Why Irish weather changes so fast
Four seasons in a day, explained: the Atlantic depressions, the fronts and the showers behind them, and how to read a forecast that says "sunny spells and scattered showers".
Visitors notice it within a day: sunshine, then rain, then sunshine again, with a rainbow somewhere in between. The reason is where Ireland sits. The island lies on the edge of the Atlantic, directly under the track of the jet stream, the river of fast air several kilometres up that steers weather systems from west to east across the ocean. Depressions form on it and arrive in Ireland every two or three days in winter, less often in summer, and each one brings a sequence of weather that plays out in a day or two.
The sequence a depression brings
A classic Atlantic depression has two fronts, and you can follow them on the radar and in the sky.
- Ahead of the warm front the sky clouds over from the west: first high wisps, then a milky sheet, then thickening grey. Rain begins, steady and light to moderate, and lasts several hours. The wind backs to the south and freshens.
- In the warm sector, between the fronts, the rain eases to drizzle, the cloud is low, the hills are in mist and it feels mild and muggy. On the west coast this can be the whole day.
- The cold front brings a narrow band of heavier rain, sometimes with a squall of wind, and then the sky clears from the west. The wind veers to the west or north-west, the air turns fresher and the visibility becomes superb.
- Behind the cold front comes the showery air: cumulus clouds building through the morning, sharp showers in the afternoon, bright gaps between them. This is the weather that makes the rainbows. The showers fade in the evening, and then the next depression's cloud appears on the western horizon.
Each stage can last hours or a day depending on how fast the system moves, and the whole cycle is why a forecast for "Tuesday" can honestly include rain, sunshine and a gale.
Showers and the sea
Showers in Ireland are mostly born over the sea. In autumn and winter the Atlantic is warmer than the air above it, so the air is heated from below, rises, and builds the clouds that drop showers on the west coast all day and all night. They weaken as they move inland, which is why Galway can have a showery day while Kildare is dry. In spring and summer the land heats faster than the sea, and the showers build inland in the afternoon instead, often over the hills, while the coast stays sunny under a sea breeze.
Why settled spells are rare, and why they happen
A settled spell needs high pressure to park itself over or near Ireland and hold the Atlantic depressions off. The Azores high drifts north often enough to give most summers a fine week or two, and in winter a block of high pressure over Scandinavia can give a week of frost and fog. Occasionally the block lasts: the heatwave of June and July 2018, the Beast from the East in March 2018 and the freezing December of 2010 all came from a pattern that refused to move. Those are the exceptions. The rule is movement.
Reading the forecast
Forecasters have a vocabulary for changeable weather, and it is more precise than it sounds. "Sunny spells and scattered showers" means most places will see both, with perhaps a one-in-three chance of rain in any given hour. "Isolated showers" means most places stay dry. "Showery outbreaks of rain" means longer wet spells with breaks, usually from a weak front. "Rain at times" means a front that is slow or stalling. "Rain clearing to showers" is the cold front passing. When a forecast gives a percentage chance of rain for an hour, it is the chance that at least 0.1 mm falls at that point in that hour: 30% is a reasonable bet on staying dry, 70% is not.
The first two or three days of a forecast are reliable for the pattern, less so for timing to the hour. Days four to seven give the trend. Beyond a week the forecast is a climate average with a tilt, and the honest answer to "what will the weather be like for the wedding" is "wait until the Monday before".
Using this site on a changeable day
Start with the radar for the next hour, the hourly forecast for your place for the day, and the written forecast from Met Éireann's forecasters for the shape of the week. Every town on the island has its own page with the forecast in words for today, tonight and tomorrow. The radar guide explains how to tell a front from showers on the loop.