Fog, mist and the mornings the midlands vanish
The three kinds of fog Ireland gets, when and where each one forms, how long it lasts, the danger of freezing fog, and the signs in a forecast that a foggy morning is coming.
Fog is cloud on the ground, and Ireland makes a lot of it: damp air, long nights, slow rivers and a sea that is warmer than the land for half the year. Most of it is harmless, but it is the most common cause of a cancelled flight, a late ferry and a crash on a straight road in good weather. Here is how it works and how to see it coming.
Mist or fog?
The difference is how far you can see. Mist is when visibility drops below a few kilometres; fog is when it falls under a kilometre, and dense fog, the sort that stops traffic, under about 200 metres. Both are tiny water droplets hanging in air that has cooled to its dew point, the temperature at which it can hold no more moisture. The closer the forecast temperature and dew point sit together, with humidity near 100%, the more likely fog is.
Radiation fog: the still nights of autumn and winter
This is the fog of the midlands. On a calm, clear night, the ground loses its heat to the sky, the air just above it cools, and from about midnight onwards the moisture condenses into a fog that thickens until dawn. It needs the same conditions as frost, which is why the two so often arrive together, and it favours the same places: the Shannon basin, the bogs of Offaly and Westmeath, the river valleys of Leinster, and anywhere low-lying with wet ground after rain. The towns along the Shannon, from Carrick to Athlone to Limerick, and the plains around Kildare and Carlow, see far more fog than the coast.
Radiation fog is at its worst an hour or two after sunrise, when a little warmth stirs the air and lifts the fog off the ground without clearing it. In October and November the sun is strong enough to burn it off by mid-morning. In December and January it may not be, and a fog that forms on a Monday night can sit under a winter high for three or four grey days, with sunshine on the hills above it and the temperature stuck at 2°C below.
Sea fog: the summer surprise
Sea fog, or advection fog, is the opposite kind. It forms when warm, moist air blows over sea that is colder than itself, so it is a fog of spring and early summer, when the land has warmed but the water, which lags by two months, has not. A warm southerly in May or June drifting across the Celtic Sea arrives on the coast of Cork, Waterford and Wexford as a thick white wall, and it can push up the Irish Sea to cover Dublin Bay and the east coast while the midlands bask in sunshine. It comes and goes with the tide of air: a sea breeze brings it in during the afternoon, and a change of wind takes it away.
Unlike radiation fog it does not need a calm night, and it does not burn off, because the sea that makes it is still cold. A beach day on the east coast can be 14°C and foggy at Bray while Naas, 30 km inland, is 22°C under a blue sky. The forecast gives the clue: a warm, humid day with a wind off the sea in May, June or early July.
Hill fog
Hill fog is simply low cloud touching high ground. The cloud base in a damp Atlantic airflow is often only 300 to 600 metres, which means the tops of the Wicklow Mountains, the Reeks, the Galtees and the Donegal hills spend a third of the year in cloud. It is the fog that catches walkers, because the valley is clear and the summit is not; the guide to hill walking weather explains how to allow for it.
Freezing fog
When fog forms at temperatures below zero, the droplets stay liquid until they touch something, and then freeze on contact. Trees, railings, windscreens and the road all grow a coat of ice. Freezing fog combines the two most dangerous driving conditions in the country, poor visibility and black ice, and it does so under a sky that gives no warning. It is mostly a midlands event in December and January, after a frosty, foggy night. A fog warning with a temperature at or below zero in the hourly forecast is the sign.
Driving in it
Dipped headlights, not full beam, which bounces back off the droplets. Fog lights only when visibility is under 100 metres, and off again as soon as it improves, because they dazzle the car behind. Slow down to a speed at which you can stop within what you can see, and leave the gap: a straight midland road in fog hides a stopped tractor until it is thirty metres away. Fog is patchy, and the dangerous moment is driving from a clear stretch into a dense one at full speed.
Reading the forecast
For radiation fog, look for a clear, calm night after a damp day, with the temperature forecast to fall to within a degree of the dew point and the humidity at 95% or more; the hourly forecast on the town page shows the cloud, wind and humidity for every hour. For sea fog, look for a humid onshore wind in late spring with sea temperatures still in single figures, which the sea swimming guide lists month by month. And for both, the written forecasts from Met Éireann on the Ireland pages will say so plainly: "mist and fog patches forming overnight, slow to clear" is a phrase worth taking seriously.