Lying on our backs in the meadow during a summer day, glancing heavenwards, we certainly do not notice any certain layers or zones that divide the atmosphere into several layers. Similarly, we do not see a wall or line when we cross the equator, telling us that we left the Northern Hemisphere and arrived in the Southern Hemisphere. Yet we divide the atmosphere into several layers, of which the lowest two are responsible for "the weather", and shall be viewed closer in the following text. The lowermost layer, reaching from the ground to about 11 km in altitude (slightly lower at the poles and higher in the tropics), is called the troposphere. The next layer is called the stratosphere.
In a simple picture, this is what it looks like:

Source: DFD-Glossary of the German Aerospace Centre DLR

The troposphere is marked light blue, the stratosphere dark blue. The boundary between the two layers is called the tropopause. In part of the stratosphere, the ozone layer exists. In the troposphere, the Earth's weather takes place, and a number of chemical reactions occur here as well. Here, we find most of the clouds, precipitation, and sometimes summer smog and car exhaust gases. Plants emit their scents into the troposphere, and chimneys emit their exhaust gases. Nearly none of these gases arrive into the stratosphere - and hardly any water either. Why is that so? Why are there layers, when we cannot see any?

Dialogue during the holidays

Anne: Hey, look! There is lots of snow left on top of the mountains!
Peter: Yeah, that's clear!
Anne: Why is that clear?
Peter: Because it's colder up there.
Anne: But why, actually?
Peter: Well, the higher you go, the colder it gets.
Anne: Colder and colder? Even if I fly real high?
Peter: Yes, certainly, up into space...there it's freezing cold. I read that in a book.

No, Peter! It's not clear! Although the book is right, it is freezing cold in space. However, with increasing altitude, it does not get uniformly colder. Instead, that counts for the troposphere initially, up to the tropopause. There, it is about -50( Celsius. Mountains won't tower beyond that - Mt. Everest is the world's highest mountain, reaching a height of about 9 km. Therefore, it is correct, that it gets colder with rising altitude in the mountains.

Why can temperatures rise again above that? No, not because it is closer to the sun. The sun is far, far away, and a lot of freezing air and space lies in between. The cause is the ozone layer. Ozone absorbs ultraviolet radiation and transforms it into heat energy. (For an explanation of the absorption mechanism please click here.)

In the diagram of the atmospheric layers at the top of the page, the temperature is shown with a red line. It shows how the temperature decreases from the surface to the tropopause, and increases above the troposphere.
This change from falling temperatures in the troposphere to rising temperatures in the stratosphere is very important as identifiers of the properties of both layers.

Let's look at a parcel of warm air while it is rising. In contrast to the balloon, where the warm air was covered by the balloon's skin, the warm air parcel can expand. Therefore, it cools more quickly. At the tropopause, the ascent finally ends. The air disperses and will descend eventually

For water as well, the transport generally ends at the tropopause, and nearly all clouds that result in precipitation or dissolve, exist in the troposphere. An exception to that are stratospheric ice clouds. We will learn more about them in the next chapters.

The fact that very few exchanges of air occur between the stratosphere and the troposphere means that only the most and least active air masses enter the stratosphere. All others are broken down in one of the thousands of chemical reactions in the troposphere, either by light or by so-called radicals (very reactive molecules which have one free electron). Because of this, the chemical diversity in the stratosphere is not great and only simple processes can take place. Along those lines are the production and destruction of ozone molecules. For this to happen, a surface of ice crystals is necessary. However, we will discuss this in the next chapters.

It is dangerous to disturb the few simple processes in the stratosphere by alien compounds. This can happen due to human activity (CFCs and aircraft exhaust), but also naturally, such as by strong volcano eruptions. Mt. Pinatubo catapulted sulphurous compounds and particles into the stratosphere when it erupted in 1991, which caused slight global cooling for the next two years.