Measure of daylight brightness

Question

I am looking for a meteorological measure of daylight brightness/luminosity. I can only find measures that are defined by the number of hours of sunlight during a day, but is there a measure that also captures the fact that particular regions of the world, far from the equator, have not only less daylight at some point during the year, but also less brightness, or luminosity, meaning that the sun is further down or up in the sky.

Maybe something close to a measure of albedo at certain points in time and place?

Answer 1

Perhaps you may be interested in downward shortwave flux? It isn't easy to compute or measure, but it should capture what you are after. You can use the equation $$R_{net}=S\downarrow-S\uparrow+L\downarrow-L\uparrow$$

where $S$ represents shortwave, $L$ represents longwave, $R_{net}$ is the net radiation, and the arrows depict upward or downward. If you assume an albedo $\alpha$, you can say that $S\uparrow$ is a fraction of $S\downarrow$: $$R_{net}=S\downarrow(1-\alpha)+L\downarrow-L\uparrow$$

If you use an IR thermometer, you can measure the temperature of the sky ($T_{sky}$) and the ground ($T_{ground}$). If you assume a corresponding emissivity (\epsilon), then you can invoke Stefan-Boltzmann's "law" and get $$R_{net}=S\downarrow(1-\alpha)+\sigma\left(\epsilon_{sky}T_{sky}^4-\epsilon_{surface}T_{surface}^4\right)$$.

If you measure $R_{net}$ using a net radiometer, you can solve for $S\downarrow$. Alternatively, you can also compute $S\downarrow$ empirically, like some of the climate models do. But that is perhaps the most computationally intensive part of climate modeling.

Answer 2

I am looking for a meteorological measure of daylight brightness/luminosity...but is there a measure that also captures the fact that particular regions of the world, far from the equator, have not only less daylight at some point during the year, but also less brightness, or luminosity, meaning that the sun is further down or up in the sky.

Consider the following -

Radio Corporation of America, 1968 (and in later years), RCA Electro-Optics Handbook. Section 6. Sources of Radiation, p.6-10ff, fig. 6-10, Atmospheric Transmittance.

Note: The image (below) is from a republication by RCA ca. 1974, p 74. Please note the original source reference, Bond and Henderson, 1963, given below.

16. Bond, D.S. and Henderson, F.P., THE CONQUEST OF DARKNESS, AD 346297, Defense Documentation Center, Alexandria, Va., 1963.