Anatomy of the Atmosphere

The atmosphere is primarily made up of two important gases: nitrogen and oxygen.

These account for about 78% of the atmosphere, and 21% of the atmosphere, respectively. The remaining 1% of the atmosphere is a diverse mix of gases such as argon (0.9%), methane, and carbon dioxide.

The atmosphere also contains varying amounts of water vapor. Take a look at the sky, and you might see some of it now in the form of a passing cloud.

The atmosphere also contains aerosols: Tiny particles including dust, volcanic ash, pollutants, spores, and pollen.

In other words, the atmosphere is a soup of different components. And it’s also important to understand how this atmospheric soup behaves. There are two main factors to be aware of: pressure and temperature.

Atmospheric pressure — also known as air pressure — is a relatively simple concept.

Take any point in the atmosphere, and think about the weight of all the air above it. This weight applies pressure as it squeezes down on that point.

As a general rule, pressure reduces as you get higher in the atmosphere because there’s less air above you, and therefore less weight pushing down. It’s just like the ocean. The pressure of the water is much higher at the bottom than it is just beneath the surface.

Millibars (mb) are a unit of pressure that meteorologists use to describe the atmosphere. At sea level, the average air pressure is just over 1000 mb. At the top of Mount Everest, air pressure is closer to 300 mb.

Speaking of mountains: if you’ve ever climbed one, you might have noticed that the atmosphere feels colder at the top than it does at the bottom.

This principle is known as the lapse rate: the rate at which the temperature falls as we get higher in altitude, just like atmospheric pressure.

But unlike pressure, this principle only takes us so far. At a height of approximately 5 miles (8 kilometers) the temperature of the atmosphere actually starts to go up. Even higher, it drops again, then it starts to go up again, yo-yo-ing back and forth.

We’ll discuss this in more detail later. For now, it’s just important to know that atmospheric temperature gradients — that’s the change in temperature in relation to altitude — aren’t as linear as changes in pressure.

The changing temperature gradients, at different points in the atmosphere, are used by meteorologists to divide the atmosphere into layers.

You can think of it like a gaseous cake with five main tiers.

The bottom layer of the atmosphere is called the troposphere. All the air you breathe? That’s the troposphere. All the weather you experience? That’s the troposphere too.

This part of the atmosphere has the highest pressure, as it’s right at the bottom of all the other layers. It’s also the densest layer, as the atmospheric pressure squeezes all the particles close together. As for temperature, it drops off as altitude increases.

Then something strange happens. The temperature levels off, before starting to increase. This change in the direction of the temperature gradient marks the start of the next layer: the stratosphere.

The stratosphere is the atmospheric layer that starts about 7.5 miles (12 kilometers) above the Earth’s surface and stops about 23.5 miles (38 kilometers) later.

Its most notable feature is the fact that it contains a high concentration of ozone.

Ozone is a type of gas made of three oxygen atoms, which is great at absorbing ultraviolet radiation from the sun. It’s like a giant blanket sucking up heat — that’s why temperature increases as you get higher and higher in this layer.

With lower pressure, the air is much thinner up here than it is in the troposphere, and we don’t see any real weather. The odd cloud might stretch into the stratosphere, but most of the action happens down below.

Above the stratosphere, temperatures start decreasing again, which marks the next layer: the mesosphere.

The mesosphere is the atmospheric layer that starts at 31 miles (50 kilometers) and stops after another 19 miles (30 kilometers). There’s less ozone here, which is why the temperature starts to drop off. The highest point of the mesosphere is the coldest place in the atmosphere.

The next layer is called the thermosphere. It’s the atmospheric layer that starts 50 miles (80 kilometers) from the surface of the Earth and continues upwards for a massive 390 miles (620 kilometers).

In some places, you might be able to glimpse it. This is where the aurora borealis and aurora australis usually take place.

Again, it’s differentiated from the layer beneath by the fact that the temperature gradient changes. The temperature starts to climb again because the atoms here are absorbing a lot of high-energy solar radiation. This is the hottest layer in the atmosphere, which is why it’s called the thermosphere.

Last but not least — right at the top of this atmospheric cake — is the exosphere.

