Question: how does the weaher man know what o say?

Ciarán O'Brien answered on 18 Nov 2014:

The weather presenter is usually a fully trained meteorologist as well as a pretty face that points at things 🙂

Meteorologists spend their lives studying the weather, watching what it does, and building models of it to try and predict what it will do next.

Before a weather forecast, meteorologists gather as much data as they can about the weather over the last few days. They look at temperature, air pressure, wind direction, cloud cover, and lots of other things, and build up a computer model of how the weather has been behaving. Then they make the model run faster to see what that model says about the next few days.

They usually make several models, each slightly different, and run them all at the same time. This is because the weather is very complicated and small differences in their calculations can make them think the weather is going to do something when it’s not. the more models they run at the same time, the easier it is to catch these mistakes.

Even with all those models and checking for mistakes, nobody can get an accurate enough picture of the weather for long, and forecasts are only accurate for a few days into the future. After that, the tiny errors (or lack of data because you can’t measure the temperature and wind speed everywhere in the world) add up to big differences in the calculations, and the predictions don’t even get close to what actually happens.

This has a little bit to do with something called Chaos Theory, which is a great way of using relatively simple rules to make a model of something that seems very complicated and random. You might have heard the saying “A butterfly flaps its wings in Brazil and changes the direction of a storm in Russia”? To get an accurate picture of the weather a few months into the future, we’d have to measure the wind caused by everything from butterfly wings to windmills to the little draft you make when you wiggle your fingers! They all add up to making the weather doing what it does, and if a weather forecast doesn’t include them, then it will only be accurate for a little while.