City streets take the chill out of winter

The waxing of our customary Yuletide euphoria is tempered, or it ought to be, by the plight of those who must spend Christmas…

The waxing of our customary Yuletide euphoria is tempered, or it ought to be, by the plight of those who must spend Christmas in the open.

They are the homeless ones, those who sleep in some semi-sheltered spot beneath the Christmas stars, and on whom the spirit of the season, whether it be to give or to receive, has little impact; they are the victims of what A. E. Housman called:

The mortal sickness of a mind

Too unhappy to be kind.

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Those who would soothe their consciences in this regard may wish to recall that on cold frosty nights, the city streets, if not a source of comfort, are at least, in some little way, a source of heat.

The temperature in a built-up area on a winter's evening is often several degrees higher than that in the surrounding countryside. The phenomenon is known to meteorologists as the "heat island" effect.

Many factors contribute to the heat island. In the first place, buildings artificially heated in wintertime lose heat to the outside air, and thus increase the outside temperature.

Second, the brick and concrete present in abundance in urban areas are good absorbers of heat; they store any warmth received from the sun during the day, and release it slowly to the surrounding atmosphere at night.

And the compacted soils beneath the city's roads and parking lots also act as "storage heaters ", much more efficiently than the loose soils of agricultural land.

The urban temperature anomaly is also enhanced by the congregation of tall buildings, which block the wind that would otherwise disperse any pockets of relatively warm air.

And finally, the rapid drainage of surface water from the city streets means that less energy is dissipated by evaporation; this energy is therefore available to raise the temperature of the urban environment.

When global warming became a major issue in the early 1980s, some scientists believed the upward trend in average temperatures might be explained by the worldwide encroachment of the urban sprawl around weather stations which had formerly been in open country.

The heat island effect, they argued, might be sufficient to account for any apparent increase in the global average temperature. Careful statistical analysis, however, has shown that despite its very noticeable effect at local level, the heat island effect is negligible on a global scale.

We must look elsewhere for reasons for the gradual increase in the temperature of our planet that has been such a prominent feature of our weather records.