IOSR Journal of Applied Physics (IOSR-JAP) e-ISSN: 2278-4861. Volume 6, Issue 1 Ver. I (Jan. 2014), PP 63-69 www.iosrjournals.org www.iosrjournals.org 63 | Page Impact of Cloud Cover on Global UV Irradiance over Baghdad, Iraq Kais J. AL-Jumaily, Natiq A. Zaki, Ali M. AL-Salihi Department of Atmospheric Sciences College of Science, Al-Mustansiriyah University Baghdad, Iraq Abstract : Six months of continuous of hourly records of Ultraviolet irradiance recorded at Baghdad, (33.34 o N, 44.45 o E, 34 m a.m.s.l.), Iraq, were combined with concurrent synoptic cloud observations to investigate the relative influence of clouds on UV irradiance. The ratio of UV irradiance to the total solar irradiance increases with cloud cover, especially for cloud cover greater than 4 octas, In order to determine the effect of cloud on UV irradiance, a cloud modification factor was considered, it is defined as the ratio between the UV measurements and the corresponding clear sky UV irradiance that would be expected for the same time period and atmospheric conditions. It was shown that the effect of cloud with 4 octas increases demonstrating the spectral dependence of the cloud radiative extinction, while the impact of cloud cover less than 4 octas is relatively constant and ranged between (3.6 – 4.3%). Keywords : Ultraviolet Radiation, Clouds, Ozone, Baghdad, Iraq. I. INTRODUCTION The study of solar ultraviolet radiation has received considerable attention in the past few years because of its biological, ecological, and physic effects produced by short-wave radiation received at the surface of the earth. Solar UV radiation is usually divided into three bands UV-C (100–280 nm) which is completely absorbed by stratospheric ozone; UV-B (280–315 nm) which is only partially absorbed or scattered in the atmosphere and UV-A (315–400 nm) which makes up most of the UV radiation received at the earth’s surface[1-8]. In outer space, UV-B and UV-A band represent only 7.45% of the total solar radiation [9]. The stratospheric ozone forms a shield around the globe protecting the biosphere from the dangerous ultraviolet radiation coming from the sun. A slight decrease in the stratospheric ozone leads to an increase in the UV-B reaching the earth. UV radiation data are of particular interest because such radiation is energetic enough to break apart several biological molecules, including DNA [4]. The increase in UV radiation affects human health [10]. There are wide spatial and temporal variations in the UV irradiance at the surface of the earth depending on latitude, solar elevation, and atmospheric and local conditions. Atmospheric aerosols also affect the UV irradiance, but their influence is small relative to cloud cover [11-13]. Among these factors influencing the UV irradiance, cloud cover present a high temporal and spatial variability. It is certain that clouds can cause large year to year variability in UV radiation and therefore possibility play an important role in determining long term trends [14]. For the cloud effect, it is necessary to have knowledge of clouds optical thickness and drop size distributions. The aim of this work is to investigate the effects of clouds on UV irradiance using data collected in Baghdad, Iraq. II. LOCATION OF STUDY AND MEASUREMENTS Broadband solar irradiance (0.3–3 μm), global, ultraviolet global irradiance, photo synthetically active irradiance (0.4–0.7 μm) and other meteorological variables were continuously recorded for the period between first of October 1994 and 30 March 1995 in the Radiometric Station of the Center of Energy and Environment Researches (33.34 0 N, 44.45 0 E, 34 m a.m.s.l.), in Baghdad, Iraq. The measurements were taken every 10 minutes. Solar global irradiance was measured using Eppely pyranometer, An Eppley TUVR radiometer was used to measure the ultraviolet irradiance (0.29–0.385 μm) on a horizontal surface. Air temperature and relative humidity were also recorded for the same period. From this database, hourly values have been generated for the mentioned period to guarantee that a wide range of seasonal conditions and solar elevation angles are covered. It is important to point out that there are significant differences in temporal and cosine response between the radiometers used for the measurements of UV and broadband solar irradiance. Consequently hourly values have been computed, in order to reduce the temporal effects that could be introduced in obtaining the ratio of UV radiation to broadband solar radiation. These undesirable effects can reach quite a significant level, especially under highly changing conditions, which are usually associated with broken cloud fields. III. RESULTS AND DISCUSSION To study the effect of cloud on UV irradiance , a cloud modification factor, which is define as the ratio between the UV measurement and corresponding clear sky UV irradiance that would be calculated for the same