Daylight
The warm light photographers chase and the cool light that follows it are the same phenomenon at two stages. Understanding the mechanism lets you predict both without an app.
Golden hour and blue hour are conventions borrowed from photography, but the underlying physics governs what any outdoor activity looks and feels like at either end of the day.
Air molecules scatter light with a strong wavelength dependence. Rayleigh scattering efficiency varies with the inverse fourth power of wavelength, meaning short blue wavelengths scatter roughly ten times more strongly than long red ones.
Two consequences follow. Scattered blue light arriving from all directions is what makes the sky blue. And light travelling directly from the sun to your eye loses its short wavelengths preferentially along the way, shifting toward red.
When the sun is high, the path through the atmosphere is short and the shift is slight. As the sun approaches the horizon, path length increases dramatically — by a factor of thirty to forty compared with overhead — and blue and green are stripped out almost entirely. What remains is the orange and red of low sun.
Golden hour is not an hour. It is the period when the sun sits between roughly 6 degrees above the horizon and the horizon itself. Its characteristics come from three simultaneous changes.
The colour temperature of direct sunlight falls from around 5,500 kelvin at midday to below 3,000 kelvin near the horizon. The light arrives at a very low angle, producing long shadows and strong side-lighting that reveals surface texture. And the ratio between direct sunlight and diffuse skylight narrows, which softens contrast and fills shadows.
Duration depends on the same geometry that governs twilight. Near the equator, the sun crosses that 6-degree band in about twenty-five minutes. At 50 degrees latitude in summer, it can take well over an hour.
After sunset, direct sunlight stops reaching the ground but continues illuminating the upper atmosphere. What reaches you is entirely scattered light, and scattered light is preferentially blue.
Blue hour corresponds roughly to civil twilight, with the sun between the horizon and 6 degrees below. Illumination is soft, shadowless and cool, and its intensity falls into a range that balances against artificial lighting — which is why cityscapes are conventionally photographed then.
The same sequence runs in reverse before sunrise: blue hour first, then golden hour, then full day.
Ultraviolet drops far faster than visible light during golden hour, for the same reason winter sun produces no vitamin D: the slant path removes UVB disproportionately. Golden hour is therefore genuinely low-risk sun exposure, and also useless for vitamin D synthesis.
Air temperature typically falls fastest during evening golden hour, particularly on clear nights when radiative cooling is unimpeded. Valley floors and basins cool faster than slopes as dense cold air drains downhill, which can produce a temperature difference of several degrees between a ridge and the valley below it within the same half hour.
Wind commonly drops around sunset as the thermal gradients driving daytime convection collapse. Water surfaces frequently go glassy in the evening for this reason.
A rough field method: the sun moves about 15 degrees per hour, so it covers 6 degrees of elevation in roughly 24 minutes when descending perpendicular to the horizon. At latitude, divide that by the cosine of your latitude for an approximation of the shallower descent angle. At 40 degrees north, 24 divided by cos(40) gives about 31 minutes.
Fist-widths at arm's length offer a cruder check: one fist spans roughly 10 degrees. Two fists between sun and horizon is a little over an hour of light remaining at mid-latitudes.
For anything where being wrong has consequences, calculate rather than estimate. The daylight window calculator returns sunset and civil dusk for your exact coordinates and date.