ON one day every year, a little" after Daffodil Day and just before the clocks roll forward into summertime, the world's weather people try to be a little more conspicuous than usual. The unaccustomed brashness of this diffident community is a benign exercise in propaganda, the occasion being the annual feast day of their science, celebrated each year on March 23rd. World Meteorological Day is intended to focus attention on the importance of meteorology to the modern way of life; each yearn it has a specific theme, and tomorrow's is "Weather and, Water in the Urban Environment".
The ultimate driving force behind the water supply in any city is the radiant energy of the sun. Heating of the sea causes evaporation, by means of which water is changed from the liquid into the gaseous state, and absorbed into the atmosphere. In due course this vapour is converted back into liquid water again by means of condensation, and falls to earth as rain. Having reached the ground, much of the water that does not immediately evaporate again, flows into streams and rivers, and may be stored in lakes or reservoirs before entering the labyrinth of pipes and channels that will eventually bring the much travelled liquid to our homes.
Current usage of fresh water in Ireland runs at about 80 gallons per person per day or enough on average, to fill five good sized baths. Less than 2 per cent of this amount, however, is actually drunk by any one: the rest is used for washing and flushing the toilet, for running our washing machines and dishwashers, for industrial purposes, and a little now and then for irrigation and hosing down the car. Whichever way this urban water may be used, most of it eventually finds its way into the drainage or sewerage systems, and then back to its origin - the sea.
The drains and gullies provided to get rid of excess water from our streets sometimes have a subtle effect upon the urban climate. In the fields and woods of the countryside, water is soaked up by the soil and vegetation, from which it subsequently evaporates. This evaporation requires energy, which is extracted from the environment in the form heat, causing a drop in temperature. But in urban areas, excess water quickly disappears down shores, and is carried away through drains; with very little energy used for evaporation, a "heat island" develops in certain conditions, whereby an urban complex may be significantly warmer than the surrounding countryside.