What is a weather forecast?

0:47 Welcome to Matt's Is Not Boring by Kidopoly.com. I'm Mira! And I'm Finn! Today we have a very special, huge hello to six-year-old Enzo and four-year-old Milo in Kansas City, Missouri. Hi Enzo, hi Milo!

1:05 I hear you guys listen to us on the way to school, at breakfast, and in the afternoon, which is so incredibly cool.

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1:22 Hey, before we jump in, I also wanted to mention a brilliant episode of our sister show, History's Not Boring.

1:30 Oh yes, the one asking what the Golden Age of Islam was. It takes you all the way back to the House of Wisdom in Baghdad centuries ago. Scholars there were figuring out incredible things about astronomy and medicine.

1:45 They even brought in the numerals we use every single day. And there was this amazing mathematician named Al-Khwa Rizmi who basically created algebra.

1:55 And his name actually gave us the word algorithm, which is an incredibly important idea in mathematics that we still use in so many ways today.

2:05 We would absolutely love it if you gave Histories Not Boring a listen and a follow. It feels like uncovering a hidden treasure chest of stories.

2:14 So Enzo and Milo, speaking of the weather, when you are sitting at the table eating breakfast and getting ready for school, you might look at a weather app and see a little dark rain cloud.

2:25 Yeah, and right next to the cloud it says something like, 70% chance of rain. I check that all the time to see if I need my raincoat.

2:33 But here is the thing that completely changes how you see the world. That 70% does not mean it will rain for 70% of the day.

2:43 Wait, it doesn't? But I always thought it meant most of the day would be wet and grey, and only a little bit of the afternoon would be dry.

2:50 I thought the exact same thing. A lot of people also think it means that 70% of your city will get rain, and 30% will stay completely dry.

3:00 That actually sounds like it makes perfect sense though. If the cloud is floating over the city, maybe it just drops rain on some of the streets and misses the others.

3:10 It sounds so right, but it is actually entirely wrong. To understand the real mathematical name for this, which is the probability of precipitation, we have to go on a little journey back in time.

3:25 Wait, we are time travelling just to understand the weather app on my phone.

3:29 We are? Close your eyes and imagine we are in London on August 1st, 1861. A man named Robert Fitzroy had just done something nobody had really done for the public before.

3:43 Who was Robert Fitzroy?

3:45 He was a brilliant explorer and a scientist. In fact, years earlier, he had captained the famous ship HMS Beagle, which carried Charles Darwin all the way around the world.

3:57 Oh wow! So he spent years and years out on the open ocean, just watching the sky and the waves.

4:04 Exactly. Sailors desperately needed to know about incoming storms. It was a matter of life and survival. But back then, trying to predict the weather was treated like a joke. People thought it was basically fortune telling. Really? They laughed at him for looking at the clouds. They did. But Fitzroy was stubborn.

4:26 He gathered information from different ports using the new telegraph machines, and on that August day, he printed a weather prediction in a newspaper called The Times.

4:36 Among the first public weather forecasts in the newspaper, people must have thought he was a wizard.

4:43 Some people definitely did. But Fitzroy was very careful with his words. He specifically called them forecasts, not guarantees. Why did he choose the word forecast? Because he knew that nobody can know the weather for certain. The atmosphere is too wild and messy. He was just casting his best guess forward into the future.

5:05 Okay, so a forecast is just casting a really good guess into the future. But today, would you not just use barometers and telegraph messages? We have giant supercomputers, right?

5:17 Yes. Today, meteorologists run massive computer models of the weather. These computers divide the whole sky into a giant 3D grid and do billions of calculations. But here is the amazing part. They do not just run the model once.

5:35 Why would they run it more than once? Doesn't the computer just do the math and give them the exact right answer?

5:41 Well, the weather is chaotic. A tiny breeze over a mountain, or a slight change in ocean temperature, can change everything days later. So scientists use something called ensemble forecasting.

5:55 Ensemble forecasting? That sounds like a band playing music together in a concert.

