How Weather Works: Part I

Today, I'm going to explain why  different parts of the world have such different climates: why some  parts of the world are really hot, while others are really cold, why some parts of  the world are very dry, while others are very wet. It all starts with the sun. That's what's driving  the weather.

Sunlight hits the globe at different angles. At the equator it hits the ground head  on. But at the poles it's coming in at an angle.

Let's look at two patches of ground: one at the  north pole and one at the equator. Both these patches are the same size, but the equator patch  gets more sunlight because the light is hitting it directly. That's why it's so hot at the equator. 

Over the course of a year the angle of the sun changes. As the earth orbits the sun, the angle  of its axis changes with respect to the sun. In July, the North pole is pointed towards  the sun giving the North more sunlight which means it's hot and it's summer.

In  January, it points away from the sun meaning it's cold and it's winter. The angle  of the sun also changes over the course of a day. That's why it's hottest in the middle of the  day because sunlight is hitting us more directly, but as the sun goes down it's coming in  at an angle.

Also notice how quickly the temperature is changing on land and  how slowly it changes in the oceans. Imagine a sunny day at the beach, the sun  is shining on both the water and the sand, but only the sand gets really hot. This is  because water has a higher heat capacity than land and because the water is circulating.

So the  land changes temperature more easily than water. You also may notice that the  mountains are especially cold. As you go up in elevation, the  pressure of the air decreases.

When you lower the air pressure, the air cools  down. When you compress air, it heats up. Expansion and compression explain how your  fridge works.

What your fridge is doing is it's using these principles to take heat from  inside the fridge and move it out of the fridge. In your fridge, you have a bunch of tubes filled  with a refrigerant. The refrigerant changes between a liquid and a gas.

When it enters  the fridge, it's expanded. When you expand the refrigerant that cools it down. Now that it's  cold, it can absorb heat from inside the fridge.

Now as the refrigerant leaves the fridge, you  compress it. This heats it up. Now we have a lot of hot gas outside the fridge.

The gas runs  through a bunch of coils and the heat dissipates away. Then the cycle repeats itself. The  refrigerant comes in.

It's expanded and it cools. Then it absorbs heat from the inside. When it  leaves, it's compressed.

This heats it up and the heat dissipates away. Pressure also affects  the temperature of the atmosphere. Air pressure is high near the surface because gravity is pulling  the air down compressing it at the bottom.

Now as you go up, there's less air pressure. Less  pressure means less temperature, so mountains are cold. Here we see lines of equal pressure. 

This line shows you where the pressure is exactly 500 millibars. Now normally these pressure  lines are pretty flat and I'll tell you why. Imagine something disrupts the air and changes  the air pressure.

Now as soon as that happens, the air begins to move. Air moves from high pressure  to low pressure. You may be familiar with this.

This is wind. By moving from high to low pressure,  the wind flattens out these pressure lines. Now let's talk about water.

There's  water vapor in the air and the amount of water depends on the temperature.  Hot air holds more water than cold air. This graph shows you how much water vapor you  can hold at a given temperature.

When it's hot, the air holds more water. Have you ever gone  outside and seen a layer of dew on the ground? Where did this water come from?

It came from  the air. During the day, it was warm and the air held lots of water. But at night, it got cold  and the air could no longer hold this water.

So it condensed on the ground and that's what  dew is. Let's imagine what would happen if we take some air and we lift it up. Now as it's  lifted, there's less pressure so the air expands and then it cools.

Now that it's cold, it can no  longer hold so much water. So the water condenses and this is how clouds are made. Now what  could cause the air to be lifted like this?

Imagine you have humid air passing over a mountain  range. As the air passes over, it's lifted up. So it expands and it cools.

The cold air can no  longer hold so much water. So the water condenses forming clouds and rain. When the air makes it  over the mountain, the water has all been sucked out of it.

And as it goes down, it gets compressed  and it heats up and we're left with hot dry air. So the windward side of the mountain gets all the  rain and the other side gets nothing. We see this effect all over the world.

Between California  and Nevada, there's a mountain range called the Sierra Nevadas. We have humid air coming in from  the Pacific. As the air goes up the mountains, the water condenses and it rains.

By the time  the air reaches Nevada, the water's gone. So on one side of the mountains, we have green  farm fields and the other side has Death Valley. And this is why California is so green and Nevada  is not.

Why did I come here? Let's look at another way we can get rain. If you've ever been to  Florida in the summer, you know that it rains there all the time.

