Probability / Likelihood
The probability of occurrence is defined as the likelihood or predictability that the risk event will occur - tomorrow there is a 60% chance of rain. There is a known probability of rain, based on known atmospheric conditions such as high and low pressure, wind speed and direction, and possibly past records of how often it has rained in the past when similar conditions were noted.
All three elements are required for a risk. If you intended to stay in tomorrow, there would still be a 60% chance of rain, but this would have no consequence.
It is also important to remember that this example could also be described as a positive risk. For example there is a 40% chance of dry weather tomorrow, and so the chance of a lovely picnic.
