The Sun's energy and Earth's weather

0:00- [Instructor] What do a sunny afternoon and a thunderstorm have in common? Well, they both begin with the energy from the sun. Every breeze, every cloud, every storm is powered by solar energy. But how exactly you ask? Let's find out. We'll start with an overview.

0:16Every day Earth receives energy from the sun. We call this the solar radiation. This energy interacts with our atmosphere, which is layer of gases between the Earth's surface and the space, basically the air, and it also interacts with the hydrosphere. This includes all of Earth's water, liquid frozen underground, and in the air as water vapor, all of it, okay.

0:42The key idea is that it's this energy from the sun that makes the air and water move. We'll see how in a second, but it's this movement that causes changes in air pressure and temperature. It affects cloud formations, wind and rainfall patterns. In short, it affects the weather.

1:03But how exactly? Let's dig deeper. First of all, a quick reminder, what's weather? Well, weather is a state of the atmosphere at a specific time and place. For example, it can be sunny, or rainy, or windy. All that is weather. It's different from climate. Climate is a long term weather pattern in specific areas, and it's measured across several decades.

1:26For example, desert is typically dry and hot, right? That's the climate. But if there's a certain dust storm in that desert, that will be the weather, okay. Anyways, let's dig deeper now. Let's try and understand how the solar radiation causes the air and water to move.

1:45To do that, here's our question. What do you think happens to all of that solar radiation when it reaches the earth? Well, it turns out about 29% of it gets reflected back into space by clouds, the atmosphere, and even surfaces like snow, they can all reflect light.

2:05About 23% of it is absorbed by the atmosphere and clouds. And nearly 48% reaches the Earth's surface. It reaches the lands, the oceans, the soils. Most of this energy is absorbed by the Earth, and it turns into thermal energy. It warms our planet, and it also fuels the weather system.

2:26But the Earth doesn't hold onto all of this energy. It emits some of it back into the atmosphere as infrared radiation. What's infrared radiation you ask? Well, it's a type of radiation given by all warm objects like our bodies, stars, even water and rocks for that matter.

2:43Now, although we can't see it, when our skin absorbs it, it raises its temperature and it makes us feel warm. Okay, but anyways, the big question is what happens to this energy? Does it all go back into space? No. This is where the greenhouse gases come in. Carbon dioxide, methane, water vapor, nitrous oxide, they're all called greenhouse gases, okay.

3:09They absorb some of that infrared radiation and they get warmed up, and then they re-emit it back in all the direction, including back towards the Earth's surface. So this naturally warms up our planet, and this effect is called the greenhouse effect. And without it, the Earth would be too cold to support life.

3:33But the big question is, how does this energy actually cause the air and water to move? Well, let's answer that. Let's first start with water. Solar energy heats up the surface of water bodies turning it into water vapor. We call this evaporation. And this water vapor rises into the air and as it rises, it cools and condenses into tiny water droplets forming clouds.

3:55Eventually, the water returns to Earth as snow or rain, or even hail for that matter. And now that water can flow into rivers and oceans and can evaporate again, forming a cycle. So we call this the Water Cycle. This is how the sun's energy can make the water move.

4:16Okay, now let's talk about the air. The key idea here is that the sun doesn't heat every part of the Earth equally. If you look carefully, you can see that the same amount of sunlight is spread over a greater area near the poles compared to near the equators, right?

4:32Which means the sun's energy is more concentrated at the equator compared to the poles. So the result is that the equator heats up more than the poles. Even different surfaces absorb heat differently. For example, dark surfaces absorb more energy than the light ones.

4:48So the result of all of this is that it creates temperature differences. But what does that do? Well, when air warms, it becomes less dense and rises. As a result, the air pressure near the surface decreases causing a low pressure system. On the other hand, when air cools, it becomes more dense and sinks.

5:11And as the air sinks, the air pressure near the surface increases creating a high pressure system. And guess what? Air moves from a region of high pressure to lower pressure, and it's this movement that we call the wind. So this is how the sun's energy can cause differences in temperature, which can cause differences in air pressure resulting in air moving.

5:36Beautiful, right? So in summary, the sun is the main source of energy for Earth's weather and even climate. It's what sets the water cycle in motion. And because the sun doesn't heat all parts of the Earth equally, it causes changes in temperature and air pressure leading to different weather patterns like wind, and rain, and storms.

5:58Finally, greenhouse gases can trap some of this heat, keeping the earth warm enough for life. So everything from a gentle breeze to a thunderstorm starts with the sun.