Equate the gravitational and drag forces to find an expression for the terminal fall velocity of the particles in the atmosphere You will find from Question 3 that the terminal fall velocity of the particles depends on air density air. Thus, to estimate the time, the particles took to fall in the atmosphere back to the surface we need to know how air density changes with height in the atmosphere. Assume an isothermal atmosphere of temperature T, with RT 9 the scale height. The gas constant for air is R = 287JK- kg-. H =