Page 35 - Beginner's Guide to Aviation

4. Stratosphere
Upper boundary: 50 km
Lower boundary: 8 km (at poles) / 10 km (at equator)
Aircrafts usually cruise pretty low in the Stratosphere at about 10 km to stay above heard weather and avoid turbulence. The Stratosphere also contains the ozone layer which protects us by absorbing most of the Sun’s ultraviolet radiation.
5. Troposphere
Upper boundary: 7km (at poles) / 17 km (at equator)
Lower boundary: Surface of Earth
The Troposphere is the lowest atmospheric layer which begins from the Earth’s surface. This layer is where most weather conditions such as thunderstorms occur.
PRESSURE DATUMS
First of all, what is pressure? Well, in aviation, it refers to the force exerted by the atmosphere. (Think about it as weight of a column of air over a given point!)
Flight path
QNE QNH QFE
1013.2mb
MSL
The diagram above shows 3 different pressure altimeter settings and the different datums that aircrafts use in flight. The pressure datum selected depends on the pressure set on the altimeter subscale.
Airfield