(b)at any time when the wet-bulb reading of the hygrometer is higher than that specified in the following schedule in relation to the dry-bulb reading of the hygrometer at that time, or as regarding a dry-bulb reading intermediate between any two dry-bulb readings indicated consecutively in the schedule when the dry-bulb reading does not exceed the wet-bulb reading to the extent indicated in relation to the lower of these two dry-bulb readings :
