Weather glossary

The terms we use in our forecasts, in plain English. 40 entries, from anomaly to wind chill.

A

Anomaly: The difference between a forecast or observed value and the normal for that place and time of year. A temperature anomaly of +1.5°C means 1.5 degrees warmer than normal. More
Anticyclone: An area of high pressure. Air sinks and spreads out, which suppresses cloud and rain: anticyclones bring settled weather, warm and sunny in summer, but often frosty, foggy or grey in winter.
Apparent temperature (feels like): What the temperature feels like to the human body once wind, humidity and sunshine are taken into account. Wind makes cold days feel colder; humidity makes hot days feel hotter.
Atlantic front: A front that has formed over the Atlantic and moves across the UK from the west, usually bringing a band of cloud and rain followed by a change in temperature and wind.

Beaufort scale: A scale from 0 (calm) to 12 (hurricane force) describing wind strength from its effects. Force 8, a gale, is a sustained wind of 39 to 46 mph.
Blocking: When a large, slow-moving area of high pressure stalls and diverts the usual west-to-east flow of weather systems. Blocks can bring long dry spells, heatwaves or cold spells, and are hard for models to forecast weeks ahead.

Climate normal: The average of a weather element over a 30-year period, currently 1991 to 2020. Normals are the yardstick for calling a month warm, cold, wet or dry. More
Cold front: The leading edge of colder air. It often brings a short spell of heavy rain, sometimes with a squall, followed by brighter, cooler, showery weather.
Confidence: On this site, how closely the 51 runs of the ECMWF ensemble agree on a day: high when they agree on the top temperature within 3 degrees and on whether it is wet, low when they spread more than 6 degrees or split on rain. More
Convection: Warm air rising, cooling and forming cloud. Strong convection on warm days builds the towering clouds that give heavy showers and thunderstorms.

Depression: An area of low pressure, also called a low. Air rises, forming cloud and rain, and winds blow anticlockwise around it. Most wet and windy UK weather comes with Atlantic depressions.
Dew point: The temperature to which air must cool for its water vapour to condense into dew, fog or cloud. A high dew point feels muggy; when the temperature falls to the dew point overnight, expect dew or fog.
Drizzle: Very fine drops of rain, smaller than half a millimetre, falling from low cloud. It rarely adds up to much, but can last for hours.

EC46: ECMWF's extended-range forecast system, run every day out to 46 days. ECMWF runs 101 versions; the feed we use (through Open-Meteo) carries 51 of them. It is the source of our six week outlook. More
ECMWF: The European Centre for Medium-Range Weather Forecasts, an intergovernmental organisation based in Reading, the UK and Bologna. Its global model has long ranked at or near the top in published verification scores. More
Ensemble forecast: A set of forecasts from the same model started from slightly different initial conditions. The spread of the runs shows how predictable the weather is: tight agreement means confidence, wide spread means uncertainty. More
Evapotranspiration: Water lost to the air from soil (evaporation) and plants (transpiration). Farmers compare it with rainfall to see whether soils are wetting up or drying out. More

Freezing level: The height above sea level at which the air temperature is 0°C. Above it, precipitation is likely to fall as snow and surfaces may ice. More
Front: The boundary between two air masses of different temperature and humidity. Fronts are where most UK rain is produced.
Frost: Ice crystals that form on surfaces when they cool below 0°C. An air frost is when the air at 1.25 metres falls below freezing; a ground frost can happen on grass when the air stays above it.

GFS: The Global Forecast System, the main global weather model of the US National Weather Service, run four times a day. Our wind and colour maps use it. More
Gust: A brief peak in wind speed, lasting a few seconds. Gusts can be 50% stronger than the average wind and cause most wind damage.

Haar: Sea fog or low cloud on the east coasts of Scotland and north-east England, usually in spring and early summer when warmer air passes over the cold North Sea. Also called sea fret.
hPa (hectopascal): The unit for air pressure on weather charts, the same as a millibar. Average sea-level pressure is about 1013 hPa; deep Atlantic storms can fall below 950 hPa.

Isobar: A line on a weather chart joining places of equal pressure. Isobars packed close together mean strong winds.

Jet stream: A fast, narrow current of air around 9 to 12 km up, flowing west to east. Its position steers depressions and decides whether the UK is unsettled or settled. More

Long range forecast: A forecast beyond about two weeks. At that range it describes tendencies (warmer, wetter than normal) for whole weeks or months. It cannot say what a given day will bring. More

Met Office warning: An official warning of severe weather from the Met Office, coloured yellow, amber or red according to the likelihood and the impact. More

Occluded front: A front formed when a cold front catches up with a warm front, lifting the warm air off the ground. Often brings a band of cloud and rain that weakens as it goes.

Precipitation: Any water falling from the sky: rain, drizzle, sleet, snow or hail.
Precipitation probability: The chance that at least a small amount of precipitation falls at a place in a given period. 30% means rain on roughly three occasions in ten in similar situations.

Reanalysis: A reconstruction of past weather made by running a modern forecast model through decades of observations. ERA5, from the Copernicus Climate Change Service, is the reanalysis behind our climate averages. More
Ridge: An elongated area of high pressure extending from an anticyclone. A ridge often brings a short spell of fine weather between depressions.

SEAS5: ECMWF's seasonal forecast system, run once a month with 51 runs out to seven months. It is the source of our months-ahead outlook. More
Sleet: In the UK, a mixture of rain and snow, or snow that is partly melted as it falls.
Sudden stratospheric warming: A rapid warming of the air high above the Arctic in winter that can weaken the polar vortex. In some cases it leads, weeks later, to blocked patterns and cold spells in the UK, but not always.

Trough: An elongated area of low pressure. A trough can bring a line of showers or longer rain even without a clear front.

UV index: A scale of the strength of the sun's ultraviolet radiation. In the UK it ranges from 0 or 1 in winter to 7 or 8 on clear midsummer days. More

Warm front: The leading edge of warmer air. Cloud thickens and lowers ahead of it, bringing a longer spell of steadier rain, followed by milder, often cloudier weather.
Wind chill: The cooling effect of wind on exposed skin. At 0°C with a 30 mph wind it feels like about minus 8°C.