At a level in the atmosphere higher by a distance dx, the specific intensity has dropped, as a result of absorption, to I+dI. (Here dI, by the convention of differential calculus, means ...At a level in the atmosphere higher by a distance dx, the specific intensity has dropped, as a result of absorption, to I+dI. (Here dI, by the convention of differential calculus, means the increase in I, and it is in this case negative. The quantity −dx, which is positive, is the decrease in I.) The linear absorption coefficient α is defined such that the fractional decrease in the specific intensity over a distance dx is given by
