across the European continent, and in the 20th century his cloud classification system was adopted, with some additions, as the international standard, but that was not his only contribution to meteorology. He wrote papers on barometers and theories of rain, and what is probably the first textbook on weather. He can also be considered to be the father of what is now called "urban climatology". Howard had realized that cities could significantly alter meteorological elements. One of these he called "city fog", which we nowadays call "smog", a combination of smoke and fog.