How do meteorologists predict the weather?

RYAN CORDOVA: You're listening to Brains On! where we're serious about being curious.

[THEME MUSIC]

MOLLY BLOOM: It's getting cold and blustery, here, in Minnesota.

RYAN CORDOVA: In Los Angeles, it's sunny but getting a little colder.

MOLLY BLOOM: There's all sorts of weather happening all around the country and the world, right now.

RYAN CORDOVA: Snow, rain, clear skies, wind gusts, sticky, dry, hot, freezing.

MOLLY BLOOM: You name it, somewhere it's happening. It may seem hard to keep track of it all, but scientists have it figured out.

RYAN CORDOVA: We're going to find out how meteorologists predict the weather.

MOLLY BLOOM: And where wind comes from.

RYAN CORDOVA: Keep listening.

[THEME MUSIC]

MOLLY BLOOM: You're listening to Brains On! from NPR News and Southern California Public Radio. I'm Molly Bloom. My co-host, today, is 12 year old Ryan Cordova from Los Angeles. Hi, Ryan.

RYAN CORDOVA: Hi, Molly.

MOLLY BLOOM: So Ryan, do you pay attention to weather forecasts or predictions?

RYAN CORDOVA: Yeah. That's pretty much what I watch most of the day-- you know, the news?

MOLLY BLOOM: And what is your favorite kind of weather?

RYAN CORDOVA: I'd say, maybe, the weather we're having, right now, in Southern California-- windy, a bit breezy, but sometimes sunny.

MOLLY BLOOM: Do you think that the weather affects your mood?

RYAN CORDOVA: No, not really. I'm pretty satisfied.

MOLLY BLOOM: So if it's rainy or cloudier, you're still a happy guy?

RYAN CORDOVA: Yeah.

MOLLY BLOOM: That's awesome.

RYAN CORDOVA: Exactly.

MOLLY BLOOM: We're going to find out all about how meteorologists make their forecasts. But first, we're going to talk about something that plays a huge part in determining what our weather is going to be like.

RYAN CORDOVA: The wind.

MOLLY BLOOM: Lots of you have written in with questions about the wind.

JOSH: My name is Josh. My question is, where does wind come from?

JUDE: Hi. I am Jude. And my question is, how does the wind blow? I've always been wondering where the wind comes from. And I know that the wind is there, because I feel it. And I can see the leaves shaking.

MOLLY BLOOM: We called Deanna Hence to help us find an answer.

RYAN CORDOVA: She's an atmospheric scientist at the University of Illinois at Urbana-Champaign.

DEANNA HENCE: Wind, at its simplest, is just moving air. And it's kind of funny in that we think of air as being this invisible, weightless thing that's around us all the time. But really, gases, which is what makes up our air, have mass. And you feel that mass when the winds hitting up against you. It's kind of pushing you over.

MOLLY BLOOM: So you may not be able to see the moving air.

RYAN CORDOVA: But the gases that make up the air have mass.

MOLLY BLOOM: And you can feel that mass when the air is moving fast enough. Phew.

RYAN CORDOVA: But what makes the air move faster or slower?

MOLLY BLOOM: Deanna says, first, you have to understand what air pressure is. She explains it like this.

DEANNA HENCE: Now you and your best friend, you line up back to back. And you lean up against each other.

RYAN CORDOVA: Your backs pushing up against each other? That's pressure, force over an area.

DEANNA HENCE: So if that pressure is equal, then you guys stay upright. But then, of course, if one of you pushes harder than the other, then that pressure would push over the person that's pushing less hard, right? So the person that's pushing harder would have the higher pressure. And then the person that's falling over would have the lower pressure, right? Wind kind of works the same way in a lot of ways.

MOLLY BLOOM: Some areas of air will have higher pressure. And others will have lower pressure.

DEANNA HENCE: So all the wind is, is the air, the atmosphere trying to balance these differences in pressures.

RYAN CORDOVA: But why do some pockets of air have more pressure than the others in the first place?

MOLLY BLOOM: Deanna says it has a lot to do with the temperature of the air.

DEANNA HENCE: If you think, like, a hot air balloon, right? You heat up the air. The air starts to rise and fill the balloon.

RYAN CORDOVA: So since that hot air is rising, it's not pressing down toward the Earth.

MOLLY BLOOM: So there's not as much pressure, at the ground, under that pocket of air. That's low pressure.

