The thin shell of gas around our planet that we call our atmosphere is divided into several layers. While the pressure falls with increasing altitude, the temperature is more dynamic, alternately decreasing and increasing with altitude.
There are six layers of the atmosphere around the Earth:
- Troposphere [surface to 10-18 km]
- Stratosphere [11 to 50 km]
- Mesosphere [40-50 to 80-90 km]
- Thermosphere [80-90 to 800 km]
- Exosphere [800 to 10000 km]
TROPOSPHERE
The troposphere is the lowest layer of Earth’s atmosphere. All life on our planet is affected by changes in this layer. Also, all the weather changes take place in the troposphere. It starts from the planet’s ground/surface (at sea level) and expands to 12 km in the sky.
Actually, the depth of the troposphere layer varies between 8 km and nearly 20 km, depending on where you live. It is the deepest above the equator and thicker above the poles.
This is the layer when we talk about oxygen levels, the one layer that contains the most of this gas every living thing on this planet needs. The higher we go in this layer of the atmosphere, the thinner the air gets. This means that humans would have a harder time breathing than in the lowest parts of the troposphere.
This gives you an idea of why it becomes so challenging when we climb high mountain peaks, and climbers take extra oxygen if needed.
STRATOSPHERE
Above the top of the troposphere and further up into the sky, the layer known as the stratosphere is placed. This layer rises from 11 to 50 km above the Earth’s surface. In this layer, the temperature normally rises as you go further up, and it has something to do with the ozone layer inside the stratosphere.
The ozone layer, however, serves a vital role in protecting our planet, as the ozone molecules prevent ultraviolet light (known as UV) from the Sun from hitting our planet without stopping. The UV light is not technically stopped in this layer, but the conversion from ultraviolet light to heat occurs there. That is why holes in the ozone layer (known as the ozone hole) are dangerous for us.
Another very important feature in the stratosphere is known as the polar vortex. This enormous, three-dimensional ring of winds surrounds the North and South poles. These winds are about 20 to 50 km above the Earth’s surface. The polar vortex often plays an important role in the winter weather across the high and mid-latitudes.
MESOSPHERE
When we reach this part, the mesosphere is the third and middle layer halfway up our atmosphere layers. The mesosphere goes up from 40/50 km to 80 or 90 km above the surface of our planet, and the temperatures here behave as they do in the troposphere. So, the higher you go, the colder it gets.
The mesosphere has very low oxygen levels, so it would be impossible to breathe there. This layer of the atmosphere also has the lowest temperature of all layers. It could get as cold as low as near -90° C.
Red sprites, a known transient luminous event (TLE) type, are also found in the mesosphere layer. They differ from the familiar lightning in the troposphere, but they occur high above the thunderstorm tops. They can be photographed from the Earth’s surface or space (e.g., International Space Station).
THERMOSPHERE
The Thermosphere layer is located between 100 and 800 km above the Earth’s surface level. And the interesting fact is that its name is ‘Thermo’ because high temperatures are found in this layer. The Thermosphere is under constant attack from the X-rays and UV radiation coming from the Sun and the space around Earth. This is also why the temperatures in this layer can even reach up to 2000° C.
The Northern Lights (also known as Aurora Borealis) and Southern Lights (known as Aurora Australis) mostly occur in the thermosphere. Charged particles (e.g., electrons and protons) from space collide at high latitudes with atoms and molecules in the thermosphere.
These collisions excite them into higher energy states. Those atoms and molecules shed this excess energy by emitting photons of light, which we see as colorful auroral displays.
EXOSPHERE
The final and the highest layer is known as the Exosphere. Unlike other lower and better-known layers, which are mostly distinguishable from one another, it is quite hard to know how far the exosphere is from the planet’s surface.
Somewhere it is around 100,000 km, but it can expand up to 190,000 km above sea level. The air here is very thin in this layer, and the conditions here are more similar to the ones we find when we leave the Earth’s atmosphere entirely. Basically, it’s similar to outer space.
Virtually all the weather happens in the troposphere, and thunderstorm clouds are (mostly) confined to the troposphere. Therefore, unless specifically stated otherwise, all phenomena discussed here occur in the troposphere.
FUN FACTS:
- The edge of space – the Karman line – is at 100 km height, within the thermosphere
- The highest-ever balloon ride and parachute jump were made by Alan Eustace in 2014: he rose up to 41.419 m and made a safe parachute jump.
- Meteors appear in the thermosphere and mesosphere, usually between 130 and 70 km. Some have been detected as high as almost 170 km, and some penetrate less than 20 km above the ground, well within the stratosphere.
Now that we have seen the structure of our atmosphere let us look at the part of the atmosphere thunderstorms (and we) live in.
Let us take a closer look at the troposphere. The pressure at sea level is approximately 1 bar or 1000 millibar (mbar or hPa). The pressure is higher if you get below sea level without diving beneath the sea surface. For example, on the coast of the Dead Sea in Israel, at 430 m below sea level, the air pressure is just above 1060 millibar.
As you go higher in the troposphere, the pressure drops. This happens because the higher you are, the less the atmosphere or air column is above you, pushing down on you. At the peak of Europe’s highest mountain, Mt Blanc (4807 m), the air pressure is only about 430 mbar.
That is less than half that at sea level. On top of Mt Everest (8848 m), the air pressure is only about 340 mbar. An aircraft cruising altitude of about 12 km is only about 200 mbar, only 1/5 that at sea level.
The troposphere cools with altitude, i.e., the higher you go, the colder the air is. On average, the air is cooler by about 0.7 °C for every 100 m higher you go. So, while it may be a pleasant 25 °C or more on the surface, it is -50 °C or below an aircraft cruising altitude at 10 km altitude or more.
The gradual temperature change with height is called a vertical temperature gradient or temperature lapse rate.
Recap: In the troposphere, the temperature drops with height. It is usually relatively warm near the surface but very cold in the troposphere. The rate at which the temperature drops is called the vertical temperature gradient.
Now, we will look at what drives the formation of thunderstorms.
NEXT: Convection and moist convection
