About WLMAC weather page

The weather page combines from various data sources to gather both current observations and an enhanced forecast in one place.

Aviation: current reported weather

This is the average of the current METAR data from Heathrow and Luton. METARs are produced by major airfields, reporting the actual observed weather at the runway. Heathrow and Luton both update them automatically every few minutes. Our server fetches the latest METARs every 10 minutes.

Since we are between the two airfields, the temperature and wind values reported here are the average. The wind direction will show "varies" if they are very different (typically when the wind is very light)

The cloud base is the lower of the 2 values, and is adjusted for the different elevations of the sites. Since Heathrow is lower-lying than Harefield it can report cloud below Harefield ground level - this might be fog at Harefield, or might sit a little higher over the high ground.

The "reported conditions" lists any specific weather effects (mostly types of rain) at each site - they are listed separately if there is anything to report

Weather services: combined forecast

This combines the forecast data from 4 of the better European weather services to give the best overall forecast:

• UK Metoffice (UKV 2km)
• Météo-France (ARPEGE Europe; 11km)
• European Centre for Mid-range Weather Forecasting (ECMWF IFS 0.25°)
• Deutscher Wetterdienst (ICON-D2; ~2km)

The forecast is for the Harefield site (Lat 51.625, Lon -0.4879). The forecast for Pickeridge will be almost identical. Each forecast model gives one predicted value for each parameter.

Our weather page reports the range of values given by the different forecasts. When they all agree they are likely to be correct. Our server fetches the forecasts every 3 hours. However most of the weather services only run the models every 6 or 12 hours, so the data here is likely to be the latest version.

Rain: This is separated into "Widespread rain" (e.g. cold and warm fronts) - in which all locations are likely to receive similar amounts of rainfall. And "Showery rain" from which there will be huge variation between locations. This is the average amount, but many areas may remain dry and others get much more than this. If "widespread rain" is forecast it is very likely to be wet. If small to moderate amounts of "showery rain" is forecast it is a pure gamble, and may remain entirely dry. Snow is also reported separately, but only when any is forecast.

Wind speed: This is the average wind speed 10m (30') above the ground. The range is the difference between forecasts, not the variation in the wind.

Wind direction: The arrow just shows the average predicted wind direction and doesn't try to represent the amount of variation in the forecasts. However if the forecasts differ wildly this will just say "varies"

Cloud cover: The percentage of the sky obscured by cloud. The models seem to differ in how thick the cloud has to be, to count as "covered", therefore there is more variation in these numbers than other parameters. However the values still give a good idea of how cloudy/sunny it will be.