**6.1.2 Direct solar irradiance, Hb**

The direct solar irradiance or solar beam **Hb**, is the component of the total solar irradiance H, which comes directly from the top of the atmosphere, through the atmosphere, to the ground surface not deviated, nor scattered nor absorbed. The ratios of it to the total H i.e Hb/H and to the extraterrestrial radiation Ho, i.e Hb/Ho, are very important atmospheric radiation parameters in the radiative property of the atmosphere. Hb/H can be used to indicate the clearness of the atmosphere while Hb/Ho may be used to indicate the cleanness of the atmosphere and to determine the transmittance property of the atmosphere.

The direct solar irradiance is similarly measured like the global solar irradiance. It is measured using the Eppley solar tracker(NIP) with calibration constant 8.42 x 10-6V/ Wm-2. Unfortunate the incessant power outage prevented the continuous functioning of this radiation sensor in many developing nations.Therefore the data of direct solar irradiance is here, as in many other stations, obtained by computation.

### **6.1.3 Diffuse sky irradiance, Hd**

This radiation flux is also known as the sky radiation. It is short wave radiation, coming from the sky covering angular directions of 1800 to the sensor. It is incident on the ground surface as a result of scattering and reflection by particles in the atmosphere. Its ratio to the total flux H, i.e Hd/H measures the cloudiness and turbidity of the sky and its ratio to the extraterrestrial radiation Ho i.e Hd/Ho is expected to measure the scattering co-efficient of the atmosphere.

This radiation flux is measured in same manner as those above. An Eppley Black and White Pyranometer model 8-48 with calibration constant 9.18 x 10-6 V/Wm-2, with a shadow ring across the sensor, is used for the measurement. Unfortunately and inevitably the shadow ring may cut off some diffuse radiation, thus making the measurement to be inaccurate. This is why eqn.6 may not be valid or suitable to obtain the correct direct solar irradiance Hb .
