THE annals do not record which honourable and eminent Roman scribe noted: Totum annum pluvit felibus et canibus. It reflects, however, a notion prevalent in Europe, even in the present times, about our Irish climate.
And of course it is a calumny: now and then it indeed rain "cats and dogs" upon our relatively recently unsceptred isle, but in general the rain falls intermittently and gently, and the abundant moisture lends our soil an enviable fertility.
It is the meteorologist's task to monitor this moisture closely. The rain that falls on Ireland every year, if it were to be spread evenly over the whole country, would provide a layer of water slightly less than 4ft deep.
About half this replenishes any deficit there may be from time to time in the reservoir of groundwater, and then runs off through streams and rivers to the sea. The remainder is returned to the atmosphere, either directly by evaporation, or indirectly by plants through a related process called evapotranspiration. Temporarily however, some of the water resides within the soil.
The amount of moisture that a soil can hold depends upon its type, on the size of the particles comprising it, and on the extent to which they are loose or well-compacted.
When it contains enough moisture for it to resemble a saturated, but not dripping, sponge, it is said to be afield capacity. If more water is added to an area of ground at field capacity, the excess liquid occupies the larger air spaces in between the individual particles of soil, and the ground is waterlogged. If, on the other hand, the ground falls below field capacity, there is said to be a soil moisture deficit.
The typical Irish winter provides plenty of rainfall, and because of the relatively low temperatures, there is comparatively little evaporation. As a result the land is at field capacity for most of the winter and early spring, and even waterlogged at times.
In summer, on the other hand, the reverse is the case; lower rainfall, and the high rates of evaporation brought about by high temperatures, mean that the moisture content of the soil drops dramatically, and a substantial soil moisture deficit often exists.
Meteorologists use temperature, humidity, wind and rainfall figures to keep track of the moisture content of the soil. These measurements allow them to calculate the extent to which the ground is experiencing a net gain or loss of water, and to provide useful estimates of the soil moisture deficit, or otherwise, on a countrywide basis for the agricultural community.