It depends on the jet or aircraft you are refering too some go into our atmosphere
The maximum altitude of any military aircraft is dependent on many things including the type of craft. The maximum altitude for a military jet is approximately 60 thousand feet. At that level the flight crew will need special equipment to allow them to breathe and stay conscious in the thin air.
Zero, if the jet plane is on the ground, otherwise it depends at what altitude the plane happens to be flying. Cruising altitude for many passenger airliners is around five to seven miles. Military aircraft often fly at much higher altitudes.
The jet stream regularly affects high altitude wind and low altitude weather.
jet streams
The cruising altitude of a normal commercial jet airplane today is around 12,000 m (35,000 ft).
Jet Streams.
8600
It's about 35,000 ft.
Jet stream
The altitude ranges from 30,000 feet to 45000 feet. This is only average altitude. This range is suitable when the plane has levelled.
There are all kinds of "military jet" . You would need to name a specific one for that answer
Military jet fighters patrol and if necessary attack the nations enemies. -They are a projection of power.