The acceleration of Gravity is less, as you get further away from the center of the earth. This would cause you to weigh less. The equation for the force of gravity is GMm/(R^2). Where G is the universal Gravity Constant which is something like 6.67x10^ (-11) and M is the mass of the earth, and m is the mass of the weight being measured. R is the distance from the center of the earth that increases as you increase in altitude.