Ask any Eskimo, and he will tell you that an igloo is not an easy thing to build. Disaster looms if the snow is not exactly right in density: if it is too soft it crumbles, and if too hard it will break up in sharp, misshapen, quite unmanageable slabs. But once built, an igloo is a masterpiece of architectural efficiency.
Firstly, it can be easily heated from within, and the heat inside causes a vapour-proof glaze to form upon the snow-packed walls. Then the powerful insulating capabilities of snow ensure that those inside enjoy a temperature regime that is at least 10 degrees warmer than the air without, and the living area is entirely draught-proof. Moreover, the igloo's hemispherical topology presents the smallest possible external surface area for a given volume, so that loss of heat by radiation is as low as it could possibly be.
When you come to think of it, of course, we ourselves in our naked thermal interface with the surrounding atmosphere are, mutatis mutandis, similarly affected by our shape. Fat people have a smaller surface area relative to their bulk than skinny people; this is unpleasant in hot weather, because of the difficulty in evaporating enough perspiration to keep cool, but it helps when the weather is cold because less heat is lost through radiation.
More important in the cold, however, are the useful insulating properties of fat. It has been calculated that a subcutaneous layer one centimetre thick has the same effect, albeit not the same sartorial elegance, as a Louis Copeland business suit. If we wished to optimise our situation, therefore, we ought be thin when it is hot and fat when it is cold, and indeed unconsciously we do our best to aim at this ideal: we adjust our food consumption during the winter months, tending to eat more and to prefer foods of higher caloric value than we do in summer.
More usually nowadays, however, rather than regulate our thermal comfort by personally expanding and contracting with the seasons, we raise the temperature of our environment indoors in winter. This has the unpleasant side-effect of automatically reducing the relative humidity, since the actual amount of moisture in the air remains the same, but its potential moisture-holding capability is significantly increased. This is why indoor plants - paradoxically unless you think about it in terms of the relative humidity - need to be watered more often during the winter months than in the summer. In a dry artificially heated indoor atmosphere, water loss is very rapid from their leaves, and also from the soil in which they grow.