# Measure of daylight brightness

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?

• The term you are looking for is "airmass", not to be confused with the meteorological concept of "air mass". Does this answer your question? Sep 26, 2022 at 11:30
• I think this is it indeed, but then are there any systematic measures of this around the world? I mean like one would easily get access to data about the number of hours of sunlight in a given city, is it reasonable to think this is possible to get this for air mass as well?
– Maël
Sep 26, 2022 at 14:29
• While albedo is a calculation of the reflection of solar radiation (percentage units), could you use the base measurement of incoming solar radiation that is used in the albedo calculation? Its units are watts/square meter so it should take into account sun distance and angle. So it's about how much energy is coming in, not how much is being reflected back out. Sep 26, 2022 at 19:06
• I now realize this might be duplicate of earthscience.stackexchange.com/questions/4987/…
– Maël
Sep 30, 2022 at 10:30

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.

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