The ratio of diffuse to direct solar irradiance (perpendicular to the sun's rays) with clear skies - a conserved quantity throughout the day.

Abstract

The ratio of diffuse irradiance to direct normal is a conserved quantity throughout the day. Though its absolute value depends on the condition of the atmosphere, ground reflection and obstruction of horizon, once this value is established (by a single measurement) in rural regions, it persists over the entire day. -from Authors