Sunshine duration or sunshine hours is a
climatological
Climatology (from Greek , ''klima'', "place, zone"; and , ''-logia'') or climate science is the scientific study of Earth's climate, typically defined as weather conditions averaged over a period of at least 30 years. This modern field of study ...
indicator, measuring duration of
sunshine in given period (usually, a day or a year) for a given location on
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
, typically expressed as an averaged value over several years. It is a general indicator of
cloudiness of a location, and thus differs from
insolation, which measures the total
energy
In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of hea ...
delivered by sunlight over a given period.
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.
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.
[
]
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
Atmospheric refraction is the deviation of light or other electromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of height. This refraction is due to the velocity of ligh ...
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
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where the Earth's rotation, Earth's axis of rotation meets its surface. It is called the True North Pole to distingu ...
receives 4,575. Because of elliptic nature of the Earth's orbit, the Southern Hemisphere is not symmetrical: the Antarctic Circle
The Antarctic Circle is the most southerly of the five major circles of latitude that mark maps of Earth. The region south of this circle is known as the Antarctic, and the zone immediately to the north is called the Southern Temperate Zone. So ...
, 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.
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 sunshine recorders. For the specific purpose of sunshine duration recording, Campbell–Stokes recorders are used, which use a spherical glass lens
A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements ...
to focus the sun rays on a specially designed tape. When the intensity exceeds a pre-determined threshold, the tape burns. The total length of the burn trace is proportional to the number of bright hours. Another type of recorder is the Jordan sunshine recorder. Newer, electronic recorders have more stable sensitivity than that of the paper tape.
In order to harmonize the data measured worldwide, in 1962 the World Meteorological Organization
The World Meteorological Organization (WMO) is a specialized agency of the United Nations responsible for promoting international cooperation on atmospheric science, climatology, hydrology and geophysics.
The WMO originated from the Intern ...
(WMO) defined a standardized design of the Campbell–Stokes recorder, called an Interim Reference Sunshine Recorder (IRSR).[ In 2003, the sunshine duration was finally defined as the period during which direct solar irradiance exceeds a threshold value of 120 W/m2.][
]
Geographic distribution
Sunshine duration follows a general geographic pattern: subtropical latitudes (about 25° to 40° north/south) have the highest sunshine values, because these are the locations of the eastern sides of the subtropical high pressure systems, associated with the large-scale descent of air from the upper-level tropopause. Many of the world's driest climates are found adjacent to the eastern sides of the subtropical highs, which create stable atmospheric conditions, little convective overturning, and little moisture and cloud cover. Desert regions, with nearly constant high pressure aloft and rare condensation—like North Africa
North Africa, or Northern Africa is a region encompassing the northern portion of the African continent. There is no singularly accepted scope for the region, and it is sometimes defined as stretching from the Atlantic shores of Mauritania in ...
, the Southwestern United States, Western Australia
Western Australia (commonly abbreviated as WA) is a state of Australia occupying the western percent of the land area of Australia excluding external territories. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to th ...
, and the Middle East
The Middle East ( ar, الشرق الأوسط, ISO 233: ) is a geopolitical region commonly encompassing Arabia (including the Arabian Peninsula and Bahrain), Asia Minor (Asian part of Turkey except Hatay Province), East Thrace (Europ ...
—are examples of hot, sunny, dry climates where sunshine duration values are very high.
The two major areas with the highest sunshine duration, measured as annual average, are the central and the eastern Sahara Desert
, photo = Sahara real color.jpg
, photo_caption = The Sahara taken by Apollo 17 astronauts, 1972
, map =
, map_image =
, location =
, country =
, country1 =
, ...
—covering vast, mainly desert countries such as Egypt
Egypt ( ar, مصر , ), officially the Arab Republic of Egypt, is a transcontinental country spanning the northeast corner of Africa and southwest corner of Asia via a land bridge formed by the Sinai Peninsula. It is bordered by the Medit ...
, Sudan, Libya
Libya (; ar, ليبيا, Lībiyā), officially the State of Libya ( ar, دولة ليبيا, Dawlat Lībiyā), is a country in the Maghreb region in North Africa. It is bordered by the Mediterranean Sea to the north, Egypt to the east, Suda ...
, Chad, and Niger
)
, official_languages =
, languages_type = National languages[Arizona
Arizona ( ; nv, Hoozdo Hahoodzo ; ood, Alĭ ṣonak ) is a state in the Southwestern United States. It is the 6th largest and the 14th most populous of the 50 states. Its capital and largest city is Phoenix. Arizona is part of the Fou ...](_blank)
, California
California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...
, Nevada
Nevada ( ; ) is a state in the Western region of the United States. It is bordered by Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. Nevada is the 7th-most extensive, ...
). The city claiming the official title of the sunniest in the world is Yuma, Arizona
Yuma ( coc, Yuum) is a city in and the county seat of Yuma County, Arizona, United States. The city's population was 93,064 at the 2010 census, up from the 2000 census population of 77,515.
Yuma is the principal city of the Yuma, Arizona, M ...
, with over 4,000 hours (about 91% of daylight time) of bright sunshine annually,[ but many climatological books suggest there may be sunnier areas in North Africa. In the belt encompassing northern Chad and the ]Tibesti Mountains
The Tibesti Mountains are a mountain range in the central Sahara, primarily located in the extreme north of Chad, with a small portion located in southern Libya. The highest peak in the range, Emi Koussi, lies to the south at a height of and i ...
, northern Sudan, southern Libya, and Upper Egypt
Upper Egypt ( ar, صعيد مصر ', shortened to , , locally: ; ) is the southern portion of Egypt and is composed of the lands on both sides of the Nile that extend upriver from Lower Egypt in the north to Nubia in the south.
