Why do planes fly at high altitude even across the oceans, when flying at lower altitudes require covering a shorter distance?

Question

Why do planes fly at high altitude even across the oceans, when flying at lower altitudes require covering a shorter distance?

In: Engineering

Answer 1

So, my best attempt at a true ELI5 answer:

You are in a swimming pool at one end, and you must get to the other end. You have two options:

You can swim all the way (staying at 5,000 feet).

You can climb out, walk to the other end, and climb back in (climb to 40,000 feet).

Even though climbing out adds distance (and requires a little more energy for the climb) you will use a lot less energy walking along the side of the pool.