Airflow Types and Rainfall in Ireland

Airflow Types and Rainfall in Ireland John G. Houghton and Michael S. O'Cinneede* IEiELAND has one of the most maritime climates for its latitude in the world. Its insular west coast position between 52° and 560N is subject to prevailing westerly winds throughout the year,1 winds with a fetch of more than 2,000 miles across the northAtlantic Ocean. Precipitation is therefore frequent, with most locations in Ireland receiving 200 to 250 measurable rain days per year. Rainfall also resembles other midlatitude marine west coast climates in its intensity (modest in the lowlands but greatly increased orographically) and seasonal distribution (heaviest in fall and winter). These characteristics vary spatially across Ireland, in response to prevailing airflow patterns which divide the country2 into two basic rainfall regions: (1) the west, which has heavy precipitation and a strong winter maximum associated with westerly airflow, and (2) a drier regime in the midlands and east, where seasonal contrasts are less pronounced and closed cyclones assume the dominant role. This study explores the spatial variation of Irish precipitation as influenced by airflow from different moisture sources. Its goal is parallel to one in an earlier study of Great Basin rainfall in which * Dr. Houghton is an Associate Professor of Geography at the University of Nevada, Reno, and Dr. O'Cinnéide is a Lecturer in the Department of Social and Environmental Studies, National College of Physical Education, Limerick, Ireland. 1 Wind roses for most stations in the Irish Republic show that winds with a westerly component are more than twice as common as those from the east. P. Butler and B. C. Farley, Surface Wind over Ireland 1961-1970, Meteorological Service, Dublin, CIimatological Note No. 2 ( 1973) . 2 In this paper the term "country" refers to the whole island, including the Irish Republic and Northern Ireland. 33 34ASSOCIATION OF PACIFIC COAST GEOGRAPHERS three types of synoptic patterns (Pacific storms, Great Basin lows, and convective showers in moist tropical air) were analyzed in terms of their effect on total seasonal precipitation and its seasonal distribution .3 Although Ireland is much smaller, more maritime, and further north than theGreatBasinarea, both areas not only showed an inland precipitation decrease but also a change from a winter rainfall maximum in the western part of the study area to a more even distribution in the eastern part, associated with comparable changes in dominant airflow types causing precipitation. Rainfall Distribution Although Ireland's geographical position and rainfall have several characteristics in common with western North America, Ireland is in some ways quite different. North America presents continuous , high mountain barriers which create very steep rainfall gradients across their crests, whereas Ireland's mountains in contrast are rather scattered and low, the highest only reaching 1000 m above sea level. These low mountains do produce sharp local precipitation increases; annual mean totals range from about 700 mm in the lowlands near Dublin to more than 3000 mm in the highest mountains of the southwest.4 The pronounced orographic lifting is due partly to the strength of winds accompanying steep pressure gradients in many storms. At the same time, gaps between the mountain ranges allow maritime rain-bearing winds to penetrate throughout the country. This effect is enchanced by Ireland's small size and its physical separation from theEuropean continent. Precipitation gradients from west to east are therefore modest across the Irish lowlands in comparison to western North America, the latter being comparable only in areas of modest relief such as the Oregon Coast Ranges. AU of Ireland, furthermore, is subject to year-round cyclonic activity owing to its high latitude, and the south coast is also exposed to frequent southerly flow from the ocean. As a result, precipitation does not decrease southward across Ireland as it does in western 3 J. G. Houghton, Characteristics of Rainfall in the Great Basin (Reno, Nev.: Desert Research Institute, 1969). * Rainfall in Ireland 1931-1960 (Dublin: Meteorological Service, 1971). YEARBOOK · VOLUME 38 · 197635 North America south of about 550N. Instead the pattern is roughly horseshoe-shaped, with heaviest rainfall near the north, west, and south coasts and lightest amounts in the midlands and east. Precipitation everywhere exhibits a characteristic common to west coast...