In ancient E ...
, annual sunshine duration is estimated at over 4,000 hours. There is also a smaller, isolated area of sunshine maximum in the heart of the western section of the Sahara Desert around the Eglab Massif and the Erg Chech
The Erg Chech ( ar, عرق شاش), is a large erg in southwestern Algeria and northern Mali.
Geography
It is an almost uninhabited part of the greater Sahara Desert, an inhospitable desert region with long, extremely hot summers and short, very w ...
, along the borders of Algeria
)
, image_map = Algeria (centered orthographic projection).svg
, map_caption =
, image_map2 =
, capital = Algiers
, coordinates =
, largest_city = capital
, relig ...
, Mauritania, and Mali
Mali (; ), officially the Republic of Mali,, , ff, 𞤈𞤫𞤲𞥆𞤣𞤢𞥄𞤲𞤣𞤭 𞤃𞤢𞥄𞤤𞤭, Renndaandi Maali, italics=no, ar, جمهورية مالي, Jumhūriyyāt Mālī is a landlocked country in West Africa. Mal ...
where the 4,000-hour mark is exceeded, too. Some places in the interior of the Arabian Peninsula receive 3,600–3,800 hours of bright sunshine annually. The largest sun-baked region in the world (over 3,000 hours of yearly sunshine) is North Africa. The sunniest month in the world is December in Eastern Antarctica, with almost 23 hours of bright sun daily.
Conversely, higher latitudes (above 50° north/south) lying in stormy westerlies have much cloudier and more unstable and rain
Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water ...
y weather, and often have the lowest values of sunshine duration annually. Temperate oceanic climates like those in northwestern Europe
Northwestern Europe, or Northwest Europe, is a loosely defined subregion of Europe, overlapping Northern and Western Europe. The region can be defined both geographically and ethnographically.
Geographic definitions
Geographically, North ...
, the northwestern coast of Canada
Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...
, and areas of New Zealand's South Island are examples of cool, cloudy, wet, humid climates where cloudless sunshine duration values are very low. The areas with the lowest sunshine duration annually lie mostly over the polar oceans, as well as parts of northern Europe, southern Alaska, northern Russia, and areas near the Sea of Okhotsk. The cloudiest place in the United States is Cold Bay, Alaska
Cold Bay ( ale, Udaamagax,; Sugpiaq: ''Pualu'') is a city in Aleutians East Borough, Alaska, United States. As of the 2010 census, the population was 108, but at the 2020 census this had reduced to 50.
Cold Bay is one of the main commercial ...
, with an average of 304 days of heavy overcast (covering over 3/4 of the sky). In addition to these polar oceanic climates, certain low-latitude basins enclosed by mountains, like the Sichuan
Sichuan (; zh, c=, labels=no, ; zh, p=Sìchuān; alternatively romanized as Szechuan or Szechwan; formerly also referred to as "West China" or "Western China" by Protestant missions) is a province in Southwest China occupying most of the ...
and Taipei Basin
The Taipei Basin () is a geographic region in northern Taiwan. It is the largest basin in Taiwan. The basin is bounded by the Yangmingshan to the north, the Linkou Plateau to the west, and the Ridge of Xueshan Range to the southeast. The shape ...
s, can have sunshine duration as low as 1,000 hours per year, as cool air consistently sinks to form fogs that winds cannot dissipate. Tórshavn
Tórshavn (; lit. "Thor's harbour"), usually locally referred to as simply ''Havn'', is the capital and largest city of the Faroe Islands. It is located in the southern part on the east coast of Streymoy. To the northwest of the city lies the ...
in the Faroe Islands
The Faroe Islands ( ), or simply the Faroes ( fo, Føroyar ; da, Færøerne ), are a North Atlantic island group and an autonomous territory of the Kingdom of Denmark.
They are located north-northwest of Scotland, and about halfway bet ...
is among the cloudiest places in the world with yearly only 840 sunshine hours. Campbell Island / Motu Ihupuku
Campbell Island / Motu Ihupuku is an uninhabited subantarctic island of New Zealand, and the main island of the Campbell Island group. It covers of the group's , and is surrounded by numerous stacks, rocks and islets like Dent Island, Foll ...
south of New Zealand is even cloudier, with only 647 annual sunshine hours, while Bellingshausen Station
Bellingshausen Station (Russian: станция Беллинсгаузен) is a Russian (formerly Soviet) Antarctic station at Collins Harbour, on King George Island of the South Shetland Islands. It was one of the first research stations fou ...
off the Antarctic Peninsula gets only 591 annual sunshine hours. Also Bear Island (Norway)
Bear Island ( no, Bjørnøya, ) is the southernmost island of the Norway, Norwegian Svalbard, Svalbard archipelago. The island is located in the western part of the Barents Sea, approximately halfway between Spitsbergen and the North Cape, Norwa ...
in the Svalbard Archipelago gets only 595 annual sunshine hours.
See also
* List of cities by sunshine duration
The following is a list of cities by sunshine duration. Sunshine duration is a climatological indicator, measuring duration of sunshine in given period (usually, a day or a year) for a given location on Earth, typically expressed as an average ...
References
External links
UNdata portal : World Meteorological Organization Standard Normals : Sunshine
Map of sunshine hours of Australia
Sunshine duration world map
(June snapshot)
{{Webarchive, url=https://web.archive.org/web/20190626210003/http://earth.rice.edu/mtpe/geo/geosphere/hot/energyfuture/Sunlight.html , date=2019-06-26 (Yearly average)
Atmospheric radiation
Climate and weather statistics