Sunshine duration

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 average of 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 general sunniness of a location compared to other places, while the latter allows for comparison of sunshine in various seasons on 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[edit]

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. However, there are physical and astronomical effects which change that picture. Namely, atmospheric refraction allows the Sun to be still visible even when it physically sets below the horizon line. 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 of 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: Antarctic Circle at 4,530 hours receives 5 per samy days less of sunshine than its antipodes. The Equator has the total daytime of 4,422 hours per year.^[2]

Definition and measurement[edit]

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[edit]

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: dry areas in the subtropical latitudes (about 25° to 40° north/south) have the highest sunshine values, as these are the locations 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 subtropical highs, which bring very high atmospheric stability, little convective overturning, and very low 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 sky is clear in these regions and fair weather is virtually perpetual. The descending branch of the Hadley cell and the long-term lack of atmospheric disturbances helps to explain the seemingly endless supply of sunny, cloudless days in the deserts. Low clouding conditions are usually associated with rainfall shortage, as seen in these dry regions.

Conversely, higher latitudes (above 50° north/south) which are in the stormy westerlies have much cloudier and unstable weather, and often have the lowest values of sunshine duration annually. Temperate oceanic climates like Northwestern Europe, the western Pacific coast of Canada, and areas of New Zealand's South Island are examples of cool, cloudy, wet climates where sunshine duration values are very low.

The two major areas with the highest sunshine duration in the world, measured as annual average, are the central and the eastern part of the Sahara Desert, covering a lot of vast mainly desert countries such as Egypt, Sudan, Libya, Chad, Niger, Algeria as well as 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 h (about 91% of time) of bright sunshine annually^[4]^[5] but many climatological books suggest there may be sunnier areas in North Africa. In fact, some desert cities such as Aswan and Luxor, Egypt have even sunnier weather than Yuma. Some years may pass without seeing any cloudy day at all in both these bone dry cities. The central and the eastern part of the Great Desert receive over 3,600 h (about 82% of the time) of bright sunshine annually amounting to nearly 4,000 h where the borders of Algeria, Libya, Egypt, Sudan, Chad and Niger meet.^[6] In the belt encompassing northern Chad with the Tibesti Mountains, northern Sudan, southern Libya and Upper Egypt, the sunshine duration easily surpasses 4,000 h and this is the sunniest vast region in the world. Percentages of sunshine soar up to 95% in these sun-drenched deserts. Also some parts in the interior of the Arabian Peninsula are said to receive 3,600–3,800 h of bright sunshine annually, that's to say 82–87% of the time.^[6] In fact, only deserts, hot and dry regions can claim the sunniest places on Earth and only deserts, except a very few places having a Mediterranean climate, can record sunshine durations over 3,500 h per year.

The sunniest month in the world is December in Eastern Antarctica, which receives almost 23 hours of bright sun daily.^[7] The two sunniest major cities in the United States are Phoenix, Arizona and Las Vegas, Nevada and both have a desert climate, of course, as they are respectively located in the Sonoran Desert and in the Mojave Desert. These two desert cities are said to bake in bright sunshine during more than 3,800 h a year, say about 87% of the daylight hours.

The areas with lowest sunshine duration annually lie mostly over the polar oceans, as well as parts of northern Europe, 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]