Sunshine duration
Sunshine duration or sunshine hours is a
Sunshine duration is usually expressed in hours per year, or in (average) hours per day. The first measure indicates the general sunniness of a location compared with other places, while the latter allows for comparison of sunshine in various seasons in the same location.[1] Another often-used measure is percentage ratio of recorded bright sunshine duration and daylight duration in the observed period.
An important use of sunshine duration data is to characterize the climate of sites, especially of health resorts. This also takes into account the psychological effect of strong solar light on human well-being. It is often used to promote tourist destinations.[1]
Daytime duration
If the Sun were to be above the horizon 50% of the time for a standard year consisting of 8,760 hours, apparent maximal daytime duration would be 4,380 hours for any point on Earth. However, there are physical and astronomical effects that change that picture. Namely, atmospheric refraction allows the Sun to be still visible even when it physically sets below the horizon. For that reason, average daytime (disregarding cloud effects) is longest in polar areas, where the apparent Sun spends the most time around the horizon. Places on the Arctic Circle have the longest total annual daytime, 4,647 hours, while the North Pole receives 4,575. Because of elliptic nature of the Earth's orbit, the Southern Hemisphere is not symmetrical: the Antarctic Circle, with 4,530 hours of daylight, receives five days less of sunshine than its antipodes. The Equator has a total daytime of 4,422 hours per year.[2]
Definition and measurement
Given the theoretical maximum of daytime duration for a given location, there is also a practical consideration at which point the amount of daylight is sufficient to be treated as a "sunshine hour". "Bright" sunshine hours represent the total hours when the sunlight is stronger than a specified threshold, as opposed to just "visible" hours. "Visible" sunshine, for example, occurs around sunrise and sunset, but is not strong enough to excite a standardized sensor. Measurement is performed by instruments called
In order to harmonize the data measured worldwide, in 1962 the World Meteorological Organization (WMO) defined a standardized design of the Campbell–Stokes recorder, called an Interim Reference Sunshine Recorder (IRSR).[1] In 2003, the sunshine duration was finally defined as the period during which direct solar irradiance exceeds a threshold value of 120 W/m2.[1]
Geographic distribution
Sunshine duration follows a general geographic pattern:
The two major areas with the highest sunshine duration, measured as annual average, are the central and the eastern
Conversely, higher latitudes (above 50° north/south) lying in stormy westerlies have much cloudier and more unstable and
See also
References
- ^ a b c d "8. Measurement of Sunshine Duration" (PDF), Guide to Meteorological Instruments and Methods of Observation, WMO, 2008, archived from the original on 2013-02-03
- ^ Gerhard Holtkamp, The Sunniest and Darkest Places on Earth, Scilogs, archived from the original on 2009-10-27
- ^ Definitions for other daily elements, Australian Bureau of Meteorology
- ^ a b Sunniest places in the world, Current Results.com
- NOAA, 2004
- ISBN 9782200246044
- ^ Antarctic climatic data, archived from the original on 2008-05-07
- ^ Cloudiest places in the United States, Current Results.com
- ISBN 9783540187684
External links
- UNdata portal : World Meteorological Organization Standard Normals : Sunshine
- Map of sunshine hours of Australia
- Sunshine duration world map (June snapshot)
- Sunshine duration world map Archived 2019-06-26 at the Wayback Machine (Yearly average)