  In this paper we calculate the delay of the arrival times of visible photons
on the focal plane of a telescope and its fluctuations as function of local
atmospheric conditions (temperature, pressure, chemical composition, seeing
values) and telescope diameter. The aim is to provide a model for delay and its
fluctuations accurate to the picosecond level, as required by several very high
time resolution astrophysical applications, such as comparison of radio and
optical data on Giant Radio Bursts from optical pulsars, and Hanbury Brown
Twiss Intensity Interferometry with Cerenkov light detectors. The results here
presented have been calculated for the ESO telescopes in Chile (NTT, VLT,
E-ELT), but the model can be easily applied to other sites and telescope
diameters. Finally, we describe a theoretical mathematical model for
calculating the Fried radius through the study of delay time fluctuations.