Why? Well, earlier we saw  that land changes temperature more quickly than water. During the day, the land gets really hot. 

The air heats up. Now hot air expands, it's less dense than cold air. So the air is expanding  and rising.

And that changes the pressure. Above Florida there's an area of high pressure.  Wind moves from high pressure to low pressure.

Now to recap: the land is getting hot, the air is  rising, and then the air moves from being over the land to being over the ocean. There's now more  air sitting on top of the ocean, pressing down. This gives us a higher surface pressure over the  ocean.

Now wind moves from high pressure to low pressure. So at the surface, there's a breeze  coming in from the sea, known as the sea breeze. We started with just a change in temperature  and this caused all the air to start moving.

At the surface, we have a low pressure zone  over the land where hot air is rising. The air that's coming in from the ocean has  lots of water. This air gets lifted up as it's lifted, it expands and it cools. 

Cold air can't hold as much water as hot air, so it starts raining. And that's why it rains  so much in Florida. Temperature differences cause pressure differences which cause wind. 

This process is called convection. Convection happens on a small scale when you build a  campfire or when you boil a pot of water. The stove is like Florida.

Convection also happens  on a much larger scale and we'll talk about this in a minute. But first, let's talk about something  else that's happening on a very large scale: the earth is spinning and this spinning motion  changes the way that objects move on earth. This is known as the Coriolis force.

Now it's a very  complicated topic and I made another video about this. But all you really need to know is this.  When the wind blows in the Northern hemisphere, it's constantly being pushed to the right. 

When the wind blows in the Southern hemisphere, it's being pushed to the left. Now let's look  at how air moves on a global scale. This is similar to what we saw in Florida.

Remember the  equator is really hot, the poles are really cold, and hot air expands. So we have a lot of  expanded air sitting high above the equator. But the air stays compressed over the poles. 

Air moves from high pressure to low pressure. So high altitude winds move the air from being  above the equator to being above the pole. But now we have more air sitting over top the pole, giving  us more surface pressure there.

Wind moves from high pressure to low pressure. So at the surface,  we expect the wind to be moving from the pole to the equator. This is what would happen if the  earth was not spinning.

But the earth is spinning. So there's a Coriolis force pushing the air  to the right in the Northern hemisphere. The air does not travel from equator to poles. 

It keeps getting deflected to the right. Instead of moving in one big circulation pattern  from equator to poles, the air actually moves in three circulation patterns. These are called cells  and this is the three cell model of the earth.

What if the earth was spinning at a different  rate? If it was spinning slower, it'd be like Venus. Venus takes over 200 earth days to spin  around.

What if earth was spinning faster? Well then it would be like Jupiter, which only  takes about 10 hours to spin around. By the way, Venus is the only planet that's spinning  backwards.

On Venus, the sun rises in the West. Venus is spinning very slowly, so the Coriolis  force is very weak. Venus has one cell going from equator to pole.

Earth is spinning faster, so the  Coriolis force is stronger and splits the air into three cells. And on Jupiter, it's strong enough  to split it into many cells. So what's the effect of all this air moving around?

What's happening  is that hot air is moving away from the equator and cold air is coming in. So this is cooling  off the equator and it's warming up the poles. It's moderating the world's temperatures.

Now you  probably think man it's doing such a lousy job. The equator is still really hot and the poles  are still really cold. But not so fast!

You don't really understand what it's up against. It  could be worse. The average temperature at the North pole is about -40° F.

At the equator, it's  about 80° F. Now imagine, what if the earth had no air and no water? Well then it would be a lot  like the moon and the moon is -200° F at night and +250° F during the day.

So earth's air and  water is actually helping us out quite a bit. In my next video, I'm going to put all the pieces  together and explain why different parts of the world have such different climates using all the  tools we've learned. Here's a quick preview.

Along the equator, we have very warm air that holds lots  of water. The warm air is expanding and rising. As it rises, it cools down and so it can no longer  hold so much water.

So it rains a lot. All around the equator, we have the world's great tropical  rainforests: the Amazon, the Congo, and Indonesia. So this air had all this water in  it, but it lost it over the equator.

The air moves North and the air comes  down about 30° N. When the air comes down, it comes down as hot dry air and so this is  where many of the world's deserts are found, like the Sahara. They're right where this dry air is  coming down.

I'll explain more in my next video. For more astronomical videos,  please click to subscribe. Mmmmm, astronomical.