Figure 1.

Annual patterns of (a) twilight duration, (b) photoperiod and (c) temperature, for different latitudinal ranges. (a) Twilight duration for Northern Hemisphere, defined as the time between −12° (nautical twilight) and 0° solar altitude (approx. 20–80% of the log light intensity change between midnight and noon). (b) Photoperiod calculation based on civil twilight times at dawn and dusk. Civil twilight (solar altitude 6° below the horizon) is the moment when log light intensity changes most rapidly [3] and is often considered as the time of ‘lights on’ and ‘lights off’ for biological systems. (c) Monthly mean dry bulb air temperatures (mostly between 1960 and 1990) from 873 weather stations around the world obtained from the World Meteorological Organization (http://www.wmo.int), and globally averaged over 10° latitudinal bands. Colours indicate mid-latitude of each band (hemispheres plotted separately).