The sky is that huge blue globe covering the earth. It is made of air and clouds. It is currently 250 miles thick. There are five layers of the sky:
Troposphere: where most clouds are
Stratosphere: where Mt. Everest’s peak is
Mesosphere: where the D-Layer is
Thermosphere: where the rest of the ionosphere is
Exosphere: Basically outer space
The ionosphere is where northern lights appear from those cosmic ray thingies.