# Chances are you have been reading the weather forecast wrong

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What if we told you that you were reading the weather forecasts all wrong?

I thought I was correctly interpreting the forecast information on the Bureau of Meteorology (BOM) website, but when it comes to the information on rainfall, I was making a big mistake – and apparently it’s a mistake that’s quite common.

If you visit www.bom.gov.au, and click on the forecast for your capital city you will see two pieces of information relating to rainfall. There is the ‘possible rainfall’, which will display as 0 to 1mm or something similar, and then there is a ‘chance of any rain’ section that displays the probability of rain (for example 40 per cent).

I used to understand this to mean that there was a 40 per cent of between 0–1mm in the above example, but this is not right.

The possible rainfall figure lists the chance of any rainfall above 0.2mm for the day in question. It is not forecasting the actual amount of rainfall, just that there will be rainfall, by the smallest margin the BOM considers to be significant.

The possible rainfall figures are forecast differently again. If the possible rainfall figure is 2–10mm, there is a 50 per cent chance that there will be 2mm of rain and a 25 per cent chance of 10mm. Thus, there will always be a 50 per cent chance of the number at the lower end of the range and a 25 per cent chance of the higher number in the range.

BOM senior meteorologist Rick Threlfall told the ABC that daily rainfall forecasts were the most viewed and misunderstood part of the metropolitan forecast.

“It is a fairly complicated subject and feedback on social media has shown there is a bit of confusion about the rainfall forecast, particularly the percentage chance of any rainfall,” he said.

“It is quite a complicated formula – unfortunately, rainfall is not like temperature where it is a single number,” Mr Threlfall said.

“Because of the way rainfall is distributed, particularly in showers and thunderstorms, there is quite a lot more to it and you do have to delve quite a bit deeper and know a bit about interpreting the numbers.”

Have you been interpreting the numbers the right way when looking at the rainfall forecast? Will this better help you plan your outdoor activities?

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## Written by Ben

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If BOM still had physical observers across the country maybe, just maybe, we would have better weather forecasts. Relying on satellites is not the be all and end all of forecasting the weather.

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Firstly, Ben, despite your 10 years of online work, you need to learn English, which uses adverbs: “…wrongly.” Your audience here is not made up of illiterate Americans.

Very few people actual comprehend risk, chance, or probability in its many forms. They are OK to drive a car at enormous risk, but fear a single Asbestos fibre. Happy to fly in plane or have a medical scan, but wouldn’t go near a nuclear power plant; swear off Strawberries for the next year, and so on. No logic, just ignorant fear.

It is dangerous to tell things to people who cannot understand the science. Meteorology is a very complex science, so they would be better off telling folk that it might rain, it should rain, or take a raincoat. Anything more complex – too hard. 40%? Forgive them, they know not what you mean.

This might be patronising, but we all know that a lot of people are basically quite dim.

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Ben, you must surely be kidding! Is there anyone who seriously thinks the ‘percentage chance of rain’ means that the ‘possible 1-4mm’ should be interpreted as how much of that possible amount is going to fall?

Sheesh! It’s glaringly obvious that when the forecast says “60% chance of rain” that means there is a 60% chance of ANY rain falling, not “60% of 1-4mm is going to fall”. *Shakes head in wonder*

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I find it wonderfully accurate and very useful.
– another amazing source is Higgins storm chasers. I am surprised at their long term accuracy and high degree of storm prediction.

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Don’t fret if you misunderstood the rainfall forecast. I worked for the BOM (not forecasting) for 30+ years and am still confused. Why confuse the issue by mentioning the percentages of the amounts likely to fall. To my mind, IF rain falls, the amount is likely to be between 1mm and 4mm. Meteorologists always seem to have trouble communicating their science to the masses.

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