Why do planes cruise at around 35,000 feet?

Because it is the altitude where fuel efficiency, weather avoidance and engine performance balance best. Fly lower and you burn more fuel; fly much higher and the engines run out of air.

Thinner air means less drag. At 35,000 feet the air is roughly a quarter as dense as at sea level, so an aircraft meets far less resistance and can travel the same distance on considerably less fuel. This is the dominant reason.

Engines still need oxygen, though. As altitude increases, there is less air to compress and burn, so thrust falls. Somewhere around 36,000–42,000 feet, depending on the aircraft and its weight, the falling drag and falling thrust cross over. That crossover is the optimum, and it is why airliners cluster in that band.

Weather is mostly below. Most turbulent weather and cloud sits in the troposphere. Cruising near the tropopause puts an aircraft above the majority of it.

Altitude buys time. In an emergency, height converts to gliding distance and gives crews minutes rather than seconds to respond.

The exact altitude is not free choice. Air traffic control assigns flight levels, and aircraft travelling in opposite directions are separated by using odd and even levels — broadly eastbound odd, westbound even — with 1,000 feet of vertical separation.

Aircraft also step-climb. A plane is heaviest at departure and cannot reach its most efficient altitude immediately, so on long flights it climbs progressively as fuel burns off and weight falls.

Concorde was the outlier, cruising near 60,000 feet where the air was thin enough for supersonic efficiency.

Short hops never reach cruising altitude at all, which is part of why they are less efficient per mile.