An alternative approach is thermal imaging, where differential water loss (which is dependent of the availability of water at the roots) can create temperature differences, which result in thermal crop marks that are potentially visible at any time during crop growth. Thermal imaging can also reveal archaeological residues as a result of thermal inertia (storage heater effect) or differential evaporation. The interaction of the processes involved can be complex and the prediction of optimal imaging time, for a given site, further complicated by environmental conditions including temperature variation and relative humidity.