So if we want a detailed complete picture, we need additional information. This is available as knowledge of the behaviour
and probable structure o fthe atmosphere. For instance the knowledge of the typical structure of a frontal depression enables a human to draw an "analysis" of the atmospheric state, based on scattered observations. To advance beyond this subjective approach, the behaviour of the atmosphere is embodied in a computer model. In particular, knowledge of the evolution with time is embodied in a forecast model. This enables us to use observations distributed in time. The model
also provides a consistent means of representing the atmosphere. Assimilation is the process of finding the model representation which is most consistent with the observations.