Quoted from:F. Ahmad and I. Ulfat, “Empirical models for the correlation of monthly average daily global solar radiation with hours of sunshine on a horizontal surface at Karachi, Pakistan,” Turkish Journal of Physics, vol. 28, no. 5, pp. 301–307, 2004.https://journals.tubitak.gov.tr/physics/issues/fiz-04-28-5/fiz-28-5-4-0404-6.pdf
A new set of constants for Angstrom-type correlation of first and second order, to estimate monthly average daily global solar radiation, has been obtained employing sunshine hours data recorded at Karachi,Pakistan (Lat. 24°54’N, Long. 67°08’E). Least square regression is performed to derive these constants.The correlation equations developed are employed to calculate the monthly average daily global solarradiation. These results are then compared with various other existing correlations and the measureddata. Excellent agreement has been found between the estimated and the measured values.
INTRODUCTION
Sunshine-based models use only bright sunshine hours as input parameter while others use additional climatological data together with bright sunshine hours. In some of the models geographical and seasonal parameters are also taken into account to reflflect the latitudinal and seasonal variation of the air mass.
lar radiation was proposed by Angstrom. The Angstrom correlation was modifified by Prescott and Page. Many researchers have employed hours of bright sunshine to estimate solar radiation. Other workers, e.g. Reddy, Sayyigh, Glover and McCullouch, derived their equations by using sunshine duration, relative humidity, temperature and latitude of the locations under study. Reddy suggested the use of the number of rainy days, sunshine hours and a factor which depends on the geographical location of the place along with the latitude. Barbaro related daily total solar radiation to the sunshine duration and the noon height of the sun on the 15th of the given month.
The object of the work reported in this paper is the development of Angstrom-type polynomials of fifirst and second order for the estimation of monthly average daily global solar radiation on horizontal surfaces at Karachi, Pakistan. The coeffiffifficients for developing these polynomials are derived by using least square regression analysis. These coeffiffifficients are generally valid for estimating the radiation in location of similar climate, latitude and altitude.
Ahmad and Ulfat also suggested the linear regression constants for Karachi, Pakistan:
\( 𝐻/𝐻_0 = 0.324 + 0.405 𝑆/𝑆_0 . \)
Ahmad and Ulfat also suggested the quadratic equation for Karachi, Pakistan:
\( H/H_0 = 0.348 + 0.320 S/S_0+ 0.070(S/S_0)^2. \)