DEANNA HENCE: Cold air likes to sink. And if we let that cold air just sink and sink and sink, then the air pressure would get higher and higher and higher as air compresses in that area.

MOLLY BLOOM: But like Deanna said, the atmosphere wants to balance these high and low pressure areas.

DEANNA HENCE: So you move the air from where it's piling up, in the high pressure, to the place where the air is leaving in the low pressure. And so that movement of that air, that large movement of air is what we know as the wind.

RYAN CORDOVA: But like we mentioned, we need hot areas and cold areas for this to work. Where do these temperature differences come from?

MOLLY BLOOM: You can thank our good friend, the sun, for that. The sun heats up the air. But not all places get the same amount of direct sunlight. So the Earth heats unevenly.

RYAN CORDOVA: And why are some areas more windy than others?

DEANNA HENCE: One of the big reasons why is how smooth or how rough the surface of that area is. Wind blows more easily when you have less things in the way, right? So say, for example, if you have a lot of trees, then the trees are going to disrupt the wind. And so the wind's not going to be able to get as fast.

If you have mountains or terrain of some kind, the wind is going to behave-- actually, it can behave quite differently when it's going over the mountains than if it's going over a flat area. Water is actually really great, because, typically, water's pretty open and quite flat. But, of course, as the wind picks up, then you start picking up waves. And actually, those waves, kind of like the trees, can also disrupt the wind close to the ground.

MOLLY BLOOM: And the wind can help scientists tell what kind of weather is on the way.

RYAN CORDOVA: So even though we can't see the wind, scientists have tools that help them measure it.

MOLLY BLOOM: Wind vanes tell us what direction the wind is coming from.

DEANNA HENCE: So you might have seen those, on top of a barn or something, where sometimes they come in the shape of an animal, like a rooster, and they're, on top of the barn, spinning with the wind. When we, as meteorologists, talk about the wind, we always talk about where it's coming from. The direction of the wind actually helps us know, sometimes, where these differences in pressure are happening.

RYAN CORDOVA: They can also measure how fast it's moving.

MOLLY BLOOM: With an instrument called an anemometer.

DEANNA HENCE: And you've probably seen these where they might have the spinning cups or they might have a little propeller. When the air hits them, it causes the propeller to spin or the cups to spin. And so that's how we measure the wind speed.

MOLLY BLOOM: They can also use radar to see how wind is moving within clouds.

RYAN CORDOVA: That helps scientists track storms in real time.

MOLLY BLOOM: But the radar doesn't work in clear air, because it needs something for the radar beams to bounce off of.

DEANNA HENCE: We bounce the radar beam off of all the little particles in the cloud, which would be the raindrops, the snowflakes, whatever is up in there. But you can use other things, too, which is actually kind of cool, too. So you can, if there's a lot of dust in the air or if there's a lot of bugs or birds, it'll also work the same for those, too, which is kind of fun.

This is a cool thing. It's like I encourage anyone to do. If you go to the National Weather Service website, weather.gov, and you can look at the national picture of all the radars put together. If you look at either the first, right around sunrise or right around sunset, you'll see these big rings of green that will be near the radar and then kind of spread out. And then there might be a big ring in the evening that starts out big and then compresses down.

That's actually-- one thing that they've noticed is that that's actually birds, taking off for the morning and coming back to roost at night. We, meteorologists, will partner with people, who study birds or study bugs, to track the migration patterns of these different animals.

MOLLY BLOOM: So even though we can't see the wind, scientists have found all sorts of ways to track it.

PLAYERS: (SINGING) Ba ba, ba-ba, ba-ba, ba ba-ba ba Brains On!

MOLLY BLOOM: Weather is complicated stuff.

RYAN CORDOVA: There are all sorts of things that can affect weather. Luckily, we don't have to figure it out all for ourselves. We can leave it to the professionals.

MOLLY BLOOM: Like Fritz Coleman-- he's a weather forecaster for KNBC TV in Los Angeles. And he's here with us now.

RYAN CORDOVA: Hi, Fritz.

FRITZ COLEMAN: Hi, guys. How are you?

MOLLY BLOOM: Great

RYAN CORDOVA: Good.

MOLLY BLOOM: Thank you for being here.

FRITZ COLEMAN: I'm honored to be here. This is a lot of fun. It's intimidating.

RYAN CORDOVA: So, Fritz, how do we know what the current weather is besides just looking out through the window?

