When sunlight passes through the atmosphere the light waves are scattered in different directions by dust particles and water vapor that is present in the air. The shorter violet and blue waves are scattered more effectively than the longer orange and red waves. As the Sun rises and sets its light travels a longer path through the atmosphere and so more of the colors at the red end of the spectrum are visible. The orange and red colors of sunsets can be intensified by air pollution or a high quantity of dust in the air.
During sunrise and sunset the Sun is low in the sky and so it is transmitting light through the thickest area of the atmosphere. When the sky is red there is a significant amount of dust and moisture in the air. When we see a red sky at night this usually means that a high pressure system blowing in from the west will be bringing stable weather conditions. Sailors like to see red skies at night. A red sky in the morning occurs when dust particles are being reflected from a system that has just passed from the west. This generally means that there is a high level of water vapor in the atmosphere and with it comes impending rain. Sailors do not like to be greeted in the morning to a bright red sunrise.
The sky is a deep blue when the atmosphere is clean and occurs when a cold front is bringing clean air in from the north, or when clean air over an ocean is brought inland. When the sky is a medium blue there is generally plenty of water vapor in the air. In certain areas of the world it is also caused emissions from certain plants and trees. A perfect example of this are the Blue Ridge Mountains which were named for the bluish haze that is caused by tiny particles in the air formed by terpenes, a chemical that is emitted by plants and trees.
When there is a considerable amount of air pollution, particularly sulfur, the sky will be a pale or milky white. This occurs quite often in summer when there is no breeze to move the pollution and it accumulates in one area.