Downward longwave radiation is a key process to understand the climate change, energy budget, and water cycle at the
earth’s surface. Cloud is a dominant factor to determine the intensity of longwave radiation. It is widely known that
cloud cover and cloud base height (CBH) have strong effects on the downward longwave radiation, however there are
not so many studies regarding the quantitative evaluation of relationship between cloud properties and downward
The intent of the present study is to quantify the impact of cloud property on the downward longwave irradiance (DLI).
We used the data obtained with CGR-4 pyrgeometer at Tateno, Japan for the period from January 2002 to December
2011. Cloud radiative contribution fraction (CRC) is evaluated with a ratio of the difference of DLI between observation
under cloudy sky without precipitation and calculation assumed clear-sky condition to the observed DLI. The difference
between calculation and observation is -4.60±3.00 W/m2, and the calculation method reproduced to observation. Cloud is classified into three types by CBH, low (CBH<2000 m), middle (2000≥CBH<5000 m), and high (CBH≥5000 m).
In the results, CRC is almost proportional and inverse proportional to cloud cover (CC) and CBH in the average,
respectively. However, CRC for low cloud shows proportion to CBH because existence of low altitude cloud is related to
large precipitable water (PW).