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Updated : 16/10/2016

GHowSAW Weather Sunshine

Introduction

How many minutes of sunshine did we have today? Almost everyone enjoys sunshine, apart from the beneficial health effects as a source of vitamin D and the synchronisation of our internal circadian clocks, there are notable phsychological benefits as well. Most people prefer a bright sunny day to one which is grey and overcast and a lot of people chooose vacation locations and dates based on an anticipation of exstensive sunshine.

Sunshine duration is the length of time that the ground surface is irradiated by direct solar radiation (i.e., sunlight reaching the earth's surface directly from the sun). In 2003, WMO defined sunshine duration as the period during which direct solar irradiance exceeds a threshold value of 120 watts per square meter (W/m2). This value is equivalent to the level of solar irradiance shortly after sunrise or shortly before sunset in cloud-free conditions.

Reporting

Sunshine is generally reported as hours of sunshine per day. However, with the extent of data recorded by the radiation monitoring system and statistical analysis it is possible to provide much more detail such as the percentage of sunshine hours between sunrise and sunset, the maximum period of sunshine, frequency of interuptions to direct sunshine etc.

Sensors

Meteorological stations measure the amount of time that the Sun shines with an electronic sensor, but until very recently a Campbell-Stokes sunshine recorder did the job. This instrument focused the Sun's rays through a spherical glass lens and on to a piece of card, scorching a line on to the "burn card" every time the Sun emerged from behind the clouds. Sunshine hours correlated to the length of the burn. Campbell-Stokes sunshine recorders and Jordan sunshine recorders have long been used as instruments to measure sunshine duration, and are advantageous in that they have no moving parts and require no electric power. Their disadvantages are that the characteristics of the recording paper or photosensitized paper used in them affect measurement accuracy, differences between observers may arise in determining the occurrence of sunshine, and the recording paper must be replaced after sunset.

As sunshine is defined quantitatively at present, a variety of photoelectric sunshine recorders have been developed and are used in place of these instruments. As the threshold value for the occurrence of sunshine is defined in terms of direct solar irradiance, it is also possible to observe sunshine duration with a pyranometer or pyrheliometer.

Whilst the WHO definition of sunshine may be fine for academic purposes the lay definition is more likely to be one of 'direct sunlight uninterrupted by clouds'. Using such a lay definition I arbitrarily decided to define bright sunshine as a solar radiation or illuminance level in excess of 80 per cent of the predicted clear sky radiation for a particular location and time. Therefore, with a suitable model for the calculation of clear sky radiation such as the Bird and Hulstrum Model, whether or not the sun is shining brightly can be determined by a simple comparison of the current radiation level with that predicted for a clear sky conditions.

Since the GHowSAW system already includes a radiation monitoring system an interpretation of the data will provide the sunshine information

Methodology

With a model of the daily solar irradiation such as Bird and Hulstrom's model from the publication "A Simplified Clear Sky model for Direct and Diffuse Insolation on Horizontal Surfaces" by R.E. Bird and R.L Hulstrom, SERI Technical Report SERI/TR-642-761, Feb 1991. Solar Energy Research Institute, Golden, CO. used in conjunction with the NOAA JavaScript solar position calculator.(N.B. if using this javascript please note the reverse convention used for longitude and time zone) the expected solar irradiation for can be calculated. If the measured solar irradiation is more than 90 per cent of the predicted value then we may assume that there are no clouds between the sensor and the sun and a condition of bright sunshine exists.

By accumulating the periods determined to be bright sunshine we arrive at the total duration of bright sunshine. This result can then be reported as sushine hours per day or as a per centage of the daylight hours.

Sources

  • Bird and Hulstrom A Simplified Clear Sky model for Direct and Diffuse Insolation on Horizontal Surfaces by R.E. Bird and R.L Hulstrom, SERI Technical Report SERI/TR-642-761, Feb 1991. Solar Energy Research Institute, Golden, CO.
  • NOAA JavaScript solar position calculator.
  • Solrad.zip MS EXCEL™ VBA version of the NOAA calculator by Greg Pelletier of the Washngton Department of Ecology
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