Sunshine duration

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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.

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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 the 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

Annual sunshine hours map of the world. Note that the data for Antarctica is accurate only for the coast. Parts of the interior receive more than 3600 hours of annual sunshine.
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< 1200 h
1200-1600 h
1600-2000 h
2000-2400 h
2400-3000 h
3000-3600 h
3600-4000 h
> 4000 h Sunshine.png
Annual sunshine hours map of the world. Note that the data for Antarctica is accurate only for the coast. Parts of the interior receive more than 3600 hours of annual sunshine.
  < 1200 h
  1200–1600 h
  1600–2000 h
  2000–2400 h
  2400–3000 h
  3000–3600 h
  3600–4000 h
  > 4000 h

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). [5] 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, [5] [6] 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. [7] 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. [8]

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). [9] 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. [10] Tórshavn in the Faroe Islands is among the cloudiest places in the world with yearly only 840 sunshine hours.

See also

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Cloud cover


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

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Arabian Desert desert located in Western Asia

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

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

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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.

2006 European heat wave

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.

Nancheng County County in Jiangxi, Peoples Republic of China

Nancheng is a county of eastern Jiangxi province, People's Republic of China. It is under the jurisdiction of the prefecture-level city of Fuzhou. Historically it has been known as Jianchangfu (Kienchang) (建昌府).

Guiping County-level city in Guangxi, Peoples Republic of China

Guiping is a county-level city in eastern Guangxi, China. It is under the administration of Guigang City, located at the confluence of the Qian and Yu rivers, which are the Xi River's primary north and south tributaries, respectively.

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.

Barcelona and its metropolitan area has a Mediterranean climate, with mild winters and hot summers. According to Troll-Paffen climate classification, Barcelona has a warm-temperate subtropical climate. According to Siegmund/Frankenberg climate classification, Barcelona has a subtropical climate. Barcelona is located on the eastern coast of the Iberian Peninsula, so Atlantic lows often arrive in Barcelona with low humidity, producing little or no rain. The proximity of the Mediterranean Sea and the relief, are the reasons why the summers are not as dry as in many other Mediterranean Basin locations.

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.

Valencia and its metropolitan area have a hot-summer Mediterranean climate with very mild winters and long warm to hot summers. According to Troll-Paffen climate classification, Valencia has a warm-temperate subtropical climate and according to Siegmund/Frankenberg climate classification, Valencia has subtropical climate.

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 has a hot dry subtropical or a typical Mediterranean climate, with short and very mild winters and warm 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.

Rome and its metropolitan area has a Mediterranean climate, with mild winters and warm to hot summers. According to Troll-Paffen climate classification, Rome has a warm-temperate subtropical climate. According to Siegmund/Frankenberg climate classification, Rome 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.

References

  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. https://www.climatestotravel.com/climate/antarctica#optical_phenomena
  5. 1 2 Sunniest places in the world, Current Results.com
  6. Ranking of cities based on % annual possible sunshine, NOAA, 2004
  7. Godard, Alain; Tabeaud, Martine (2009), Les climats: Mécanismes, variabilité et répartition (in French), Armand Colin, ISBN   9782200246044
  8. Antarctic climatic data, archived from the original on 2008-05-07
  9. Cloudiest places in the United States, Current Results.com
  10. Domrös, Manfred; Peng, Gongbing, The Climate of China, pp. 75–78, ISBN   9783540187684