FRITZ COLEMAN: Well, looking out the window is always the failsafe way. But there are a lot of things that we can measure to know exactly what the weather is. There is a barometer that measures barometric pressure. There's the thermometer that measures temperature. There is an anemometer that measures wind speed and direction. There is a rain gauge.

So we have all these various tools, plus tools that you can't see, like a satellite that is 50,000 miles, in what we call geosynchronous orbit, over the Earth that takes pictures for us. So we have a whole arsenal of tools we can use to figure out what the weather is. But it's still an educated guess.

There are people that don't call weather forecasting a science. They call it a "technical art," because it's still such a variable. But we give it our best shot.

RYAN CORDOVA: Forecasters often talk about warm fronts and cold fronts. What are those?

FRITZ COLEMAN: OK, well a cold front, if you think of the atmosphere, again-- and the whole job of weather forecasting is figuring out the relationship between cold dry areas of air and warm moist areas of air. And when a cold dry area of air is moving and overtaking a warm moist area of air, that line, just at the leading edge of that area of cold dry air, is called a cold front.

Now, if it was a warm moist area of air that was overtaking a cold dry area of air, what do you think they would call that?

RYAN CORDOVA: The warm front.

FRITZ COLEMAN: Exactly.

RYAN CORDOVA: So how far into the future can we predict the weather?

FRITZ COLEMAN: Well, they're getting pretty good. I mean we have lots of computer models that go way out. For instance, right now, we're in the midst of predicting this El Nino season. And so they can look toward the end of winter in a general sense. That's not the most accurate forecasting. We're probably 90% accurate up to 48 hours, and then it sort of drips off after that.

RYAN CORDOVA: We talked about this before, about the hot air rising. So if hot air rises, why is it hot at the ground and colder up high?

FRITZ COLEMAN: During the day, the sun heats the Earth, right? And the Earth absorbs some of the heat and reflects some of it back into the atmosphere. When this air, this warm air rises up in the atmosphere, the farther away from the heat source it gets-- that is the reflective ground-- it cools. And when air cools, as you know, when a gas cools, it condenses. And when it does that in the atmosphere, the moisture in the atmosphere condenses into clouds. And if it keeps condensing, at the upper elevations of the atmosphere, you get thunderstorms.

MOLLY BLOOM: You'd mentioned El Nino. And that's a lot in the news right now. Can you just give us a brief description of what El Nino is exactly?

FRITZ COLEMAN: El Nino, which means "the child," or "the Christ child," is a phenomenon that was noticed back in the 1600s by Peruvian fishermen. And these Peruvian fishermen were noticing every 7 to 10 years, which tends to be the El Nino cycle, that they were getting bountiful fishing. They called it El Nino, because this tended to happen over the winter right around Christmas.

El Nino is an abnormally warm area of water in the Pacific. And why that's important is the temperature of the ocean changes the weather above it. So this abnormally warm ocean water was changing the path of the jet stream-- think of the jet stream as the railroad tracks along which storms ride-- changes the path of the jet stream and brings it into our area a little farther south.

And if streams come into us a little farther south, that means they're warmer, because they're closer to the equator. And if they're warmer, they have more water. That's why El Nino winters are very wet at least for the Southwestern United States. But in other parts of the United States, El Nino can mean abnormally dry months, abnormally cool months. So if you look at the United States, it affects each quarter of the United States in a slightly different way.

MOLLY BLOOM: Thank you so much for being here today, Fritz. We really appreciate it.

FRITZ COLEMAN: I was honored to be a part of it.

RYAN CORDOVA: Thank you.

[MUSIC]

MOLLY BLOOM: What's your burning question about the world?

RYAN CORDOVA: Send it to us and maybe we'll answer it in a future episode.

MOLLY BLOOM: Or you can email us a mystery sound, a drawing. Or maybe you just want to say, hi. Whatever it is, we love hearing from you.

RYAN CORDOVA: Our email is brainson@ M-as-in-Minnesota P-R dot org.

MOLLY BLOOM: And you can send us good old fashioned mail, too, with a stamp and everything. Our postal address is on our website, brainson.org.

RYAN CORDOVA: While you're there, sign up for our newsletter. It's full of fun stuff.

MOLLY BLOOM: Plus we'll tell you about new episodes and any live events we do.

