日照时数是指太阳每天在垂直于其光线的平面上的辐射强度超过或等于120W/m2的时间长度。The solar duration is the length of time in which the solar radiation falling on a plane perpendicular to the direction of the sun is greater or equal to 120W/m2.再给出一个世界气象组织（World Meteorological Organization）的标准定义：According to WMO (2003), sunshine duration during a given period is defined as the sum of that sub-period for which the direct solar irradiance exceeds 120 W m–2.

日照时数的拼音