Why do cowboys in movies put their ears to the ground to listen for enemies?

Because sound travels faster and is more distinct when transmitted through solids compared to gases. This happens because solid particles are closer to each other than particles in gases. So the sound of enemies moving will be easier to hear when their ears are to the ground.

If you want to try this for yourself, hit a table when you're standing, then put your ear to the table and hit it again. It'll be much louder when your ear is to the table.

Hope this helps!

Jet streams are relatively narrow bands of strong wind in the upper levels of the atmosphere. the winds blow from west to east in jet streams but the flow often shifts to the north and south. jet streams follow the boundaries between hot and cold air. the earth's rotation is responsible for the jet stream as well.