RYAN CORDOVA: So don't miss out. Now a warm welcome to the latest group of Brains On! Honor Rollees. These are the fans that keep us going with their excellent questions, drawings, emails, and high-fives. Here they are.

[MUSIC PLAYING]

[LISTING HONOR ROLL]

[MUSIC PLAYING]

RYAN CORDOVA: OK, back to the weather. What's next, Molly?

MOLLY BLOOM: Oh, just, you know, the mystery sound.

[WEIRD NOISES]

PLAYERS: (WHISPERING) Mystery sound.

MOLLY BLOOM: Here it is.

[WHOOSHING]

Any guesses?

RYAN CORDOVA: A rainstorm?

MOLLY BLOOM: Ooh, good guess. Should we hear it one more time? It was kind of a short one.

[WHOOSHING]

RYAN CORDOVA: Thunder?

MOLLY BLOOM: Excellent guess. Well, keep thinking. We're going to be back with the answer soon. But first, we're going to answer another weather related question.

AVA: My name is Ava. I'm from Baltimore, Maryland. And I'm seven years old. My question is, how do meteorologists predict what the weather will be?

RYAN CORDOVA: Here to help answer that is our pal, Sanden Totten.

SANDEN TOTTEN: Hey, guys. A lot of science is really about predicting the future. You look at stuff that happened in the past, then you make informed guesses about what will happen next based on that. So predicting the weather is kind of the same.

Explorers would predict the weather based on their past observations and experiences.

MOLLY BLOOM: Right, but as we're learning, there are a lot of things that can affect the weather, from the wind and sun, to the shape of the Earth in a certain area. That's a lot to factor in.

SANDEN TOTTEN: Right. Our planet is really complex. But thankfully, we don't have to do it all by ourselves. These days, a lot of weather prediction is done with the help of-- dun da-da duh-- supercomputers!

RYAN CORDOVA: Supercomputers? You mean like really big, fast, and powerful computers?

SANDEN TOTTEN: Yeah, yeah. I mean, they don't have capes or superpowers to fly or anything like that. But they are sort of like superheroes. Because they can make massive calculations based on equations and observations. And these calculations can help predict the weather down where we live. This is the kind of math that would take humans years to do on their own. And that's how supercomputers help us predict the weather.

Let me explain. Say you've got a big bad storm over the ocean, and you want to know where it's headed.

PLAYERS: [GROWL] I'm a storm. Puff, puff, bluster and blow, I'm going to bring the rain and snow. But when I strike, you'll never know. [LAUGHS] [THUNDER]

SANDEN TOTTEN: Of course, we're able to track the storm, as it moves, in real time, thanks to all kinds of weather tools, like satellites, radars, and weather balloons.

[RADIO SQUELCH] [BEEP]

PLAYERS: This is radar. We have winds blowing from the south to northeast at the moment.

PLAYERS: Roger that, radar, this is your eye in the sky, the satellite. I can see clouds over the southwestern part of the ocean. Looks like a big bad storm.

PLAYERS: Copy that, satellite. Over and out.

SANDEN TOTTEN: Forecasters also use data gathered by weather stations around the world. They get readings on temperatures, air pressure, the amount of water vapor in the air, the wind at different levels of the atmosphere, all kinds of stuff.

RYAN CORDOVA: Wow. That's a lot to take in.

SANDEN TOTTEN: Exactly. So that's where our friend, supercomputer, comes in.

[BEEPING]

PLAYERS: Have no fears, supercomputer is here. What's the situation, radar?

PLAYERS: Well, there's a nasty storm right out over the ocean. We want to know what will happen when it hits land. Can you help us out?

PLAYERS: Of course, it's what supercomputers do. Give me all your data, and I'll work my magic.

SANDEN TOTTEN: All this information we have about the current weather, around the world, as well as information we have about how weather behaved in the past, is combined into a program called a mathematical model. It uses equations that describe how the air moves around and warms and cools. And this model doesn't just look at the weather around the storm. It looks at weather everywhere to see if that might impact the storm down the line.

PLAYERS: Let's see, we've got sun in South America, rain in Indonesia, high pressure in the Sahara desert. Let's see how this might play out.

SANDEN TOTTEN: Using these models, supercomputers give a best guess about what will happen over the next week or so. But of course, this method isn't perfect. Because modeling the weather is not easy even for a supercomputer. There are always little uncertainties about the weather that can end up making big changes.

