How Weather Footprints works

A footprint is an area where one kind of weather, such as snow or fog, could cause noticeable problems on a particular day. It is drawn around everywhere that weather reaches at least a low level of risk, so each footprint is one joined-up patch of the map.

Each footprint has a peak: the spot inside it where the risk is highest. The peak is marked with a number on the maps, and the footprints of the day are numbered in order, with 1 the most serious.

Footprints are worked out for six kinds of weather: snow, heavy rain, high winds, thunderstorms, frost and ice, and fog (see Risks). Different kinds of weather can leave footprints in the same place on the same day.

Weather Footprints uses a set of 31 forecasts from the United States' national weather service. Each one starts from very slightly different conditions, because nobody can measure the atmosphere perfectly. Where the forecasts agree, the weather is more certain; where they disagree, it is less so.

A new set arrives four times a day, and the site is updated with each one, looking a week ahead. The buttons above the days let you look back at the last few sets, to see how the forecast has changed.

Forecasts are not perfect, so for frost, fog, thunderstorms and strong winds we compare past forecasts with what airports across the UK and Ireland actually reported, and correct the chances to match. This carries on all the time: every night the site collects the latest airport reports, and every week it checks whether an updated correction would have done better on the days since. It only switches to the new one if it would.

For every kind of weather, every place and every day, Weather Footprints asks three questions.

How likely is it? The chance comes from how many of the 31 forecasts bring that weather to that place. If 15 of them bring snow, the chance is about one in two. Very small chances, of less than one in twenty, are left out.

How bad could it be? Each kind of weather is looked at in three steps, each more serious than the last: for snow, a light covering, enough to block roads, and deep snow. The more serious the step, the bigger its impact.

What is in its way? The impact is then raised or lowered by what the weather would hit: how many people live there, and the roads, railways, power lines, hospitals, care homes, schools and airports nearby. Each kind of weather counts the things it really affects: wind and lightning bring down power lines, so those count for high winds and thunderstorms; fog matters most on roads and at airports (see Impacts). A busy place can raise the impact by up to two steps, and the emptiest countryside can lower it by as much.

The answers are combined into a single level from 0 to 10 for each place. The level goes up the more likely the weather is, the more serious it could be, and the more there is in its way.

This means the chance needed to make a footprint depends on where the weather is. A light covering of snow with about a one-in-three chance stays below footprint level on an empty moor, but over a city, with its people, roads and railways, the same snow and the same chance make a footprint.

It also means that something unlikely but serious can matter as much as something likely but mild. A one-in-ten chance of deep snow can score about the same as a near-certain light dusting. Weather Footprints looks at all three steps of each kind of weather, and the one that scores highest sets the level.

The levels are grouped into bands: 0 to 1 is little or nothing, 2 to 3 is low, 4 to 6 moderate, 7 to 8 high and 9 to 10 very high.

The levels are worked out across a fine grid covering the UK and Ireland, then gently smoothed, so that a footprint shows the general area at risk rather than jumping from one spot to the next. Only land counts: the sea has no one in the way.

Every joined-up area at level 2 or above becomes a footprint. Very small patches are left out, so that a footprint shows a real area of concern rather than a single hilltop.

The footprints are then ranked, the highest level first, and where two have the same level the more likely one comes first.

The main map shows every kind of weather at once. The colour at each place is the worst band of any kind of weather there, and outlines show which kind of weather it is where a footprint is more than low. Beside it, the list gives each footprint's number, what could happen, its chance, its impact and how large an area it covers.

The grid shows why each footprint ranks where it does: each number sits across at its chance and up at its impact. The further up and to the right, the more serious.

The six smaller maps show each kind of weather on its own, with the times, on the UK clock, when each footprint is most likely to be happening.

On a phone the maps are laid out to fit the screen; turn it on its side, or open a map, to see the full-size version.

Weather forecasts become less certain the further ahead they look, and the forecasts used here cannot see every valley, hillside or single storm. Weather Footprints shows where trouble is more likely; it cannot say that something will or will not happen in a particular place.

• Forecasts: NOAA Global Ensemble Forecast System (GEFS), from NOMADS and the NOAA Open Data Dissemination programme
• Airport observations: Iowa Environmental Mesonet
• Population: WorldPop (2020)
• Roads, railways, power lines, substations, hospitals, schools and care homes: © OpenStreetMap contributors (Open Database Licence)
• Airports: OurAirports
• Terrain: Copernicus DEM, produced using Copernicus data and information funded by the European Union
• Land cover: ESA WorldCover 2021, containing modified Copernicus Sentinel data
• Surface-water flood maps: © Environment Agency (England), © SEPA (Scotland), © Natural Resources Wales; Open Government Licence v3.0
• Map outlines: Natural Earth