Sunshine

Sunshine generally refers to the sunlight that reaches the Earth's surface and the daylight conditions associated with it. It comprises visible light as well as infrared and ultraviolet radiation emitted by the Sun and modified by the Earth's atmosphere. The solar constant, the energy arriving at the top of the atmosphere, is about 1,360 watts per square meter; at the surface, the experienced irradiance varies with time of day, season, latitude, and cloud cover, and can exceed 1,000 W/m^2 on a clear noon sky. The spectrum is broad, with substantial contributions across the visible range and notable infrared and ultraviolet components, giving the light a white appearance when unfiltered.

Sunshine is a primary driver of climate and weather and supports life through photosynthesis. It influences human circadian rhythms and contributes to vitamin D synthesis in the skin. In meteorology, sunshine duration measures how long direct sunlight is experienced, often recorded with a sunshine recorder and expressed in hours per day or per year; irradiance and insolation quantify the energy received from the Sun.

Common measurements include irradiance (watts per square meter), insolation (energy per area, such as watt-hours per square meter), and luminous levels in lux, which reflect human visual perception. Direct sunlight can reach about 100,000 lux under clear skies, while indoor daylight is typically much lower.

Protection from excessive ultraviolet radiation is advised, through sunscreen, hats, sunglasses, and appropriate clothing. In energy contexts, sunshine is a major source of solar power and a focus of renewable energy development.