4.2 Long wave radiation 
Downward and upward long wave radiation was 
calculated from the empirical models. 
4.2.1 Downward long wave radiation: 
Downward long wave radiation (D) was calculated from 
the following empirical model. 
L = oT*{0.61-0.058 le} ....(9) 
where T: temperature (K^), e: vapor pressure (mmHg). 
Vapor pressure e is computed using the equation: 
ezew(t)-CpP/0.6221(t-t) ...... (10) 
where e: vapor pressure (mb), Cp: specific heat of air at 
constant pressure, P: air pressure, 1: 580 cal/g, t;' t: 
temperature of wet and dry thermocouple thermometers, . 
ew: saturated vapor pressure. 
In remote sensing, difficulties with the measurement of 
wet-bulb temperature make it necessary to assume that 
both wet and dry bulb thermometers' temperatures are 
equal. Therefore, we applied the above Seino and 
Uchijima vapor pressure model based on data for a 
humid climate. Similar vapor pressure models are 
expected to develop for application in regions with 
different climates. 
Figure 12 shows daytime downward long wave radiation 
in the Hokkaido Island, Japan. 
4.2.2 Upward long wave radiation: Upward long 
wave radiation is estimated by the following equation: 
U- 8o(1Is4 273.8) 4 (1,8) L 2 (11) 
where à: emissivity, ©: Stefan Boltzmann constant, 
Ts: surface temperature (C^), L: downward long wave 
radiation 
Figure 13 shows daytime upward long wave radiation in 
the Hokkaido Island, Japan. 
4.2.3 Effective radiation: Then, effective 
radiation (F) is calculated using the following equation: 
F=L-U ....(12) 
where F:effective radiation, L: downward longwave 
radiation, U: upward longwave radiation. 
4.3 Net radiation 
Net radiation (S) is obtained by the following; 
S. (Cnm ) RocR renes (13) 
where S: net radiation, a": albedo (estimated albedo in 
each pixel), R: downward short-wave radiation, F: 
effective radiation. : 
Each net radiation during the day and at night was 
calculated by this process, and the amplitude of net 
radiation dS was obtained by subtracting net radiation at 
night from that during the day. 
Figure 14 shows daytime net radiation in the Hokkaido 
Island, Japan. 
888 
  
Fig. 12 Distribution of downward longwave 
radiation in and around Hokkaido 
Island, Japan (Oct. 17, 1990). 
  
Fig. 13 Distribution of upward longwave 
radiation in and around Hokkaido 
Island, Japan (Oct. 17, 1990). 
  
Fig. 14 Distribution of net radiation in and 
around Hokkaido Island, Japan (Oct. 
17,1990). 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996 
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