PLAYERS: Ha, ha, Captain Uncertainty, here. I see my nemesis, supercomputer, is trying to track a storm. Well, I'll just start a little wind over in Mexico. [PUFFING] And maybe that will turn into a big gust that messes up the models. Hee, hee, hee, hee.

SANDEN TOTTEN: Of course, scientists know there are little uncertainties in the world, so they usually don't make just one prediction about the weather. They have their computers make many predictions, an ensemble of predictions or a family of predictions. They take the average of all of them to see what future weather is most likely to occur. They can also adjust what the supercomputer tells them, based on their own weather experiences and how the supercomputer usually predicts the weather. So maybe they notice that the supercomputer usually predicts things as too dry or too wet. They can take that into account and adjust for it.

PLAYERS: Math-tastic. I've got it. Based on my calculations, that storm is most likely headed to California. It'll be there in five days.

PLAYERS: Thanks, supercomputer.

PLAYERS: You saved the day.

PLAYERS: Boo, to you, supercomputer. Now, they'll know I'm coming. There goes all the fun. They'll have their umbrellas ready and stay dry until I'm gone. [RUMBLING]

SANDEN TOTTEN: Of course, as powerful as these models are, they aren't perfect. But scientists are always improving them. So weather forecasts get better and better over time.

RYAN CORDOVA: Cool. Thanks, Sanden.

PLAYERS: Re-run!

MOLLY BLOOM: One thing I can predict with 100% certainty is that we're going back to the mystery sound. Here it is one last time.

[WHOOSHING]

Any final guesses?

RYAN CORDOVA: Um, is it a rainstorm? Yeah, I already guessed that, I think.

MOLLY BLOOM: It's a good guess. Well, here, with the answer is Evelyn Taft. She's a meteorologist with CBS 2 News In Los Angeles.

EVELYN TAFT: The sound you just heard was a breaking news "whoosh," or what we call a sting. So we'll run the whoosh before we put up an animation that says "breaking news." And when something's breaking, it means it's happening right now. And we tell you the latest details of what's happening.

Here, in California, we have breaking weather news. That happens a lot when we have wildfires big storms, flooding, mudslides, debris flows, all possible things that could happen in a big storm.

MOLLY BLOOM: So it was weather-related, kind of, so you were close.

RYAN CORDOVA: Yeah. Yeah.

MOLLY BLOOM: But it's not a naturally occurring sound. So if you see Evelyn Taft on TV doing her forecast, she'll have maps and graphics behind her. But all of that is added by computers as the image is broadcast. When she's actually taping, there's nothing but a blank wall behind her called a green screen.

EVELYN TAFT: There's literally nothing. So if you see me doing the weather, it looks like I'm pointing to all these places. But I'm really pointing to nothing at all but a green screen. So it looks really funny. But if you take a step closer, you'll see that I'm actually looking at the weather graphics that I put together, on a monitor, in front of me, to my right and to my left.

The graphics I produce beforehand. So I come in, and I put together my forecast. And I crunch all the numbers. And I put everything in a show. Once I'm presenting it, I click through it like a slide show. I have a slide for right now temperatures. I have a slide for forecast highs. I have a slide for your seven-day extended forecast. I have slides for pretty much everything. I have satellites. I have radars. And I decide what goes in the slide show and what doesn't go in the slide show.

[MUSIC PLAYING]

RYAN CORDOVA: Even though our weather is constantly changing, we have a pretty good idea of what the weather will be like later today and even five days from now.

MOLLY BLOOM: That's thanks to models that take data collected from all over the world.

RYAN CORDOVA: Supercomputers crunch the numbers, and they give us their very best predictions.

MOLLY BLOOM: Telling us when to get ready for storms or to stock up on sunscreen.

RYAN CORDOVA: That's it for our episode of Brains On!

MOLLY BLOOM: If you like what you heard, tell the world about us. We love when you tweet or post about us on Facebook or blogs.

RYAN CORDOVA: And if you are on iTunes, please leave a review.

MOLLY BLOOM: It really helps other kids and parents find out about the show.

[LISTING HONOR ROLL]

RYAN CORDOVA: You can find past episodes at our website, brainson.org. You can also keep up with us on Instagram and Twitter.

MOLLY BLOOM: We're at brains_on.

RYAN CORDOVA: And we're on Facebook, too.

MOLLY BLOOM: We'll be back with more answers to your questions, really soon.

RYAN CORDOVA: Thanks for listening.