I enjoyed delivering the last Simplify Stats course for 2021 – Intro to Stats, without the maths. This course has a few fixed examples and exercises (some edible!) that we can all relate to, but the real value is the opportunity for participants to drive the discussion and topics covered.

The stats that come up for discussion each time are different, and this time there was interest in weather. Perhaps because of the number of rainy days lately?! Do you know what the forecast numbers and terminology used by the Bureau of Meteorology (BOM) mean?

The ‘chance of any rain’ always refers to the probability of more than 0.2 mm of rain, which is the minimum amount that can be accurately measured. The lower number in the ‘possible rainfall’ indicates a 50% chance of at least that amount of rain, and the higher value indicates a 25% chance of at least that amount of rain.

Here’s today’s forecast for Sydney: There’s an 80% chance of at least 0.2 mm of rain from midnight to midnight, a 50% chance of at least 3mm of rain, and a 25% chance of at least 8mm of rain.

But Sydney’s a large place. Does this mean there’s a 50% chance that all of Sydney will receive at least 3 mm of rain? Digging around on the BOM website there is a glossary of terms that provide more details. Under the ‘Wet or Dry’ tab we can get a sense from the words as to the intensity, distribution, and duration of this rain.

Terms such as “isolated”, “patchy”, “scattered” and “widespread” give us an indication of how much of the area will receive the forecasted rain. For example, the term widespread showers means showers occurring extensively throughout the area – generally covering over 55% of the forecasted area. Whereas isolated showers refer to showers that are separated geographically, covering about 10 to 25% of the forecasted area.

Today’s forecast has the key words of “cloudy”, “showers” and “chance of a thunderstorm” without reference to the distribution (i.e. there is no specific term to tell us how much of Sydney will get wet). Showers in today’s forecast mean that the wet weather will come in bursts with cloudy spells in between. There’s only a slight change of a thunderstorm, so those under its path will get the higher end of the forecasted mm of rain and those who are not will enjoy a drier day. In summary, I’ve got my umbrella with me today!