Sunshine duration

Last updated

Sunshine duration or sunshine hours 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 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 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. [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

Diagram showing displacement of the Sun's image at sunrise and sunset Refraccion.png
Diagram showing displacement of the Sun's image at sunrise and sunset

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

Campbell-Stokes recorder measures sunshine Campbell-Stokes recorder.jpg
Campbell–Stokes recorder measures sunshine

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 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. [3] 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 (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

Mean cloud cover map of the world (from July 2002 to April 2015) Globalcldfr amo 200207-201504 lrg.jpg
Mean cloud cover map of the world (from July 2002 to April 2015)

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, the Southwestern United States, Western Australia, and the Middle East—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—covering vast, mainly desert countries such as Egypt, Sudan, Libya, Chad, and Niger—and the Southwestern United States (Arizona, California, Nevada). [4] The city claiming the official title of the sunniest in the world is Yuma, Arizona, with over 4,000 hours (about 91% of daylight time) of bright sunshine annually, [4] [5] but many climatological books suggest there may be sunnier areas in North Africa.[ citation needed ] In the belt encompassing northern Chad and the Tibesti Mountains, northern Sudan, southern Libya, and Upper Egypt, 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, along the borders of Algeria, Mauritania, and Mali where the 4,000-hour mark is exceeded, too. [6] 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. [7]

Conversely, higher latitudes (above 50° north/south) lying in stormy westerlies have much cloudier and more unstable and rainy weather, and often have the lowest values of sunshine duration annually. Temperate oceanic climates like those in northwestern Europe, the northwestern coast of Canada, 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, with an average of 304 days of heavy overcast (covering over 3/4 of the sky). [8] In addition to these polar oceanic climates, certain low-latitude basins enclosed by mountains, like the Sichuan and Taipei Basins, can have sunshine duration as low as 1,000 hours per year, as cool air consistently sinks to form fogs that winds cannot dissipate. [9] Tórshavn in the Faroe Islands is among the cloudiest places in the world with yearly only 840 sunshine hours. Campbell Island / Motu Ihupuku south of New Zealand is even cloudier, with only 647 annual sunshine hours, while Bellingshausen Station off the Antarctic Peninsula gets only 591 annual sunshine hours. Also Bear Island (Norway) in the Svalbard Archipelago gets only 595 annual sunshine hours.

See also

Related Research Articles

Climate of the United Kingdom Overview of the climate of the United Kingdom

The United Kingdom straddles the higher mid-latitudes between 49° and 61°N on the western seaboard of Europe. Since the UK is always in or close to the path of the polar front jet stream, frequent changes in pressure and unsettled weather are typical. Many types of weather can be experienced in a single day. In general the climate of the UK is cool and often cloudy and rainy. High temperatures are infrequent.

Cloud cover Fraction of the sky obscured by clouds when observed from a particular location

Cloud cover refers to the fraction of the sky obscured by clouds when observed from a particular location. Okta is the usual unit of measurement of the cloud cover. The cloud cover is correlated to the sunshine duration as the least cloudy locales are the sunniest ones while the cloudiest areas are the least sunny places.

Desert climate Arid climate subtype in the Köppen climate classification system

The desert climate or arid climate, is a climate in which there is an excess of evaporation over precipitation. The typically bald, rocky, or sandy surfaces in desert climates hold little moisture and evaporate the little rainfall they receive. Covering 14.2% of earth's land area, hot deserts are the second most common type of climate on earth after polar climate.

Vostok Station Russian research station in Antarctica

Vostok Station is a Russian research station in inland Princess Elizabeth Land, Antarctica. Founded by the Soviet Union in 1957, the station lies at the southern Pole of Cold, with the lowest reliably measured natural temperature on Earth of −89.2 °C. Research includes ice core drilling and magnetometry. Vostok was named after Vostok, the lead ship of the First Russian Antarctic Expedition captained by Fabian von Bellingshausen. The Bellingshausen Station was named after this captain..

Oceanic climate Climate classification

An oceanic climate, also known as a maritime climate or marine climate, is the Köppen classification of climate typical of west coasts in higher middle latitudes of continents, generally featuring mild summers and cool but not cold winters, with a relatively narrow annual temperature range and few extremes of temperature. Oceanic climates can be found in both temperate and subtropical areas, notably in Western Europe, parts of central and Southern Africa, North America, South America, parts of Asia, and as well as parts of Australia and New Zealand.

Shaoguan Prefecture-level city in Guangdong, Peoples Republic of China

Shaoguan is a prefecture-level city in northern Guangdong Province (Yuebei), South China, bordering Hunan to the northwest and Jiangxi to the northeast. It is home to the mummified remains of the sixth Zen Buddhist patriarch Huineng. Its built-up area made up of Zhenjiang, Wujiang and Qujiang urban conurbated districts was home to 1,028,460 inhabitants as of the 2020 census.

Qinzhou Prefecture-level city in Guangxi, Peoples Republic of China

Qinzhou is a prefecture-level city in Guangxi, China, lying on the Gulf of Tonkin and having a total population of 3,304,400 as of 12 31 2018 estimation whom 1,296,300 lived in the built-up area made of Qinbei and Qinnan urban Districts.

