Hello, Renee,
I'll assume that "the psi" refers to atmospheric pressure. As elevation increase, the atmospheric pressure decreases. Pressure is due to the weight of gas molecules being pulled to Earth by gravity. The suction by the vacuum of outer space isn't enough to totally strip away a planet's atmosphere (unless you are a very small planet with little mass, and even then, there will be some pressure due to remaining gas molecules that are held near the surface).
The average air pressure exerted on an individual on the Earth's surface at sea level averages 16.7 psi, but will depend on temperature and wind. We don't feel that because the body is pushing back constantly, by the same amount, and we're used to it. As the elevation increases, there are fewer molecules above you and Earth's attraction of gravity is lowered slightly. The force of gravity is inversely proportional to the square the the distance between two bodies of mass. So the molecules higher up are drawn back to Earth with reduced force. That means less pressure.
Water boils on Mt Everest at only 68C, down from 100C at sea level, due to the reduced pressure of the atmosphere at that height. [A problem for a backpacker's freeze-dried foods since the foods need a high temperature, not just boiling water].
Bob
