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 \(\alpha\) is defined such that the fractional decrease in the specific intensity over a distance \(dx\) is given by