Orcadas Base Antarctic base

Base Orcadas is an Argentine scientific station in Antarctica, and the oldest of the stations in Antarctica still in operation. It is located on Laurie Island, one of the South Orkney Islands, at 4 meters (13 ft) above sea level and 170 meters (558 ft) from the coastline. Established by the Scottish National Antarctic Expedition in 1903 and transferred to the Argentine government in 1904, the base has been permanently populated since, being one of six Argentine permanent bases in Argentina's claim to Antarctica, and the first permanently inhabited base in Antarctica.

Wuzhou Prefecture-level city in Guangxi, Peoples Republic of China

Wuzhou, formerly Ngchow, is a prefecture-level city in the east of Guangxi Zhuang Autonomous Region, People's Republic of China.

Campbell–Stokes recorder

The Campbell–Stokes recorder is a type of sunshine recorder. It was invented by John Francis Campbell in 1853 and modified in 1879 by Sir George Gabriel Stokes. The original design by Campbell consisted of a glass sphere set into a wooden bowl with the sun burning a trace on the bowl. Stokes's refinement was to make the housing out of metal and to have a card holder set behind the sphere.

Longzhou County County in Guangxi, Peoples Republic of China

Longzhou County(Chinese: t龍州,s龙州,pLóngzhōu Xiàn,wLung-chou Hsien; Zhuang: Lungzcouh Yen) is a county of southwestern Guangxi, China, bordering Cao Bằng province, Vietnam. It is under the jurisdiction of the prefecture-level city of Chongzuo.

2006 European heat wave Heat wave in Europe

The 2006 European heat wave was a period of exceptionally hot weather that arrived at the end of June 2006 in certain European countries. The United Kingdom, France, Belgium, the Netherlands, Luxembourg, Italy, Poland, the Czech Republic, Hungary, Germany and western parts of Russia were most affected. Several records were broken. In the Netherlands, Belgium, Germany, Ireland and the United Kingdom, July 2006 was the warmest month since official measurements began.

Climate of Ireland Climate of Ireland

The climate of Ireland is mild, humid and changeable with abundant rainfall and a lack of temperature extremes. Ireland's climate is defined as a temperate oceanic climate, or Cfb on the Köppen climate classification system, a classification it shares with most of northwest Europe. The country receives generally warm summers and cool winters.

Climate of Egypt

Egypt essentially has a hot desert climate. The climate is generally extremely dry all over the country except on the northern Mediterranean coast which receives rainfall in winter. In addition to rarity of rain, extreme heat during summer months is also a general climate feature of Egypt although daytime temperatures are more moderated along the northern coast.

Climate of Barcelona

Barcelona has a maritime Mediterranean climate Csa according to Köppen-Geiger classification, a warm-temperate subtropical climate according to Troll-Paffen climate classification, and a subtropical climate according to Siegmund/Frankenberg climate classification.

Solar power in Africa

Africa is often considered and referred as the "Sun continent" or the continent where the Sun's influence is the greatest. According to the "World Sunshine Map", Africa receives many more hours of bright sunshine during the course of the year than any other continent of the Earth: many of the sunniest places on the planet lie there.

Bilbao and its metropolitan area has an oceanic climate according to the Köppen climate classification with mild winters and warm summers. According to the Troll-Paffen climate classification, Bilbao has a temperate climate and according to the Siegmund/Frankenberg climate classification, Bilbao has a subtropical climate. According to the European Environment Agency, Bilbao lies within the Atlantic biogeographical region. The climate of Bilbao and the rest of the north-western part of Spain is different from the rest of the country, characterized by a higher amount of rainfall and precipitation days, fewer sunshine hours and mild temperatures, in summer comparable to northern half of Europe with temperate climate.

Lisbon and its metropolitan area feature a mild subtropical or a Hot-summer Mediterranean climate, with short and mild winters and warm to hot summers. According to Troll-Paffen climate classification, Lisbon has a warm-temperate subtropical climate. According to Siegmund/Frankenberg climate classification, Lisbon has a subtropical climate.

Climate of Melbourne

Melbourne, the state capital of Victoria and second largest city in Australia, has a temperate oceanic climate and is well known for its changeable weather conditions. This is mainly due to Melbourne's geographical location. This temperature differential is most pronounced in the spring and summer months and can cause strong cold fronts to form. These cold fronts can be responsible for all sorts of severe weather from gales to severe thunderstorms and hail, minor temperature drops, and heavy rain. The city experiences little humidity in summer, except at the end of hot spells following thunderstorms and rain.


  1. 1 2 3 4 "8. Measurement of Sunshine Duration" (PDF), Guide to Meteorological Instruments and Methods of Observation, WMO, 2008, archived from the original (PDF) on 2013-02-03
  2. Gerhard Holtkamp, The Sunniest and Darkest Places on Earth, Scilogs, archived from the original on 2009-10-27
  3. Definitions for other daily elements, Australian Bureau of Meteorology
  4. 1 2 Sunniest places in the world, Current
  5. Ranking of cities based on % annual possible sunshine, NOAA, 2004
  6. Godard, Alain; Tabeaud, Martine (2009), Les climats: Mécanismes, variabilité et répartition (in French), Armand Colin, ISBN   9782200246044
  7. Antarctic climatic data, archived from the original on 2008-05-07
  8. Cloudiest places in the United States, Current
  9. Domrös, Manfred; Peng, Gongbing, The Climate of China, pp. 75–78, ISBN   9783540187684