It’s the atmospheric layer that starts 440 miles (700 kilometers) up in the air, then continues upwards for a staggering 5760 miles (9300 kilometers).

Here, the temperature drops again, as the atmosphere starts to merge with outer space. The higher you get, the fewer gases there are, until there’s nothing there at all.

In the context of meteorology, the most important layer of the atmosphere is the troposphere.

This is where all the action happens. The air in the troposphere is constantly in motion, like a churning, gaseous sea.

It mostly comes down to temperature differences. We’ve already discussed the lapse rate in the tropospheres, but this isn’t the only factor that influences the troposphere’s temperature.

Think about it: the air by the sea is usually cooler than the air a little further inland. The altitude might be the same in both places, but the temperature is still quite different.

This is because the land has a lower heat capacity than water — that’s the energy it takes to raise a body’s temperature. In other words, it takes less energy to raise the temperature of land than it does to raise the temperature of water. They could both receive the same amount of sunlight, but the land would warm faster, mainly because it’s more dense and opaque.

And as the land gets warmer, so does the air above it. This leads to local differences in temperature — and these differences lead to some very important side effects.

When air gets hotter, it gets lighter. This causes the air to rise.

When air gets cooler, it gets heavier. This causes the air to sink.

This simple process is an essential part of meteorology. When a patch of air warms up — for example, over a landmass — it rises upwards. At the same time, cooler air from neighboring areas will flow in to replace it.

We experience this as wind: a flow of air from one part of the atmosphere to the other.

As that hot patch of air gets higher in altitude, the lapse rate will cool it down again. As it starts to get heavier, it will sink back down again.

This is why the troposphere is constantly in motion: patches of air are always heating and cooling, rising and falling and flowing.

In meteorology, a patch of hot, rising air is called a low-pressure system. A patch of cold, sinking air is called a high-pressure system.

Local movements of air, caused by differences in temperature, are sometimes referred to as microscale wind patterns.

Sea breezes and land breezes are common examples. During the day, the land heats up faster than the water, and a breeze pulls in from the sea.

At night, the land cools faster than the water, so a breeze flows back out to the sea.

Mountain breezes are another example. During the day, the air on the sunlit slopes of a mountain heats up, while the shaded valley remains cool. This leads to a breeze flowing up from the valley to the mountain.

These different breezes are good examples of the interconnectedness between the atmosphere, the troposphere, and the lithosphere. By looking at the distribution of land and water in a specific area, a meteorologist should be able to predict the behavior of microscale winds.

Along with local temperature differences, like those between sea and land, there are also global differences in atmospheric temperatures. These also have a major impact on the movement of air in the troposphere.

The Sahara Desert and the North Pole are at roughly the same altitude. But is the temperature of the air in the Sahara Desert the same as the air in the Arctic? Of course not.

This uneven heating is mainly due to the tilt of the Earth's axis. This tilt means the equator receives more sunlight than the poles, and consequently, the troposphere in equatorial regions will be warmer.

On a global scale, the temperature differences between the equator and the poles lead to massive movements of air. These are referred to as global wind patterns, or atmospheric circulation.

As a general principle, hot air rises at the equator, then flows north and south towards the poles. At the same time, cold air from the poles flows back towards the equator. This happens at a lower elevation than the hot air, and leads to global winds.

This model was devised by William Hadley, a British meteorologist, in the 1700s. But it doesn’t tell the whole story. Other factors influence global wind patterns. For example, the rotation of the Earth.

The Coriolis Effect is an important phenomenon that results from the Earth's rotation. It causes moving air to veer to the right in the northern hemisphere and veer to the left in the southern hemisphere.

Actually, that isn’t quite accurate. The air is still moving in a straight line, but it appears to be veering because the Earth is rotating underneath it.

This is why Hadley’s model of atmospheric circulation isn’t entirely accurate. As well as moving from the equator in the direction of the poles, global winds also veer, or curve, to one side.

In the northern hemisphere, they veer west as they travel from the Arctic to the equator, giving an overall southwesterly direction. In the southern hemisphere, they also veer to the west, giving an overall northwesterly direction.