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.