Dear all I wonder if any body let me know what does mean this term in CERES- Wheat 2.0 which Ritchie and Godwin used to photoperiod calculation as following: "In the model, calculated photoperiods include civil twilight". Many thanks Abdolali Abdolali Ghaffari PhD student (MCR) Wye College, University of London Wye, Ashford Kent TN25 5AH UK Tel:+44-(0)1233 812401 ext. 391 Fax:+44-(0)1233 812855 E-Mail: [log in to unmask]