Sunshine duration

Sunshine duration, also called sunshine hours, is a way scientists measure how much sun a place gets over a certain time, like a day or a year. We usually find out this information by looking at many years of data to get an average. This helps us understand how cloudy or sunny a place usually is. It is different from insolation, which looks at how much energy the sun gives us, not just how long it shines.

We can talk about sunshine duration in hours each year or in average hours each day. This helps us compare how sunny different places are or how sunshine changes with the seasons in one place. Sometimes we also look at how much of the daylight time was filled with bright sun.

Knowing about sunshine duration is important for describing a place’s climate, especially for places that are known for being good for health or for visitors. The amount of sunshine can also affect how happy and healthy people feel, which is why it is often used when talking about great places to visit.

Daytime duration

Main article: Daytime (astronomy)

If the Sun were above the horizon half the time in a year, the maximum daytime would be 4,380 hours everywhere on Earth. But natural effects change this. For example, Earth's atmosphere bends light, so we can still see the Sun after it appears to go below the horizon. This means places near the poles see the Sun for longer. Areas on the Arctic Circle have the longest total daytime at 4,647 hours each year, while the North Pole gets 4,575 hours. Because Earth's path around the Sun is not a perfect circle, the Southern Hemisphere is not the same. The Antarctic Circle has 4,530 hours of daylight. The Equator sees the Sun for 4,422 hours each year.

Definition and measurement

Sunshine duration is the time when the sun shines brightly enough to be measured. Scientists use special tools called sunshine recorders to track these hours. One tool is the Campbell–Stokes recorder. It uses a glass lens to focus sunlight onto a special tape. When the sunlight is strong enough, it leaves a mark on the tape. The length of the mark shows how many bright sunshine hours there were.

In 2003, scientists agreed that sunshine duration is when direct sunlight energy reaches more than 120 W/m². This helps them compare sunshine data from different places around the world. Sunshine recorders
Campbell–Stokes recorders
Lens
Jordan sunshine recorder
World Meteorological Organization
Solar irradiance

Geographic distribution

Sunshine duration depends on where you are on Earth. Places near the subtropics, between about 25° to 35° north and south, get the most sunshine. These areas are often dry and have clear skies because of high pressure in the atmosphere. Examples include deserts like North Africa, the Southwestern United States, and parts of the Middle East.

Some of the sunniest places in the world are in the central and eastern Sahara Desert, including countries like Sudan, Libya, and Egypt. Yuma, Arizona is often called the sunniest city, with lots of sunshine each year. In contrast, places farther from the equator, like northwestern Europe and parts of Canada, get much less sunshine because they have more clouds and rain. Some of the cloudiest places include Cold Bay, Alaska and areas near the Sea of Okhotsk.