CLOUD COVER ESTIMATION 
USING MULTITEMPORAL HEMISPHERE IMAGERIES 
M. Yamashita *, M. Yoshimura?, T. Nakashizuka " 
* Survey College of KINKI, 1-5-9 Yata, Higashi-Sumiyoshi-ku, Osaka 546-0023 Japan - yamashita@kinsoku.ac.jp 
? Research Institute for Humanity and Nature, 335 Takashima-cho, Kamigyo-ku, Kyoto 602-0878, Japan — 
(yosh, toron)@chikyu.ac.jp 
Commission VII, WG VII/6 
KEY WORDS: Atmosphere, Estimation, Monitoring, Radiation, Parameters, Multitemporal, 
ABSTRACT: 
The solar radiation is the origin of energy for the photosynthesis and forest productivities. On the other hand, clouds constantly 
appear, move and disappear all the time and influence atmospheric and radiation conditions at the same time. Therefore, to 
understand cloud parameters which consist of cloud covers, its distributions and types etc. for a short time interval can help us to 
estimate solar radiation or photosynthetically active radiation (PAR) with high accuracy. 
Authors have been observed and measured various parameters related with photosynthesis and carbon exchange for tropical rain 
forest in Borneo Malaysia so far. Concerning to the cloud conditions, we also monitor the whole sky using the commercial digital 
camera with the fisheye converter lens and can obtain hemisphere imageries about every two minutes interval. 
As the experimental study about cloud parameters, in this paper, we present 1) the observation system of whole sky conditions, 2) 
how to analyze the obtained hemisphere imageries and 3) the method to estimate cloud cover and understand cloud distribution with 
sun position. Here, we use Sky Index which is calculated using visible RGB image taken by digital camera. This index is useful to 
separate whole sky area to clouds and the blue sky areas. Then multi-temporal cloud covers can be estimated from the Sky Index 
generated by multi-temporal hemisphere imageries. 
Through this study, we also could recognize the efficiency to understand cloud cover, cloud distributions and types by using 
hemisphere imageries and Sky Index. As the continuous works, we examine the relationship between the solar radiation and cloud 
conditions such as cloud covers, distribution and types. 
1. INTRODUCTION tower. Our purpose of this study is to estimate cloud covers and 
understand cloud distributions multi-temporally by using 
The solar radiation is the origin of energy. Thus estimating ^ hemisphere imageries. We present, in this paper, the whole sky 
solar radiation at the surface is needed in not only atmospheric observation system, the methods to detect clouds from the 
science but various fields such as the photosynthesis, carbon hemisphere image and understand the distribution of clouds, 
exchange, energy budget, water cycling and productivities of ^ and cloud cover estimation by using clouds detection imageries. 
forest and agriculture. On the other hand, clouds control total As for the method of clouds detection, we used Sky Index, 
incoming irradiance at the surface. On the condition of the clear which is generated by using blue and red channels of original 
sky, which means cloudless, it possible to estimate solar RGB images, and extracted the clouds area clearly. In the future 
radiations using cosine of sun zenith angle and some existing works, we will examine the relationship between the global 
models (Charles et al., 2000). However in reality, the clouds radiation and cloud conditions which consist of cloud covers 
constantly appear, move, change shapes and disappear, that and distribution estimated as the results of this study. 
influence to the atmosphere and radiation environment at the 
same time. Therefore, in order to estimate the global radiation 2. WHOLE SKY OBSERVATION 
in short time interval and with high accuracy, it is necessary to 
understand the effects of clouds as input parameters. Especially 2.1 System of whole sky camera 
cloud cover, which is defined as the ratio of cloud area to whole 
sky area at the observed point, and cloud distribution in whole The system and observation for taking multi-temporal 
sky are the strong impact to the global radiation that consists of hemisphere images are described. This system consists of the 
direct, defuse and reflected solar radiations. commercial digital camera installed with the fisheye lens and 
We have been observed and measured various parameters the remote control cord. The details are explained as follows. 
related with photosynthesis and carbon exchange for tropical E ia : 
rain forest in Borneo Malaysia (Yoshimura et al., 2001), which Digital camera: Used digital camera is Nikon COOLPIX5000 
is carried out on the project entitled ‘Mechanisms of Which has color CCD with effective pixels of 2658 x 1970. The 
Atmosphere-Ecosphere Interaction in Tropical Forest Canopy’ image sizes are selectable from 2560 x 1920, 1600 x 1200, 1280 
(Nakashizuka et al., 2001). Clouds in tropical rainforest area are X 960, 1024 x 768, and 640 x 480. The file formats of image 
moving at multi layers. Here is little chance of cloudless in quality are uncompressed TIFF (RAW) and compressed JPEG 
whole sky all day long trough a year. Therefore, as regards the at four levels as Hi, Fine, Normal and Basic. The image files 
observation of cloud conditions in this project, we setup the are saved to Compact Flush card in the camera. 
camera system which takes whole sky image at the top of tree 
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