The Weather Notebook: Flipping the Weather Coin

Flipping the Weather Coin
Listen in RealAudio
Email your weather question

What does rolling the dice or flipping a coin have to do with weather? Well it shows that when predicting something the best way to describe the potential of a certain outcome is to give it some odds. But even though heads and tails, or for example, fair or foul weather may be said to have a fifty percent chance of occurring, the information needed to determine those odds is hugely different. Hi, I'm Dave Thurlow for the Mount Washington Observatory and this is The Weather Notebook.

One of the most common misconceptions of weather forecasting is that when forecasters say there is a fifty percent chance of rain, they are simply guessing, saying take your pick. Well that's wrong and here's why.

It's true that heads will always have the same probability as tails, but it is as far as can be from the truth that rain has the same probability as lets say not rain. In most of the world, at any given spot it is less likely to rain than it is to not rain. In other words, rain is a bit of a rarity. In the rainiest parts of the country rain occurs about 30 percent of the time, in the driest areas about 3 percent of the time. So, in most places a fifty- percent chance of rain represents a significant increase in the average odds. As opposed to flipping coins and rolling dice, it takes a skill to come up with the weather odds. It takes a knowledge of weather systems, it takes observation, reasoning... In flipping coins one does forecast but it's what is called zero - skill forecasting. It's like playing the lottery, it doesn't take any skill.

Forecasting the weather however does, and the next time you hear fifty-percent chance of rain in the forecast, especially if you're in the desert, pay attention because that forecast is not just a flip of the coin.

The Weather Notebook is supported by the National Science Foundation.