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.

Cloud visible mass of liquid droplets or frozen crystals suspended in the atmosphere

In meteorology, a cloud is an aerosol consisting of a visible mass of minute liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body. Water or various other chemicals may compose the droplets and crystals. On Earth, clouds are formed as a result of saturation of the air when it is cooled to its dew point, or when it gains sufficient moisture from an adjacent source to raise the dew point to the ambient temperature. They are seen in the Earth's homosphere. Nephology is the science of clouds, which is undertaken in the cloud physics branch of meteorology.

Energy quantitative physical property transferred to objects to perform heating or work on them

In physics, energy is the quantitative property that must be transferred to an object in order to perform work on, or to heat, the object. Energy is a conserved quantity; the law of conservation of energy states that energy can be converted in form, but not created or destroyed. The SI unit of energy is the joule, which is the energy transferred to an object by the work of moving it a distance of 1 metre against a force of 1 newton.


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]

Atmospheric refraction deviation of light or other electromagnetic wave from a straight line as it passes through the atmosphere

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 light through air, decreasing with increased density. Atmospheric refraction near the ground produces mirages. Such refraction can also raise or lower, or stretch or shorten, the images of distant objects without involving mirages. Turbulent air can make distant objects appear to twinkle or shimmer. The term also applies to the refraction of sound. Atmospheric refraction is considered in measuring the position of both celestial and terrestrial objects.

Horizon apparent line that separates earth from sky

The horizon or skyline is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. The true horizon is actually a theoretical line, which can only be observed when it lies on the sea surface. At many locations, this line is obscured by land, trees, buildings, mountains, etc., and the resulting intersection of earth and sky is called the visible horizon. When looking at a sea from a shore, the part of the sea closest to the horizon is called the offing.

Arctic Circle Boundary of the Arctic

The Arctic Circle is one of the two polar circles and the most northerly of the five major circles of latitude as shown on maps of Earth. It marks the northernmost point at which the centre of the noon sun is just visible on the December solstice and the southernmost point at which the centre of the midnight sun is just visible on the June solstice. The region north of this circle is known as the Arctic, and the zone just to the south is called the Northern Temperate Zone.

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.

Campbell–Stokes recorder

The Campbell–Stokes recorder is a kind 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.

Lens (optics) optical device which transmits and refracts light

A lens is a transmissive optical device that 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), usually arranged along a common axis. Lenses are made from materials such as glass or plastic, and are ground and polished or molded to a desired shape. A lens can focus light to form an image, unlike a prism, which refracts light without focusing. Devices that similarly focus or disperse waves and radiation other than visible light are also called lenses, such as microwave lenses, electron lenses, acoustic lenses, or explosive lenses.

Jordan sunshine recorder

A Jordan sunshine recorder is a sunshine recorder in which the movement of the sun provides the occurrence of the event.

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]

World Meteorological Organization Specialised agency of the United Nations

The World Meteorological Organization (WMO) is an intergovernmental organization with a membership of 192 Member States and Territories. Its current Secretary-General is Petteri Taalas and the President of the World Meteorological Congress, its supreme body, is David Grimes. The Organization is headquartered in Geneva, Switzerland.

In radiometry, irradiance is the radiant flux (power) received by a surface per unit area. The SI unit of irradiance is the watt per square metre. The CGS unit erg per square centimetre per second is often used in astronomy. Irradiance is often called intensity because it has the same physical dimensions, but this term is avoided in radiometry where such usage leads to confusion with radiant intensity.

Geographic distribution

Annual sunshine hours map of the world
< 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
  < 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). [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 western 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.

See also

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The climate of Ireland is mild, moist 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 cool summers and mild winters. It is considerably warmer than other areas on its latitude, because it lies in the northeastern Atlantic Ocean, and as a result is warmed by the North Atlantic Current all year.

Climate of Vietnam

Vietnam's climate, being located in the tropics and strongly influenced by the South China Sea has a monsoon-influenced tropical climate typical of that of mainland Southeast Asia. In the north, the climate is monsoonal with four distinct seasons while in the south, the climate is tropical monsoon with two seasons. In addition temperate climate exists in mountainous areas, which are found in Sa Pa, Da Lat while a more continental climate exists in Lai Chau Province and Son La Province. The diverse topography, wide range of latitudes, and influences from the South China Sea lead to climatic conditions varying significantly between regions.

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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 Cyprus

Cyprus has a subtropical climate - Mediterranean and semi-arid type - according to Köppen climate classification signes Csa and BSh, with very mild winters and warm to hot summers. Snow is possible only in the Troodos mountains in the central part of the island. Rain occurs mainly in winter, with summer being generally dry.

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 west winds often arrive in Barcelona with low humidity, producing no rain. The proximity of the Atlantic, its latitude, and the relief, are the reasons why the summers are not as dry as in most 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.

Madrid and its metropolitan area has a Mediterranean climate which transitions to a cold semi-arid climate (BSk). According to Troll-Paffen climate classification, Madrid has warm-temperate subtropical climate and according to Siegmund/Frankenberg climate classification, Madrid has subtropical climate. However, the most widespread climate classification is that Madrid presents a climate of transition between the Mediterranean climate and the cold semi-arid climate (BSk), with warm summers and relatively cold winters with frequent frosts and occasional snowfalls.

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 Subtropical-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.


  1. 1 2 3 4 "8. Measurement of Sunshine Duration", Guide to Meteorological Instruments and Methods of Observation (PDF), 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