How weather phenomena are calculated

Inputs

precipitation rate, mm/h air temperature relative humidity visibility wind and gusts cloud cover and daylight apparent temperature and forecast context model weather code, when available

Display priority: thunderstorm or precipitation → fog and mist → strong wind → strong heat or cold → extreme UV → sky condition. This prevents hazardous rain or snow from being hidden by a less important concurrent signal.

When there is no significant weather

Instead of a “No weather” message, the table shows only a coloured sky-condition icon: clear, partly cloudy or cloudy. Day and night icons differ for clear and partly cloudy conditions. The visible label is omitted, while the meaning remains available in the tooltip and to assistive technology.

Strong heat and cold

The signal uses apparent temperature, accounting for humidity in heat and wind in cold. Its threshold adapts to the selected point's complete forecast range using the median and the 10th/90th percentiles. Conservative bounds also apply: heat is never marked below 30 °C and severe cold never above −10 °C. For persistently hot forecasts the heat threshold can rise to 40 °C; for persistently cold forecasts the cold threshold can fall to −25 °C.

This is a contextual user signal, not an official heatwave or cold-wave warning. An official warning requires long-term climate normals and a locally validated temperature–health relationship.

A rare “Extreme UV” signal is also shown at UV Index 11 or higher, the top category of the WHO/WMO global scale. Levels 3–10 remain in the dedicated UV field and do not overload the phenomena column.

How the daily outlook is formed

A daily card no longer copies a single timestamp around noon. For quiet weather, cloud cover is averaged across all available daylight timestamps. Rain, snow, fog and strong wind describe the day when they occupy at least two forecast hours; strong heat or cold requires at least three hours. Brief but important peaks—thunderstorm, hail, freezing rain, heavy precipitation, dense fog and UV 11+—remain visible regardless of duration. Temperature minimum and maximum, precipitation total, peak gust and UV continue to use all available timestamps for that day.

Intensity thresholds

Phenomenon	Light	Moderate	Heavy
Rain	< 2,5 мм/ч	2,5–<10 мм/ч	≥ 10 мм/ч
Snow (water equivalent)	< 1 мм/ч	1–<5 мм/ч	≥ 5 мм/ч

The boundaries follow ICAO/WMO automated present-weather rules. If the API supplies accumulated precipitation, the service first converts it to an interval amount and divides it by the interval's actual duration. An instantaneous rate from the API takes precedence.

Rain, snow or mixed

Precipitation phase is estimated using wet-bulb temperature. It is calculated from temperature and humidity with Roland Stull's 2011 approximation; the published mean absolute error is below 0.3 °C within its validity range.

T_w ≤ 0 °C → snow
0 °C < T_w ≤ 1 °C → mixed
T_w > 1 °C → rain

The 0–1 °C transition zone follows an NWS study of observed precipitation phase. This is a surface diagnosis: freezing rain or ice pellets cannot be inferred reliably without a vertical atmospheric profile.

Fog, mist and wind

Fog is shown below 1 km visibility with humidity of at least 90%; below 400 m it is marked dense. Mist is shown from 1 to 5 km with the same high-humidity check. The 1 km boundary is the WMO definition; humidity reduces the chance of confusing fog with dust or smoke, which the current API does not provide.

Strong wind is shown from a mean speed of 10.8 m/s or gusts of 17.2 m/s. These are the Beaufort 6 (strong breeze) and Beaufort 8 (gale) boundaries published by the Met Office.

Known limitations

Thunderstorms, hail and freezing rain are shown only when the model supplies the corresponding weather code. We do not infer them from weak proxies: responsible diagnosis needs lightning data, CAPE and vertical temperature/humidity profiles. Smoke, dust and sandstorms are likewise not inferred without aerosol data. An ordinary sky-condition icon only means that the available inputs did not cross the documented significant-weather thresholds.

How we classify weather phenomena