The productivity of Ireland is influenced by climate even more than by relief. Ireland possesses an extreme marine climate. Along the west coast the winter temperatures average between 42° and 44° F., and even the interior runs between 39° and 40° F. The summers are cool, the average July temperature along the west coast being 58° or 59° F., and inland 60° or 61° F. The difference between summer and winter is remarkably small, particularly in the southwest where the range between the average temperatures of the warmest and coldest months is no greater than 15°. Rainfall is heavy in the west where there are from 60 to 80 inches, but decreases to the east in the rain shadow of the mountains, falling below 30 inches around Dublin. The distribution is fairly uniform the whole year round.
The influence of regular and abundant rainfall, high humidity, and prevailing cloudiness is shown by the great amount of moorland and bog. Moorland covers the higher parts of the mountain upland as in Great Britain, and parts of the central plain as well. Monotonous low lands covered by peat are typical of widespread sections, and peat is still the commonest Irish fuel. But where the drainage is sufficient, the climate gives rise to the grassland par excellence that accounts for the legendary green of Erin. In former time, at the phases of great climatic cycles when the climate was drier, the European forest extended into Ireland. This was the case, for example, at the end of the sixteenth century. Grass, however, was abundant and probably dominated the landscape even then, particularly in the west. The combined result of climatic cycles and deforestation by man has been that only the typical park landscapes of big estates with scattered trees in the midst of grassland remain to show the former extent of the forest. At present only 1.4 per cent of the whole area is in real forest, the lowest figure among the countries of Europe.