How to read the rain radar
What the radar colours mean, how to tell a front from showers, how far ahead you can trust the loop, and the tricks radar plays.
The rain radar on the forecast page shows where rain was falling over the last two hours, updated every few minutes. It is the most useful tool on the site for the next hour or so, which is exactly the window a forecast is worst at. Here is how to get the most from it.
What you are looking at
A weather radar sends out a pulse and listens for the echo from raindrops, snowflakes and hail. The stronger the echo, the heavier the rain. The colours run from the pale shades for drizzle and light rain, which barely wet the ground, through greens and yellows for moderate rain, to oranges, reds and purples for heavy rain and hail, the kind that floods a road in half an hour. Ireland's radars are at Dublin Airport and Shannon Airport, and the picture is a composite of them and of neighbouring radars in Britain and Northern Ireland.
Fronts and showers
Most Irish rain comes in two shapes. A front is a band of rain hundreds of kilometres long, often curved, that moves steadily across the country, usually from the west or south-west. If a front is coming, the rain will arrive on schedule and last for hours, and the only question is when it reaches you. Play the loop, see how far the band moved in two hours, and project that speed forward.
Showers look completely different: scattered blobs, each a few kilometres across, drifting with the wind. Each one passes in ten to thirty minutes, and they form and die as they go, so you cannot track a single shower for long. What you can read is the pattern. If the blobs are lined up along the coast and fading inland, the showers are being fed by the sea and the midlands will stay mostly dry. If they are growing as the afternoon goes on, the heating of the land is driving them and they will fade after sunset.
Which way is it moving?
Rain moves with the wind a few kilometres up, not the wind at the ground, and the two can differ. On a day with a southerly breeze at the surface, the showers may still be coming in from the west. Trust the loop rather than the flag on the pole: the direction the echoes travel over two hours is the direction the next ones will take.
How far ahead can you trust it?
Projecting the loop forward works well for about an hour, and for a steady front up to two or three. Beyond that, rain forms and decays faster than it travels, and you need the forecast. The rain layer on the map shows where the forecast models expect rain over the next twelve hours, and the hourly forecast for your place gives the chance of rain each hour. The radar tells you what is actually happening; the forecast tells you what is likely. When they disagree over the next hour, believe the radar.
Tricks the radar plays
- It misses drizzle far from the radar. The beam rises as it travels, and 100 km out it is about a kilometre above the ground. Drizzle and light rain from low cloud sit under it, so the hills of Kerry, west Cork, Connemara and Donegal can be wet while the radar shows nothing. The forecast for your town will still have it.
- Mountains cast shadows. The beam cannot see through a ridge, so rain behind high ground can be hidden or underestimated.
- Calm clear nights show ghost echoes. In high pressure, with a clear sky and a temperature inversion, the beam bends down and bounces off the ground or the sea. The speckle that appears does not move, and it is not rain.
- Melting snow looks heavier than it is. Where snowflakes melt into rain a few hundred metres up, the echo is unusually strong. In winter a band of apparently heavy rain can be ordinary rain with sleet in the hills.
- Snow looks lighter than it is. Dry snow is a weak reflector, so a snow shower that is settling can show as the palest shade.
- Heavy rain hides what is behind it. A torrential downpour near the radar weakens the beam, and the rain beyond it looks lighter than it really is.
Putting it together
For a walk, a match or a wash on the line, the routine is simple: look at the loop, decide whether what is coming is a front or showers, read the direction and speed, then check the hourly forecast for the hours beyond. On a showery day, a gap in the echoes upwind of you is worth about half an hour of dry weather, which is usually all you need.