[intro plays]

Sabrina: Last night, I checked the weather on my phone and the app said clear skies all day, but this morning I got caught in a shower-- not the one in my bathroom, the one that comes from the sky. Curse you faulty weather app!

But I can't be too mad, weather is crazy difficult to predict. We've already learned that the Earth's shape, its distribution of land and water, and its rotation all affect the way that wind moves around the world. Air, water, land and heat energy from the sun interact in really complex ways to create weather, both in your area and around the world.

A cool breeze is a small, local weather pattern, the jet stream on the other hand is part of a global cycle. Jet streams form at the boundary between different cells of rotating air, which as you know from last time are caused by the uneven heating of Earth's atmosphere at the equator and poles.

And we talked about how jet streams are currents of fast-moving air about 10 kilometers above the Earth's surface. Think of it as a tube of ridiculously fast air whipping around the world. I mean, jet streams move, people. And since they're the boundaries between two air masses, their boundaries affect us a lot. Check out the weather report and you'll see jet streams front and center on the maps. If the jet stream dips down from the poles in the winter, we're in for some serious cold for a few days, but if it lifts back up, it'll be more mild. So you can see how moving air can affect the weather where you live, but what about moving water?

[text: Big Question]

Ocean currents form another kind of cycle. They transport warm water from the equator toward the poles, and bring cold water from the poles back down to the tropics. Without those two movements, jet streams and ocean currents, the climate and weather conditions would stay pretty much the same, and things would get unpleasant.

The equator would be incredibly hot all the time, and the poles would be unspeakably cold. In fact, a lot of the Earth would be uninhabitable. So thank goodness for the currents! But to see how they work, let's take a look at how one region of Earth is affected by ocean currents and jet streams.

[text: Investigation]

Welcome to the British Isles. Take a look at the British Isles on the globe. This group of islands is pretty far north and so we might expect it to have a pretty cold climate, but there's something else at work here, namely the ocean current. One current in particular helps keep the British Isles warm: The gulf stream.

This massive current pulls warm water from the equator up past Florida, then all the way up to the British Isles. Meanwhile, cold water cycles down, past the northwestern coast of Africa, and back to the equator. This flow of warm water helps keep winters in the British Isles milder compared to other places that are so far north. So this place has a pretty mild climate, but what about the weather? That's where the jet streams come in.

Jet streams are the main way that weather systems get around, and the British Isles are particularly affected by the polar jet stream. In a typical Summer, the jet stream lifts above the British Isles, closer to the north pole, creating hot and dry conditions with an average temperature between 16 and 21 degrees Celsius. But in one three year spell, from 2007 to 2009, the jet stream actually ran unusually far to the South, which made for really cool, wet summers. Now, if the jet stream is sitting right on top of the Isles, that's when you get some interesting weather systems.

Because when two masses of air, each with different temperature and pressure meet, they don't mix, they clash. This is called a front. When a mass of cold air moves into a mass of warm air, it's called a cold front. The warm air is forced to quickly rise causing heavy rains, hail, thunder, and lighting. A warm front, on the other hand, generally brings clear weather, but some serious humidity.

The British Isles have such variable weather because they're at the center of some very different air masses. Wet and cold air is coming down from the polar regions, while wet and warm air moves in from the southwest. Meanwhile, hot and dry air pushes up from the southeast  and more dry air comes in from the east. These fronts cause major weather events, but they're difficult to predict.

As we've learned, the jet stream isn't a straight line, it's a wiggly wavy band that boogies its way around the Earth. And while the deep ocean currents tend to flow in a more orderly way, the surface currents are at the whims of the wind, moving in strange and nearly unpredictable patterns day to day. As the air interacts with land and water, exchanging heat and moisture, it forms high and low pressure zones, in turn forming weather systems.

[text: Conclusion]

So Earth is complex. We've got deep ocean currents, crazy fast jet streams, cold fronts and warm fronts, and sea breezes too. So it's probably a lot to ask of my weather app to get the forecast right every single time. Sorry for yelling at you, phone.

[endscreen]