How to read a Bureau of Meteorology Forecast.
How to read a Bureau of Meteorology Forecast.

70% chance of 40mm? Why you’re reading forecast wrong

More often or not, we are confused and frustrated at weather predictions, particularly when it comes to rainfall.

But it turns out, most of us aren't reading the forecasts correctly.

James Thompson, a meteorologist with the Bureau of Meteorology, has given the Gatton Star some advice on reading rainfall predictions, and why a 90 per cent of 10-20mm doesn't mean what it means.

Rainfall predictions need to be broken down into two steps.

Firstly, the percentage. The rain per cent is the likelihood we will see rain - whether that's half a millimetre or 20mm.

Secondly, the amount. For example, if the forecast is 20-40mm, it doesn't mean that's what you will get.

It's broken down into a 50 per cent chance and a 25 per cent chance.

How to read a Bureau of Meteorology Forecast.
How to read a Bureau of Meteorology Forecast.

 

Mr Thompson said the way to read the prediction is there's a 50 per cent chance of getting 20mm, followed by a 25 per cent chance of 40mm.

"They're not really meant to be used together," Mr Thompson said.

And what about the little icons?

Suns out guns out, a sun with clouds, a sun with a few rain spots, or the umbrella - they all have a meaning.

Mr Thompson said when the forecast is a 40 per cent chance or more of rain, the BOM will use a sun with a cloud and droplets.

Bureau of Meteorology forecast.
Bureau of Meteorology forecast.

 

When the forecast is less than 30 per cent chance of rain, and it's predicted to be hot and windy, the BOM will use the sun with a cloud.

"When the symbol has the sun, the rain can be a bit hit and miss," Mr Thompson said.

The obvious, when it's predicted to rain for the majority of the day, the cloud with strong rain indicates it's a day to pack the umbrella.


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