Smith tionally been taken to mean events causing at least 100 deaths or direct economic losses of at least US$1 million. As extreme events deviate further and further beyond the threshold of tolerance (i.e. increase in magnitude and also increase in duration), they create a progressively greater potential for disaster. However, with our present level of understanding, it is rarely possible to define a quantitative relationship between the magnitude of severe events and the exact scale of loss which they will create. This is because the actual losses also depend on many human factors, such as the local population density, the social conditions which affect the vulnerability of people to disaster and the amount of economic wealth exposed to atmospheric processes.