Risks

What it is. Snow that falls and settles during the day. We look at how much fresh snow could build up on the ground, not just whether it might snow at all.

When and where. Most likely from late autumn to early spring, and far more often on high ground and in the north and east, though a cold snap can bring snow to towns and cities anywhere. Snow showers blowing in off the sea can bring heavy falls to coasts that face the wind.

Why it matters. A few centimetres is enough to cause trouble where snow is rarely seen, because roads, railways and people are not ready for it. Deep snow can isolate rural places for days, and the cold that comes with it is hard on older people.

On the maps it counts people, main roads, roads over high ground, railways, care homes and airports (see Impacts).

Good to know. Whether snow settles depends on things a forecast finds hard to judge, such as the exact temperature at the ground, so snow footprints can change more than most from one forecast to the next.

What it is. A lot of rain in one day. We add up all the rain expected over the day, because it is the total that floods roads and fills streams and rivers. Snow, and rain that freezes as it lands, are counted under their own headings instead.

When and where. Heavy rain can come at any time of year. In autumn and winter it usually comes with long spells of rain from the Atlantic, heaviest over hills facing the west and south-west. In summer it more often comes in sudden downpours, which can hit anywhere.

Why it matters. Water collects quickly on the hard surfaces of towns and cities, where drains cannot keep up, and on roads and railways in cuttings and dips. Where official flood maps exist, places known to flood count for more.

On the maps it counts people, main roads, railways, electricity substations, care homes and homes and roads that flood (see Impacts).

Good to know. A summer downpour can flood a street in under an hour while missing the next town completely. Forecasts can say that such downpours are likely in an area, but not exactly where they will fall.

What it is. Strong gusts of wind. We look at the strongest gusts expected during the day, and compare past forecasts with what weather stations at airports really measured, so that the chances are fair.

When and where. Most common from autumn to early spring, when storms sweep in from the Atlantic. Gusts are strongest on coasts and on hills facing the wind, and in the north and west, but a storm can bring damaging gusts far inland.

Why it matters. Falling trees block roads and railways and bring down overhead power lines, which can leave homes without electricity for days. Ferries, bridges and flights may be stopped.

On the maps it counts people, main roads, railways, power lines, electricity substations and airports (see Impacts).

Good to know. Trees in full leaf catch the wind, so a summer gale can bring down more trees than a winter one of the same strength.

What it is. Showers in air unstable enough to produce thunder and lightning. Storms are too small and short-lived to forecast for a single town, so we give the chance of thunder somewhere nearby, and compare past forecasts with the thunder that airports reported.

When and where. Most common on warm summer afternoons and evenings, especially inland in the south and east of England. In winter, storms sometimes form over the sea and drift onto western and northern coasts.

Why it matters. Lightning damages power lines and equipment and can start fires; downpours flood roads and drains within minutes; hail and gusts damage crops, cars and buildings.

On the maps it counts people, power lines, electricity substations, hospitals and airports (see Impacts).

Good to know. Even when storms are likely, any one place may stay dry while the next town has a downpour. A thunderstorm footprint shows the area where storms could break out, not the path of a particular storm.

What it is. Temperatures falling below freezing, usually overnight, and wet roads and pavements turning to ice, including rain that freezes as it lands. We compare past forecasts with the temperatures measured at airports, so that the chances are fair.

When and where. From autumn to spring, most often on clear, calm nights, when the ground loses its warmth to the sky. Valleys and hollows, where cold air settles, are often the coldest places, while towns stay a little warmer than the countryside around them.

Why it matters. Ice causes falls and broken bones, especially among older people, and crashes on untreated roads. Freezing rain is rare here, but it is among the most dangerous winter weather of all.

On the maps it counts people, main roads, roads over high ground, railways, hospitals and care homes (see Impacts).

Good to know. Main roads are usually gritted, so ice is often worst on pavements, side roads and car parks.

What it is. Fog thick enough to make it hard to see far ahead. We compare past forecasts with how far airports reported they could see, so that the chances are fair.

When and where. Most common on calm, clear nights and mornings in autumn and winter, especially in valleys and low-lying places, and in midwinter it can linger all day. Sea fog can drift onto coasts in spring and early summer.

Why it matters. Fog hides hazards from drivers until the last moment, so it can lead to serious crashes on motorways and fast roads, and it disrupts airports.

On the maps it counts people, main roads, railways, schools and airports (see Impacts).

Good to know. Fog is often patchy, clear in one street and thick in the next, so a fog footprint shows where fog is likely, not that it will be thick everywhere inside it.