Large two-ribbon flares are frequently accompanied by a system of (post)-flare loops, cooler structures being visible in several optical and UV lines. We investigate tlie non-LTE formation of hydrogen lines in stationary loop structures, taking into account the influence of macroscopic velocity flows along the loop. We demonstrate some diagnostically important effects on hydrogen Hα line and identify them using narrow-band Hα filtergrams. This diagnostics provides us with a reasonable description of physical conditions prevailing in the cool flare loops. However, we finally stress the importance of even more complex (and difficult)
radiation-hydrodynamical approach.