The Wind Rose at Home
Walk out on a calm autumn morning and you’ll notice something subtle: the wind doesn’t blow from where it did in July. Over weeks, sometimes almost imperceptibly, the predominant direction shifts. For Monkton Heathfield and the wider Somerset Levels, that shift runs from westerly in summer toward south-westerly as autumn matures.
Why does this matter? Because wind direction determines what kind of air arrives at your doorstep. Westerly winds bring maritime air from the mid-Atlantic — relatively mild, often cloudy, carrying moisture that falls as drizzle or steady rain. South-westerlies pull that same moisture closer to warm Gulf Stream waters before it reaches us, often intensifying rainfall and raising overnight minimums. Northerlies, by contrast, deliver colder, drier continental air — rare in early autumn, common in January. Our weather station measures wind direction every few seconds via a rotating vane. That raw data aggregates into hourly averages, then daily summaries, then the cumulative wind rose charts you can browse on the Data page. Over time, the pattern becomes clear: summer dominates the west, winter dominates the north and east, and autumn occupies the swinging middle ground.
The Jet Stream Connection
The driver behind this seasonal migration is the jet stream — a narrow band of fast-flowing air high in the atmosphere that separates polar cold from tropical warmth. In summer, the jet sits farther north, steering systems across Scotland or northern England. By October, cooling land masses in Europe and North America push the jet southward, bringing its influence directly over Southwest England. As the jet descends, it guides low-pressure systems along more southerly tracks. Those lows spin counter-clockwise, so their eastern flanks draw south-westerly airflow over Somerset. That’s why storms arriving from the Atlantic in late autumn often hit us harder than equivalent systems in summer: the prevailing wind corridor has tilted right toward us. This isn’t speculative. Our barometric pressure readings track falling pressure ahead of incoming systems. Wind direction records show the shift before the rain arrives. When both align — pressure dropping, wind veering south-westerly — you can expect a frontal system within 24 hours.
What We Measure Locally
At Monkton Heathfield, we don’t rely on modelled predictions for prevailing direction. The Davis Vantage Vue station reports instantaneous wind direction via magnetic compass bearing (0° = north, 90° = east, 180° = south, 270° = west). The WS40 plugin on this site converts those bearings into cardinal directions (N, NE, E, SE, S, SW, W, NW) and stores them in the daily log. Over a full month, we can generate a wind rose histogram — a circular chart showing the percentage of time wind came from each sector. Summer roses peak sharply at 270° (due west). Winter roses flatten across the north-west quadrant. Autumn roses bridge the gap, often showing twin peaks: one at west, one at south-west, with the balance tipping toward the latter as November approaches.
Why does the station beat satellite estimates? Because local topography matters. The Somerset Levels lie in a low-lying basin surrounded by slightly higher ground to the north and east. Those ridges channel wind, accelerating flow through gaps and creating micro-scale deviations from regional forecasts. A model averaging conditions across a 12-kilometre grid square misses that nuance. Our gauge sees it minute-by-minute.
Practical Implications for Residents
Understanding prevailing wind direction helps anticipate more than just rainfall. It affects:
- Air quality: Westerly winds wash pollutants out over the Bristol Channel. Southerly or south-easterly flow can trap smog from urban centres upwind.
- Pollen dispersal: Tree pollen arrives from specific regions depending on wind sector. Grass pollen tends to ride westerly fronts in late spring; ragweed (when it appears) comes from the continent via easterlies.
- Fire risk: Dry westerlies lower humidity faster than humid south-westerlies. That influences vegetation drying rates and, indirectly, wildfire warning thresholds.
- Visibility: Clear mornings often follow northerly flows overnight. Persistent mist tends to settle during stagnant south-westerly periods with high moisture content.
The Visibility Warning Widget on the Today page responds partly to wind direction logic — sustained southerly flow with high humidity raises the probability of reduced visibility, especially at dawn.