The requirements divide the meteorological events into two major groups: extreme meteorological phenomena and rare meteorological events. The extreme values of phenomena such as wind velocity, precipitation, snow packs, temperatures, seawater levels and storm surges should be measured to determine the design criteria for the affected structures and components. When using the probabilistic approach the probability of such values being exceeded should be given together with the associated uncertainties.

Rare meteorological events include lightning, tornadoes and tropical cyclones. The hazards associated with the above phenomena should include the expected frequency of occurrence and the expected maximum values of each phenomenon’s defining parameters: maximum rotational wind speed, pressure differences and rate of change of pressure for tornadoes, and wind speed pressure and precipitation for tropical cyclones. In both cases, missiles with potential to harm the plant generated by the phenom­enon itself should also be contemplated.

An IAEA safety guide on meteorological events in site evaluation (IAEA, 2003b) describes how data should be collected from extreme mete­orological phenomena and rare meteorological events, how to derive the hazards associated with such phenomena and events, and how to obtain the values needed for the design of structures and components potentially affected.