This paper suggests that the actual sky,with the exception of a few par-ticular weather conditions,such as rain and snowstorm in which artificial lighting is needed.The sky can bc divided into four types according to their sunlight conditions and sky luminance distrbutions,they arc clear sky,pscudc-clear sky,pseudo-overcast sky and overcast sky.It is inadequate to classify sky merely from cloudage and to affirm that all sky conditions except clear and overcast belong to only one type.From the viewpoint of statis-tical average over many years,when sun disk is covered with cloud,luminance distribution of the sky resembles that of overcast sky,and when there is no cloud over sun disk,it is sim-ilar to that of clear sky.The luminance distributions of pscudo-clear sky and pseudo-over-cast sky are quite.In pseudo-overcatssky,the maximum skydifferent.In pseudo-clear sky,it gets maximum sky luminence can not appear in the region around the sun in considera-tion of the problem from the viewpoint of statistial average over many years.Furthermore,the differences of horizont.al diffuse illuminancc between pseudo-clear and pseudl-ouercast sky are obvious.Therefore it is more suitable to study all kinds of skies,that is pseudo-clear and pseudo-overcast sky.