Wind chill, humidity and what "feels like" means
Why 8°C in a westerly feels like 3°C, how the feels-like figure is worked out, what the Beaufort wind numbers mean on your skin, and how to dress for an Irish day.
Ask anyone who has stood on the Cliffs of Moher in March: the thermometer does not tell the whole story. Ireland's temperatures are gentle on paper, with winter days around 8°C and summer days around 18°C, but the wind that comes with them changes everything. This guide explains the "feels like" figure in the forecast, where it comes from and what to do with it.
What the body feels
Your skin does not measure temperature; it measures how fast it is losing heat. Still air at 8°C is a thin blanket that warms up next to your skin and stays there. A 40 km/h wind strips that warm layer away as fast as your body can make it, so you lose heat as if it were far colder. Damp air makes it worse in winter, because wet skin and wet clothes conduct heat away, and in summer humidity does the opposite, stopping sweat from evaporating so that a warm day feels stickier and hotter than the figure suggests.
How "feels like" is calculated
Forecasters combine temperature, wind and humidity into a single figure that answers the question "what would still air at this temperature feel like?". Below about 10°C with a breeze, the wind chill formula is used, and the figure drops with every extra 10 km/h of wind. Above about 25°C, a heat index takes over, adding to the figure as the humidity rises. In between, the two are blended. The hourly forecast on the town pages shows the wind speed and the humidity beside the temperature, so you can see which of the three is doing the work on any given hour.
A few examples from an ordinary Irish winter:
| Temperature | Wind | Feels like |
|---|---|---|
| 8°C | Calm | 8°C |
| 8°C | 20 km/h, Force 3 | 5°C |
| 8°C | 40 km/h, Force 6 | 3°C |
| 3°C | 40 km/h, Force 6 | -3°C |
| 0°C | 60 km/h, Force 7 | -8°C |
| 14°C | 30 km/h, Force 5 | 12°C |
The last row is the one that catches people out. A May day of 14°C with a brisk sea breeze is a coat day on the beach, whatever the sunshine says.
The Beaufort scale on your skin
The forecast gives wind in km/h; the written forecasts from Met Éireann use the old Beaufort words, and they are worth knowing because they describe what the wind does rather than how fast it is. Force 3, a gentle breeze of 12 to 19 km/h, moves leaves and flags. Force 4, moderate, 20 to 28 km/h, raises dust and makes the day feel cool. Force 5, fresh, 29 to 38 km/h, is the ordinary windy Irish day: small trees sway, there are white horses at sea, and a walk into it is work. Force 6, strong, up to 49 km/h, makes umbrellas useless. Force 7, near gale, up to 61 km/h, is hard to walk against. Force 8, a gale, breaks twigs off trees, and the guide to storms takes it from there. On the forecast map, the wind and gust layers show all of this hour by hour across the country.
Gusts
The wind figure in a forecast is an average over ten minutes; gusts are the peaks within it, and on a blustery day they run 50% or more above the mean. A forecast of 40 km/h with gusts to 70 means the walk feels like Force 6 most of the time and Force 8 for a few seconds every minute. Gusts are what knock cyclists sideways, lift a tent and slam a car door on your hand, and they are what the orange and red wind warnings are written for.
Dressing for it
Because the wind is the variable, the layer that matters most in Ireland is the outer one. A windproof shell over a light jumper beats a heavy coat with no shell on most days from October to April, and in the west for most of the year. A hat makes a surprising difference, because the head loses heat fastest. Wet clothes lose their insulation, so on a showery day the shell needs to be waterproof too. In summer, the sea breeze along any coast can take five degrees off the afternoon, and a beach in Donegal or Clare is a different climate from a garden 20 km inland; the county temperature tables show the gap in the averages.
When feels-like matters most
For the elderly and small children, who lose heat faster, a feels-like figure near zero is the one to plan around rather than the air temperature. For the hills, where wind and cold stack up with height, see the hill walking guide. For sailing, kayaking and cycling, the Sports weather page already rates each day of the week by the wind each sport can take. And for a wash on the line, a breezy 12°C dries clothes faster than a still 18°C, which is one thing the wind is good for.