6:01 It kind of is! Instead of one single song, they run the computer model many, many times. And every single time they run it, they change the starting readings just a tiny microscopic bit.

6:15 Wait, but if that's true, then they are basically creating a bunch of different fake futures.

6:21 Exactly! They are looking at lots of possible futures playing out side by side. One future might have the wind blowing a little faster. Another future might have the air a tiny bit warmer.

6:34 So they just watch all these different futures play out inside the computer. Like watching different movies of tomorrow.

6:40 Yes, and then they literally just count them. Let's imagine they look at ten of those computer features, and they zoom right in on your exact location. Like Enzo and Milo's house in Kansas City.

6:54 Okay, I am picturing ten different versions of Kansas City floating in the air, each with its own little sky.

7:02 Now they count how many of those futures bring rain to that exact spot. If seven out of the ten futures have rain falling right on your house, that is a 70% chance. No! The MetOffice and other forecasters mean it is the chance of rain at a given place in a given period. It means on days that start out looking exactly like today, it rains where you are about seven times out of ten.

7:37 Seven times out of ten. So there are three times where those exact same clouds roll in, but it just doesn't rain at all.

7:45 Exactly! The 70% is not a measure of how much water is falling or how many hours it falls for. It is purely a measure of chance.

7:55 That seriously bends my brain. When I see 70%, I shouldn't picture a puddle covering 70% of my street. I should just imagine 10 identical days, and on seven of them, I need my umbrella.

8:08 You got it perfectly. Every single time you make an umbrella decision, you are holding a little piece of probability right in your hand.

8:17 That actually makes me feel so much better. It turns chance into a kind of play, like deciding whether to pack a picnic and risk it, or just bring a raincoat and be safe. Exactly.

8:28 It is just a friendly game of chance with the sky. And the beautiful thing is, nobody knows the final answer until the drops actually fall on your head.

8:38 Did Robert Fitzroy's first forecasts actually work out? I mean, without supercomputers doing ensemble forecasting, how did he do?

8:47 Well, that forecast on August 1, 1861, which was among the first, predicted pleasant weather for London, and it was actually surprisingly accurate. It proved to the world that watching the sky mathematically wasn't just a magic trick.

9:03 Oh wow! He just looked at telegraph messages from some sailors and got it right. That is pretty heroic.

9:10 He really was a pioneer, and his work paved the way for those incredible algorithms we have now, the ones checking billions of weather points every second.

9:20 And those algorithms trace all the way back to the mathematician Al Khwar Rizmi we mentioned in the intro.

9:26 Yes, it is all one giant connected story. From scholars in Baghdad to sailors braving the ocean, to a newspaper in 1861, right down to the weather app in your pocket today.

9:38 So next time Enzo and Milo are eating breakfast and they see a high chance of rain, they can just picture those ten different futures racing each other to see which one comes true.

9:48 Alright, Finn, it is time for the quiz. Let's see how much of this weather journey stuck with you.

9:55 Okay, I am ready. Let's do this.

9:58 First question. What was the exact date of Fitzroy's prediction, which was among the first public weather forecasts? Next one. What was the name of the captain who printed that early forecast? Question 3. What famous scientist did he want sail all the way around the world with? Here is number four. When an app says 70% chance of rain, how many times out of 10 will it rain on days just like today?

10:41 Last question. What is the real mathematical name for the chance of rain? Great job! Let's see how you did. The first answer is, it was printed on August 1st, 1861. For question two, the captain's name was Robert Fitzroy. The third answer is he sailed with Charles Darwin. For number four, it means it will rain seven times out of ten.

11:13 And the final answer, it is called the probability of precipitation.

11:18 I am seriously never going to look at my weather app the same way again. I used to think the little percentage just meant the city was partially covered in puddles.

11:28 Same here. Knowing that it is actually ten possible futures racing to happen makes the rain feel so much more extraordinary. It is like holding a little piece of the future.

11:39 It really does. Hey, if you love learning about Robert Fitzroy and the real meaning of chance, please leave us a five-star review. Just scroll down and tap the stars.

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12:08 See you next time on Maths Is Not Boring. Bye!