Climate of Ireland
The dominant influence on Ireland's climate is the Atlantic Ocean. Consequently, Ireland does not suffer from the extremes of temperature experienced by many other countries at similar latitude. The warm North Atlantic Drift has a marked influence on sea temperatures. This maritime influence is strongest near the Atlantic coasts and decreases with distance inland. The hills and mountains, many of which are near the coasts, provide shelter from strong winds and from the direct oceanic influence. Winters tend to be cool and windy, while summers, when the depression track is further north and depressions less deep, are mostly mild and less windy.
The polar front is a feature of the atmospheric circulation which plays an important part in determining Irish weather. It's a zone of transition between warm, moist air (sometimes of tropical origin) moving northwards and colder, denser, drier air (usually of polar origin) which is moving southwards. The flow of air between the equator and the pole is complicated and indirect. The air masses separated by the polar front are sometimes considerably modified on their paths from their respective source regions. In the North Atlantic the polar front can often be traced on weather maps as a continuous line over thousands of kilometres. In winter, it usually extends north eastwards from the east coast of the United States, in summer it is less well-defined and can be difficult to locate. Disturbances on the front (waves), sometimes amplify and deepen to form the large scale depressions of the middle latitudes. These depressions often move north eastwards across the North Atlantic and pass to the northwest of Ireland. Ahead of the depression centres, warm moist air is swept northwards while behind them colder, drier air is swept southwards. This gives the sequence of cloudy, humid weather with rain, followed by brighter, colder weather with showers so typical of the Irish climate.
Ireland experiences a range of air masses with different sources and tracks, giving us our variable weather. Air masses of polar origin are most common but they usually have a long track over the Atlantic before reaching Ireland. Even southerly or south-westerly winds can bring us returning polar air, albeit highly modified by its excursion into the warm waters of the mid Atlantic. Air masses of direct tropical or polar origin are uncommon. An examination of synoptic charts for a ten-year period showed that on average 170 fronts, which could be identified as part of the general synoptic situation, reached the south-west of Ireland each year (Rohan).
Recurrent Features of the Irish Climate
In general the Atlantic low-pressure systems are well established by December, and depressions move rapidly eastward in December and January, bringing strong winds with appreciable frontal rainfall to Ireland. Occasionally the cold anticyclone over Europe extends its influence westwards to Ireland, giving dry cold periods lasting several days.
Between February and June, the influence of continental and Greendland anticyclones make these the months of least rainfall. The sea near Ireland is at its coldest in February and March and consequently the rise of mean air temperature is slow in spring. However, on clear days with light winds, afternoon temperatures can reach summer values even in March. In such situations the nights are cold. Air frost is not infrequent at inland locations, even in May. Continental anticyclones blocking Atlantic depressions are usually responsible for dry periods in late spring.
Towards late June or early July the rise in pressure over the ocean and a corresponding fall in pressure over Europe results in the general wind flow at the surface becoming westerly, bringing air with a long ocean track over Ireland, so that cloud cover, humidity and rainfall increase. From mid July, clear nights tend to be accompanied by heavy dew. Warm air masses of high humidity and daytime heating sufficient to cause thunderstorms are a feature of mid to late summer weather.
With the advance of August there are occasional incursions into the Atlantic of cold northerly air masses and these produce active depressions in late August and September. In September the humid air is exposed to increasing periods of cooling by night and fog is frequent around dawn in low-lying districts.
In October and November westerly winds from the Atlantic pass over relatively warm seas, and frontal rain and post cold-frontal showers tend to be moderate to heavy. The development of anticyclones extending over Ireland in these months can produce very pleasant weather by day. However, fog in these situations is slow to clear in the morning, particularly in November when solar radiation income is low.
From late summer through Autumn there is a risk of former tropical depressions mixing in with the North Atlantic weather pattern depressions to produce severe storms. These are quite rare but are very significant weather events.
The broad sequence described above does not recur regularly each year. In an oceanic climate at the latitude of Ireland, variability of weather in some of the features referred to above may be completely missing over several months in an individual year.
Snowfall in Ireland
Many Irish winters are free from major snowstorms, but because of its infrequent and irregular occurrence, snow in large quantities causes serious disruption. It can completely disrupt traffic, close airports and seriously damage overhead power lines and communication lines.
