The variables involved in thermally delineating archaeological residues can be numerous, complex and interdependent. However, in practice, I have made some simple observations over the years here in the UK.
1. Choose your thermal imager wisely. When reviewing published work, look at the images that have been produced by different products. Consider the number of pixels and angle of view in relation to your intended maximum height of flight, but remember that image quality does not depend on the number of pixels alone. I always shoot video with the temperature 'span' unlocked. Try to keep the temperature range as small as possible by avoiding temperature extremes within the field of view.
2. If flying over grass (or other vegetation) in the daytime, choose a time when the UV/sunlight/temperature levels are peaking and the Relative Humidity is around a minimum for the day. A compromise usually has to be made on timing. If it is a breezy, washing on the washing line day, then it is a good day to fly. The grass should not be wet, but previous rainfall can be helpful if there has been sufficient time for differential drying of the ground.
3. If flying over grass (or other vegetation) after sunset, choose a time after the steepest drop in temperature. Superficial features will show first. Timing for optimal delineation can be critical with the imager pushed to the limits of its sensitivity if the timing is not the most appropriate.
4. If flying over bare earth, or over a vegetation-free industrial site, you can take the approach outlined in (3) or you can fly at anytime after rainfall during the period of drying. If a road is beginning to dry, then that can be a good indication that you should be out at the site. Timing is usually less critical after sunset as the drying process is often slower.
Note the additional battery on the selfie stick which allows me to fly longer than the hour provided by the imager's battery.