Frost, ice and snow in Ireland
How often it freezes, where the cold settles, why the roads ice over on a clear night, what a real snowfall takes, and the winters people still talk about.
Ireland is mild for its latitude, but it is not frost-free. Between November and March most inland places freeze on a few nights a week, and once or twice a winter a spell of cold arrives that closes schools, cancels matches and empties the bread shelves. This guide explains what to expect, where, and how to read the forecast for it.
How often it freezes
Frost is a night-time event, and it is a question of distance from the sea. The coast is kept mild by water that rarely drops below 8°C in winter, so Roches Point in Cork and Valentia in Kerry record only a handful of air frosts a year. Move inland and the count climbs fast: the midlands, the Shannon basin and the inland counties of Leinster see forty to sixty frosty nights a year, and the coldest hollows more. Kilkenny, Mullingar, Athlone and Birr are the places that top the list, not the north. The county temperature pages show the average night-time low for every month; where it falls below about 2°C, frost is routine.
Ground frost, on the grass and the car windscreen, is commoner still, because the ground cools faster than the air a metre and a half above it. On a clear, calm night the grass can be white while the thermometer in the box still reads 3°C.
The recipe for a frosty night
Frost needs three things: a clear sky, light wind and dry air. Cloud acts like a blanket and keeps the day's warmth in; a breeze stirs the air and stops the cold pooling; moist air gives up its heat more slowly. The classic frosty night follows a cold front. The rain clears through in the afternoon, the sky opens, the wind drops as high pressure builds, and by midnight the temperature is falling a degree an hour. The forecast cues are the ones to watch in the hourly forecast for your town: a cloud figure near zero, a wind under about 10 km/h and a temperature already down to 3 or 4°C by late evening.
Cold air is heavy and flows downhill like water, so valleys and bogland catch it. A house at the bottom of a hill can be three or four degrees colder than one at the top on the same night, which is why the frost pockets of the midlands and the river valleys of Leinster record the lowest temperatures in the country, and why a sheltered garden can lose its dahlias a fortnight before the one up the road.
Ice on the roads
Black ice forms when a wet road freezes, and the dangerous nights are the ones that follow rain. Bridges and flyovers freeze first, because cold air gets at them from below as well as above; shaded stretches, cuttings and roads beside rivers and lakes freeze next and thaw last. The hour before dawn is the coldest of the night, and the morning commute meets the ice at its worst. Councils grit the main routes when the road surface is forecast to fall below zero, but most minor roads are never gritted. If the forecast shows a clear night after a wet day, with temperatures heading for 1°C or below, treat every country road as icy until the sun has been on it.
Freezing fog adds a second hazard: it coats everything it touches, including the road, with a thin layer of ice, and it does so under a sky that looks calm and harmless. See the guide to fog and mist.
What snow takes
Snow falls somewhere in Ireland most winters, but lying snow is rare at low levels. The lowlands average only a few days a year with snow on the ground, and some winters pass without any; the Wicklow Mountains, the Dublin hills, the Sperrins and the hills of Donegal see it far more often. The reason is the Atlantic. Our winter rain comes on mild south-westerly winds, and snow needs cold air at the same time as the moisture, which means an easterly or north-easterly wind from the continent, or a northerly straight down from the Arctic, meeting a front coming the other way. Those set-ups happen only a few times a decade.
When they do, the east gets it. In an easterly, showers pick up moisture over the Irish Sea and drop it as snow on Wicklow, Dublin, Louth and Wexford, while Galway and Kerry sit under a dry sky. A northerly does the opposite, feeding snow showers into Donegal, Mayo and the north coast. Heavy snow in the midlands and south needs a front to stall over the country in cold air, and that is the rarest of all.
The temperature to watch is about 2°C. Above it, snow usually turns to sleet or rain before it reaches the ground; below it, snow settles, and in a strong easterly it drifts. Hills get snow when the valleys get rain: the air cools by roughly 1°C for every 150 metres of height, so 4°C at the roadside is snow on the tops of the Comeraghs.
The winters people remember
Ireland's record low is -19.1°C, set at Markree Castle in Sligo in January 1881, and the weather records page has the rest. The winter of 1947 brought snow that lay for weeks and cut off whole towns; January 1982 buried Dublin under drifts; and December 2010 was the coldest month in living memory, with temperatures below -15°C in the midlands and ice on rivers that had not frozen in a generation. Storm Emma, at the start of March 2018, brought the Beast from the East to the whole country, with red warnings, drifts over a metre deep along the east coast and a nation that stayed at home for three days.
Reading the warnings
Met Éireann issues yellow, orange and red warnings for snow and ice, and for low temperatures, on the thresholds explained in the warnings guide. A yellow ice warning is the common one: a clear night with wet roads. A snow warning gives the expected depth. The hourly forecast on this site shows the temperature, the cloud and the wind for each hour, and the sky symbol switches from rain to sleet to snow as the forecast temperature falls, so you can see at a glance which hours will settle and which will wash away.