Jälkhehkun

Jälkhehkun is a Finnish term that translates to "afterglow" or "lingering light." It refers to the residual light that remains in the sky after the sun has set below the horizon. This phenomenon occurs because the sun's rays, though no longer directly visible, still illuminate the upper atmosphere.

The appearance of jälkhehkun can vary significantly depending on atmospheric conditions. Factors such as dust particles, water vapor, and clouds can scatter and refract sunlight, influencing the colors and intensity of the afterglow. Typically, it is observed in shades of red, orange, and pink, though yellow and even violet hues can sometimes be present. The duration of jälkhehkun also varies, lasting from a few minutes to over an hour.

Jälkhehkun is most prominent during twilight, specifically during civil twilight and nautical twilight. Civil twilight is the period when the sun is between 0 and 6 degrees below the horizon, and sufficient light remains for most outdoor activities. Nautical twilight follows, with the sun between 6 and 12 degrees below the horizon, where the horizon is still discernible.

The scientific explanation for jälkhehkun involves Rayleigh scattering and Mie scattering. Rayleigh scattering is responsible for the blue color of the daytime sky, scattering shorter wavelengths of light more effectively. As the sun sets, longer wavelengths, such as red and orange, are scattered less and can travel further through the atmosphere, contributing to the warm tones of the afterglow. Mie scattering, caused by larger particles like dust and water droplets, can also play a role, influencing the intensity and specific colors observed. The presence of aerosols in the atmosphere can enhance and prolong the afterglow effect.