By their nature crop marks are visible only seasonally and may not be visible at all except in exceptionally wet or dry years. Droughts can be especially useful to cropmark hunters, as the differential growth can become apparent in normally hardy species such as grass. The drought of 2010 produced particularly good conditions for observing crop marks in the UK. Pre-parching stress in crops and grass, and others factors that may affect plant health, can be captured in near infra-red photography.