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 indicated general sunniness of a location compared to other places, while the latter allows for comparison of sunshine in various seasons on 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 still used to promote tourist destinations.
The naive expectation is that, for every place on Earth, the Sun will appear to be above the horizon for exactly half the time. Thus, for a standard year consisting of 8760 hours, apparent maximal daytime duration would be 4380 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 4647 hours, while the North Pole receives 4575. Because of elliptic nature of the Earth's orbit, the Southern Hemisphere is not symmetrical: Antarctic Circle at 4530 hours receives 5 days less of sunshine than its antipodes. The Equator has the total daytime of 4422 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 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. 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). In 2003, the sunshine duration was finally defined as the period during which direct solar irradiance exceeds a threshold value of 120 W/m².
Since sunshine duration depends chiefly on concentration of clouds and fog in the observed area, locations with arid climate naturally correlate with high sunshine duration values, and, conversely, lowest values of sunshine duration are encountered in areas with wet, west-coast oceanic climate.
The two areas with highest sunshine duration in the World, measured as annual average, are southwestern United States (Arizona and Nevada) and northeastern Africa (Egypt, Sudan and Chad). The city claiming the title of the sunniest in the World is Yuma, Arizona, with over 4,000 hours (90% of time) of bright sunshine annually. The sunniest month in the World is May in the base of Eureka in the far north of Canada, which receives 16.5 hours of bright sun daily.
The areas with lowest sunshine duration are more difficult to determine, since concentration of humidity in the air may highly vary locally. For example, narrow valleys and ravines often retain ground fog even when the surrounding area has clear skies, while prominent mountains often create clouds because of orographic lift. Also, high mountains may keep relatively small parts of terrain in a shadow for a long part of the day.
The cloudiest place in the United States is probably Cold Bay, Alaska, with an average of 304 days of heavy overcast (covering over 3/4 of the sky). The foggiest place in the World are the Grand Banks on the easternmost tip of Newfoundland Island, Canada, due to mixing of cold and warm ocean currents.
- "8. Measurement of Sunshine Duration", Guide to Meteorological Instruments and Methods of Observation, WMO, 2008
- Gerhard Holtkamp, The Sunniest and Darkest Places on Earth, Scilogs
- Definitions for other daily elements, Australian Bureau of Meteorology
- Sunniest places in the world, Current Results.com
- Ranking of cities based on % annual possible sunshine, NOAA, 2004
- Cloudiest places in the United States, Current Results.com