When altostratus clouds block the sun at sunset, they can sometimes produce a beautiful, colorful display of light known as a "sunset glow" or "afterglow".

Here's why:

* Scattering of Light: Altostratus clouds are high-altitude clouds composed of ice crystals. These crystals scatter sunlight, and at sunset, the scattered light is often red and orange due to the way the sun's rays pass through the atmosphere at an angle.

* Cloud Coverage: While the clouds block the direct sunlight, they are often thin enough to allow some light to pass through, creating the colorful glow.

The intensity and colors of the sunset glow can vary depending on the thickness and composition of the clouds, as well as the amount of dust and moisture in the air.