6 Diffuse Irradiance

A little later in the morning, if one hides the solar disk by holding the tip of a single finger at arm’s length before his eyes, the deep blue sky color holds right up to the edge of the sun itself.

Abbot Charles Greeley


6.1 Introduction

Diffuse horizontal irradiance (DHI) is defined as the solar irradiance that has been scattered by molecules, aerosols, and clouds in the atmosphere and received on a horizontal surface (refer to Figure 6.1). Another perspective on diffuse irradiance is that DHI is the skylight portion of the global horizontal irradiance (GHI) remaining after removing the direct normal irradiance (DNI). DHI is measured by a properly shaded pyranometer, or it can be computed from measurements ...

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