Why does the sky appear red in the morning?

At both sunrise and sunset the sky may appear red due to a combination of atmospheric effects, which include the way light is bent in different ways by particles depending on its wavelength. When the sun is near the horizon, this makes more red light bend towards the surface of the earth. (During the day, it is mostly blue that comes our way, which is why the sky is usually blue.)
https://en.wikipedia.org/wiki/Sunrise#Colors

The effect is intensified if there are clouds in that part of the sky. This is behind the old sayings about red at night being a delight and red in the morning being a warning - clouds are obviously related to weather.
https://en.wikipedia.org/wiki/Red_sky_at_morning