For a man given in life to sonorous and lofty turns of phrase, Banquo's dying words, shortly before he made his ghostly guest appearance at the dinner party, chez Macbeth, were singularly uninspiring: "Oh, treachery! Fly, good Fleance, fly, fly, fly. O slave!"
But then he had little time on that occasion to compose his thoughts. More typical were his musings on the previous night when he remarked, gazing skywards from the inner yard at Dunsinane: "There's husbandry in heaven; their candles are all out." It was Banquo's way of saying "It is dark", of telling us that the stars had been obscured by cloud.
But on a winter's night, a star-obscuring layer of cloud results in good husbandry on Earth as well as in the realms above. Our fuel bills in wintertime would be significantly higher without the frequent presence of a blanket of stratocumulus or altostratus.
Clouds have a marked effect on night-time temperatures in winter, especially when the winds are light. In the absence of cloud, Earth loses heat by long-wave radiation. The energy is radiated into space, causing the Earth's temperature, and that of the air in contact with it, to fall, ensuring that central heating thermostats stay in the "on" position. The cooling process continues through the night, with the temperature reaching a minimum near dawn.
But a layer of cloud prevents this heat loss to a large extent. The Earth continues to radiate energy as usual, but the outgoing radiation is absorbed by the clouds a little way above, and the lost heat is unable to escape. The clouds in turn re-radiate heat downwards, and this downward radiation may often be enough to balance the heat loss from the Earth. The net result is that the minimum temperature in overcast conditions may be as much as 10 degrees higher than that on an otherwise similar but cloud-free night.
During cloudy conditions in wintertime, there is little difference between night-time and day-time temperatures. The cloud, of course, also prevents day-time temperatures from rising as high as they otherwise might, by reflecting the incoming solar radiation and preventing it from reaching the Earth. But this makes less difference to the daily temperature cycle than the night-time effect, because winter days are relatively short.
From an economic viewpoint, therefore, clouds are welcome visitors at this time of the year. A clear starry night with a full moon is a pleasure to look at, but we pay a price in terms of larger fuel bills, as central heating systems try to cope with the freezing temperatures which normally accompany a cloudless sky.