Heating Our Atmosphere!
Our planet is surrounded by a thin layer of gases called the atmosphere. The atmosphere provides us with the oxygen we need, holds water vapor that influences our weather and serves as a protective blanket from the dangerous rays of the sun. The atmosphere is actually composed of about 78% nitrogen and 21% oxygen. Now if math serves me, that adds up to being 99%, therefore, all the other gases, including carbon dioxide, and water vapor make only 1% of the atmosphere.
Now, if you look at the picture above, you will see that the atmosphere is actually quite thin. Technically, it is about 500 km thick, but the part that really matters to us is only about 50 km thick. When you look at the picture, notice that there are five layers. The layers are from bottom to top, the troposphere, the stratosphere, the mesophere, the thermosphere, and the exosphere.
The troposphere is the lowest or bottom layer. It is where we live and where ALL weather takes place. Now it is only 10 km thick but it contains over 99% of the water vapor and 75% of the gases that are found in the atmosphere. Because air has mass (weight), it causes pressure. You know when you dive to the bottom of a swimming pool and your ears hurt, that's because of all the water pressure. Well, all the air in the atmosphere is pressing down all over Earth causing pressure. If all the air was pressing down equally, there would be no wind. BUT, there is wind and since wind is caused by a difference in air pressure, what can cause differences in air pressure? I'm glad you asked! First of all, we need some heat. Hey, how about the Sun?
The Sun is the source of almost all energy here on Earth. Let's talk about how it gets here and how it gets moved around. First of all, not all the Sun's energy makes it down to the Earth. Check out this picture. As you can see, right off the bat, about 6% of the Sun's energy gets reflected back into space by the atmosphere. Then another 25% gets reflected back into space by the clouds. 15% gets absorbed by the upper atmosphere and the surface actually reflects about 4%. So, that means only about 50% of the Sun's energy is absorbed by the Earth's surface.
OK, the really neat thing about the Sun's energy is that it travels to Earth as radiation. Radiation is energy that is transferred as rays or waves. This radiation is absorbed by the ground, buildings, trees, oceans, you, and anything else it may strike. Think about standing outside in the Sun. If it is a cold but sunny day, you can feel the Sun warming your skin. The sunlight actually changes into heat as it strikes you skin. Now, the sunlight is just pounding into the ground where it is changing into heat and warming up the ground. We now have warm ground, so how does the atmosphere get warm? Thanks for asking. The ground warms the atmosphere. Sounds strange doesn't it. How can the ground, the dirt, Mother Earth, down here under our feet warm the atmosphere which is everywhere BUT under our feet? Simple. Remember that we read how air has weight or pressure. All of the miles and miles of air is pressing down onto the ground. When the air touches the warm ground, the air starts to warm up. It gains heat from the ground by touching it. This process of gaining heat by touching something is called conduction. Heat is being conducted from the ground to the air.
These hot air balloons look like a lot of fun! These balloons are flying because they are filled with hot air. Hot air RISES!!! Just as surely as these balloons go up, the air being warmed up by the ground starts to go up or rise. As the warm air rises up through the atmosphere, it touches more air and helps to heat it up. Meanwhile, more air is touching the ground, getting warmed up and also rising, which helps heat more air. This process of warm air rising and heating other air is called convection. When heat moves through a gas or liquid, it is called convection. Convection causes currents. This further helps to move the heat throughout the atmosphere. Take a look at this picture to see how it works.
This picture shows the three main ways that the Sun's energy gets into and spread through the atmosphere, radiation, conduction, and convection. These three processes are the driving forces that cause all the weather on the Earth. Here's how. As we said earlier, when the air is heated by the Earth, it starts to rise. Memory moment: think about the last time you started walking across a hot street or sand at the beach, while barefooted. I'll bet that the farther you walked, the faster you walked because your feet started burning. The hot street was pouring heat energy into your feet. So in a way, the heat was making your feet move faster. OK here's the deal, anytime we heat something, we are putting energy into it. When we heat air, we are pouring energy into it and that causes the molecules of air to move faster and as those molecules move faster, they spread farther apart. As the molecules spread farther and farther apart, the air becomes less dense. Less dense air will begin rising. As heated air rises, cooler air sinks down below it where it gets heated and also begins to rise.
Heated air gains energy, the molecules start moving faster which makes them move farther apart, making the air less dense, which causes it to rise. As the air rises, it begins to cool, which means it is losing energy. As the air loses energy the molecules begin moving slower and slower which causes them to get closer together. The closer together the molecules get, the denser the air becomes. Dense cold air is heavier than warm air, so cold air sinks.
Check out this picture and you will start to get the idea of how wind is formed. The warm air rises leaving space beneath it. Cold air sinks into the empty space, while cooling, sinking air fills the space where the cold air was. The movement of the cold air into the space where the warm air was is wind. Wind is moving air.
Highs and Lows
When the weatherman or meteorologist starts talking about the weather map, he often has these symbols on it, "L" and "H". The letter "L" represents areas of warm air and the "H" represents areas of colder air. Here's a memory trick. Warm air rises because it weighs Less and cold air sinks because it is Heavier. So when you see the "L" think weighs less and when you see the "H", think heavier. So far so good. Now, if one thing is heavier than something else, usually, the heavier thing can push the lighter thing out of the way. This is very true of the atmosphere. The cold part of the atmosphere can push the warm part of the atmosphere out of the way. Think for a minute about the last time it was kind of warm and it started turning cold. Remember how it started getting windy and the longer the wind blew, the colder it got, well that wind was acutally the cold air pushing the warm air out of the way. After a few days, it started warming up again. That was because the cold air was touching the warm ground and slowly warmed up.
As we move into springtime, our days get longer and longer. Longer days mean more sunshine, which means more radiation striking the ground making it warmer and warmer. Warmer ground means warmer air, means warmer weather. Neat Huh?!