The sky is the upper part of the earth's surface, and is classified as a separate layer called the atmosphere. The sky is made up of a lot of gas and air, with different compositions in each layer. The sky often looks blue in the morning and afternoon, because the air refracts blue light from sunlight more than red light. The sky can change color under certain conditions, for example red at dusk or black when it rains.
Together with pressure trends, sky conditions are one of the important parameters used to forecast weather in mountainous areas. Thickening of the clouds is an indication of rain in the near future. At night, tall, thin cirrostratus clouds can cause a halo around the Moon, indicating the arrival of warm air currents and rain. Fog in the morning signals clear conditions and can be attributed to the marine layer, an indication of a stable atmosphere. Rainy conditions are preceded by wind or clouds which prevent the formation of fog. The phenomenon of thunderstorms can indicate the arrival of cold air currents. Cloud-free skies are an indication of sunny weather in the near future. Weather forecasting by looking at the sky has given rise to various folklore.