In the days of sailing ships they will have been rightly viewed as likely forerunners of stormy weather. Evening red and morning grey, two sure signs of one fine day - Morning grey probably refers to early morning mist, fog or shallow low cloud which often form on clear, near clam nights, but soon disperse after sunrise on a fine summer day. More extensive cloud especially with strong winds prevents the red sunset and overnight mist or fog, and of course a fine day is much less likely. If clouds be bright, 'twill clear tonight. These clouds often dissolve towards sunset to give a clear, cold night.
Dark clouds are usually deep and more extensive.
They do not usually vary much with local solar heating or the lack of it, and often bring rain. A green Christmas makes a fat churchyard. The second may be explained by cold, wet springs encouraging sturdy if slow crop growth and discouraging early insect pests. These strong plants may have produced better harvests in those years, leaving more to store through the following winter and better nutrition for the populace. Cast not a clout 'till May be out, 'Till April's dead, change not a thread - A clout here means a piece of clothing.