Why did the whole sky turn red in the evening before the rainstorm?

The whole sky turned red in the evening before the rainstorm, mainly because of the scattering of sunlight by the atmosphere.

When the weather is clear, air molecules in the atmosphere scatter sunlight strongly, mainly scattering blue light and rarely scattering red light. This makes the sky we see appear blue.

However, before the rainstorm, due to the increase of water vapor and suspended particles in the atmosphere, the scattering of solar light by air molecules in the atmosphere increased, especially for blue light. This leads to a decrease in blue light and a relative increase in red light, thus making the whole sky appear red.

In addition, before the rainstorm comes, it is often accompanied by the development of gas cyclone field, which will roll up a lot of water vapor and suspended particles, further enhance the scattering in the atmosphere and make the sky appear deeper red.

In a word, the whole sky turned red in the evening before the rainstorm because the scattering effect of the atmosphere on solar light increased, which led to the decrease of blue light and the relative increase of red light. This phenomenon is usually considered as a precursor of heavy rain.