The number of hours with a light intensity above the critical level for photoperiod sensitive reactions in plants is greater than the number of sunlight hours. In temperate latitudes, dark periods calculated from time of sunset to sunrise are nearly 1 hour longer than the effective dark period with an intensity less than 1 ft.‐c.; conversely, the effective photoperiod is longer by 1 hour. The length of the twilight period was used to calculate the number of “photoperiod effective” hours above critical intensities of 10, 5, 2, and 1 ft.‐c. (108, 54, 22, and 11 lux). These photoperiodic day lengths are presented for latitudes from 70° N to 60° S and all months of the year. Heavy cloud cover reduces the effective photoperiod, while scattered high clouds may reflect enough light from the sun below the horizon to lengthen the day slightly. These physical day lengths may be used to predict plant response for different planting dates in a given location when critical intensities of light and critical photoperiods which influence reactions in each crop species are known.