中文摘要：

就坡面天文辐射日总量而言,在一定纬度、坡向和时段范围内,可能存在坡面天文辐射日总量等于同纬度的水平面上天文辐射日总量的坡度,即临界坡度.临界坡度不仅存在于向阳坡,也存在于背阴坡.一般而言,中低纬度的坡面,临界坡度只有一个,在从地面到临界坡度的坡度范围内,天文辐射日总量大于水平面上的天文辐射日总量.特别地,在春秋分时,在纬度0 ～ 45°之间的南坡存在临界坡度,而且临界坡度等于坡面所在纬度的2倍.而极圈内的中高纬度,可能存在2个甚至3个临界坡度.

英文摘要：

As far as daily insolation on slope is concerned,a critical slope angle may exist where the daily insolation on slope is equal to that on the horizontal surface for the given latitude,slope angle and period.Critical slope angle exists not only on the slope toward equator but on the slope entirely shaded from the sun.Generally,there is only one critical slope gradient in the low and middle latitude slope.From the horizontal surface to the critical slope gradient,the daily insolation on slope is much more than that on the horizontal surface.Specially,when the latitude is 0-45°at equinoxes,the critical slope gradient of the slope toward equator is equal to two times of its latitude which the slope belongs to,while two or even three of critical slope angles within the polar circles are possible.