The clear sky clearness index ([[.bar.K].sub.T]) is a nondimensional ratio of the amount of clear sky radiation
that hits a horizontal surface ([[.bar.H].sub.clear]) over an entire day to the amount at the top of the atmosphere ([[.bar.H].sub.0]):
The clear sky radiation
on horizontal surface values were obtained with the use of the models proposed by Hottel and the model proposed by Liu and Jordan, as cited by Duffie and Beckman (2006).
In a typical house, the roof area is large with high exposure to solar gain and night sky radiation
, with minimal vapor control separation to the interior space.
The sky radiation
is ruled by the Plank Law, considering the concept of effective sky temperature, which can be defined as the temperature of a blackbody that emits the same amount of radiation as the sky .
Aubinet, "Longwave sky radiation
parametrizations," Solar Energy, vol.
As previously mentioned, for our experiment we used different radiometers at different frequencies, in order to measure the brightness temperature ([T.sub.B]) or a power measurement of the incoming wave reflected by the sample when pointed at a cold target, which is the zenith cold sky radiation
in our case.
Togrul IT, Togrul H, Evin D (2000) Estimation of global solar radiation under clear sky radiation
3) Long-wave sky radiation
toward the upper leaf surface [L.sub.d] was calculated as a function of air temperature [T.sub.a] and the water vapor pressure of the air [e.sub.a] according to Brutsaert's (1982) expression for clear skies:
In passive systems, such as FLIRs or mm-wave radiometers, radiation from the runway and its surroundings, including reflected sky radiation
, traverses a section of the atmosphere on its way to the sensor and in the process experiences a reduction in its intensity.
In fact, cloud cover has a strong influence on sky radiation
The sky temperature used to calculate the sky radiation
was assumed to be 10[degrees]C (18[degrees]F) below that of the ambient temperature, and that the panel's emissivity would be valid for both the long wave radiation leaving and entering it.