Why would you weigh less on a mountain than at sea level?

Thats because of the Earth center of gravity and can be also known as the gravitational potential energy, the farther an object from center of the earth the lower the gravity but the less farther an object on the center of the earth the more gravity which is 9.8 m/s^2! 
Hope it helps

B) raising the temperature and reducing the volume of the gas.|-/