Phase Lag of Temperature behind Solar Radiation in Iraq


Jasim H. Kadhum, Ahmed S. Hassan*, Ali J. Mohammed


Department of Atmospheric Sciences, College of Science, Mustansiriyah University, Baghdad, Iraq

Corresponding Author Email: [email protected]

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Solar radiation is the ultimate source of energy to the Earth and its surrounded atmosphere. In the multitudes, the seasonal variations of solar radiation reaching the Earth’s surface is almost sinusoidal resulting in nearly sinusoidal variations in air temperature. A phase lag exists between the two cycles due to the heat capacity of the Earth. The aim of this work is to determine this phase lag in four cities representing different climatic regions in Iraq. Monthly and daily data of all sky solar radiation incident on a horizontal surface (SW) and air temperature at 2m (T2m) were analyzed for the period 1990-2019. The results showed that SW peaks occurred in the last week of June while the T2m approached its peak sometime between mid-July and mid-Aug depending on the location of the city. The average phase lag for the entire 30 year’s period was ~29 days in northern city of Mosul, 44 days in western town of Rutba and ~36 days in the central and southern cities of Baghdad and Basra respectively. Diurnal SW and T2m data during the equinoxes (spring and autumnal) and solstices (winter and summer) of 2019 were used to determine the diurnal phase lag. The finding of this paper indicated that SW peaked at 1 PM during all seasons and in all four cities and the phase lag was varying between 2 to 3 hours depending on the season and location.