January and February are the months in which snow is most frequent but it’s not uncommon to have snow in any of the months November to April. Snow has been reported in May and September. On some of these occasions the falls have been considerable but the snow melted quickly. Generally snowfall in Ireland lasts on the ground for only a day or two. Some of the more notable snowfalls in recent times had snow lying on the ground lasting from 10 to 12 days. The number of days with snow cover tends to increase northwards through the Midlands corresponding to the decrease in winter air temperatures. During the winter, sea temperatures are warmer than land which can often lead to rain around the coasts but snow a few miles inland. Rain showers may fall as snow on higher ground as temperature generally decreases with altitude. The number of days with snow cover is quite variable from year to year more...
The change from winter to spring or from summer to autumn is gradual and the general trend is subject to reversals which may last for a week or more. For Climatological purposes, on the basis of air temperature, seasons are regarded as three - month periods as follows: December to February - winter, March to May - spring, June to August - summer and September to November - autumn. This is a common grouping in the meteorological practice of many countries in the middle and northern latitudes.
What does Climate Change mean for Ireland?
Weather and climate have been observed in a systematic way in Ireland for over 150 years. We know a great deal about past weather and climate patterns. When it comes to the future we must rely on computer models. These models provide a range of projections of the future state of the climate. The range is needed to take into account the uncertainty in the future level of greenhouse gases emitted, and also uncertainties in the models themselves.
These projections indicate that the rise in global temperature is likely to exceed 2 °C by the end of this century and that could rise to as much as 4.5 °C depending on the level of greenhouse gas emissions. This global temperature increase so far has not been uniform, e.g. temperatures in the Arctic have been rising much faster than the global average. Sea levels will rise and rainfall patterns will change with significant regional and local variations expected.
In Ireland the average air temperature has risen by approximately 0.8°C in the last 100 years, with much of the warming occurring towards the end of the 20th century, all seasons are warmer. Some of the impacts can already be seen; the start of the growing season for certain species is now up to 10 days earlier, there has been a decrease in the number of days with frost and increase in the number of warm days (days over 20°C).
All seasons are projected to be significantly warmer (1 to 1.5°C) by mid-century, this will lead to a further increase in the length of the growing season, with a knock-on effect on natural ecosystems which have evolved gradually to suit our climatic conditions, the rate at which these changes are expected to take place may not allow ecosystems time to adapt, e.g. migrating birds arrive in spring and take advantage of insects emerging after winter, if the insects hatch earlier fewer chicks will survive. Fragile habitats in vulnerable upland, peatland and coastal areas will come under increasing stress. Milder winters will lead to a reduction in winter mortality due to fewer cold spells but the increasing likelihood of heatwaves and hot days (days over 30 °C) may have the opposite effect in summer.
Over the last 30 years or so rainfall amounts have increased by approximately 5%, and there is some evidence of an increase in the number of days with heavy rain in the west and northwest. Climate projections for rainfall have greater uncertainty than for temperature, they indicate that overall rainfall amounts in Ireland might decrease slightly, summers are likely to become drier while winters may be wetter especially in the west and north. There are also indications of an increase in the number of very wet days (days with rainfall >20mm).
These projections, applied to river flows, show an increased risk of winter flooding, an increased risk of short duration ‘flash’ floods and to possible water shortages in summer months due to higher temperatures and lower rainfall. The rise in sea levels will make low lying coastal areas more prone to flooding, especially from storm surges.
Changes in our climate regime will continue to be incrementally small and barely noticeable on a year to year basis, and will occur against the background of natural climate variability such as El Nino and variations in the sea temperature of the north Atlantic. It is also possible that declining Arctic sea ice might affect regional weather patterns. This means that we are still likely to have periods of colder weather which appear to go against the trend. Extreme weather events will continue to occur, while it might not be possible to explicitly attribute these to human induced climate change, the probability of occurrence of extreme events is expected to increase.
The impacts of climate change on Ireland may be less severe than those expected in other parts of the world, nevertheless we need to put adaptation and mitigation strategies in place now to ensure the best societal outcome for Ireland.