Probability is expressed in a variety of ways. Forecasting disasters is an inexact science, but we do see forecasts of expected levels of activity. These are probability statements measured in terms of the confidence level of the method and the confidence which you have in the person or organization issuing the forecast. More easily recognized as probability statements are numerical expressions (usually percentages) of the probability of a certain event, either generally or within a specific time frame. These can be seen operationally as alert levels which cover a range of probability values. The United States terrorism alert levels set by the Department of Homeland Security is one example; the use of alert levels for impending volcanic eruptions is another. One confusing expression of probability is the use of time frames as a way to express probabilities. The 100 year flood, for example, is commonly misinterpreted as meaning that, if this is year 99 since the last such flood, the 100 year flood will occur next year. In reality, this means that there is a 1% chance of a flood of that magnitude occurring each and every year, but that you could have two 100 year floods in consecutive years, and no more floods of any sort for 400 more years.