The weathervane

Liz du Parcq explains how millers find out which way the wind is blowing

Look carefully at the cap of Brixton Windmill and you will see a little red arrow – a small and insignificant feature, but crucial to the process of generating maximum power by positioning the sweeps (sails) to catch the wind. This is the weathervane.

Image: Ann Lee

Weathervanes (from Old English ‘fane’, meaning flag) were probably first used in ancient Mesopotamia over 3,500 years ago, and the oldest surviving example is from the Tower of the Winds in Athens, dating from about 50 BC.

For weather forecasting, agriculture, navigation, and for gaining maximum advantage from windpower, finding the direction of the wind has always been important and remains so today. In 1588 the defeat of the Spanish Armada in 1588 owed much to a change in wind direction; in the 21st century, wind direction influences take-off and landing courses at our airports.

'The North Wind doth Blow, and we shall have Snow' is a nursery rhyme dating from the 16th century which still holds good today. In the UK, the prevailing winds come from the south west for much of the year, bringing mild, damp weather from across the Atlantic. When winds are northerly (from the Arctic) or easterly (from Siberia) we have cold snaps, but southerly winds bring hot, dry weather from the Sahara.

The language we use for wind direction can be confusing. We travel in a northerly direction from Brixton into central London. But a northerly wind is a wind FROM the north, and a weathervane will incorporate an arrow or pointer to the direction the wind is coming FROM. Some weathervanes (like the Brixton heron pictured below) incorporate fixed compass points.

A perfect balance

How does a weathervane work? It has to rotate quite freely on a vertical shaft and it has to be perfectly balanced, but crucially it has to have an unequal surface area on the two sides divided by the shaft, the tail and the head.

This is traditionally achieved using an asymmetric, mainly two-dimensional, design created in metal, or another material with similar properties. The thickness of the material may vary to ensure the asymmetric design can balance correctly.

The wind forces the feature with the larger surface area round into its path, so that the arrow on the head points back in the direction from which the wind is blowing.

But as at Brixton Windmill, the design can also be very basic – it simply has to have an equal mass but unequal surface area on the two sides. And of course, it has to be located on the top of a tall structure, as far as possible from other buildings.

These days Brixton Windmill’s flour is milled using the modular mill installed by the Ashby family in 1902. It was powered by steam initially, then later by gas. Today it is powered by electricity, mostly generated from fossil fuels.

But renewable energy sources, such as the sun’s heat and windpower, are being introduced, and about 17% of UK electricity is now generated from windpower. Occasionally the sails at Brixton Windmill are freed to catch the wind again, and this involves first aligning the sweeps to the wind, using the little red weathervane as a guide.

Image: Urban 75

Old and new

A recent addition to the Brixton skyline is the large heron weathervane created by artist Maggi Hambling, situated on top of the Prince of Wales pub/KFC, opposite the Town Hall.

At Lords cricket ground, there is a famous silhouette of Old Father Time removing the bales from the wicket.

On the spires or towers of many older churches you can see weathervanes with designs of cockerels. This is because in the 9th century AD there was a papal edict that every church in Christendom must have a weathervane with this design, as a reminder of Peter’s denial of Christ. Such ‘weathercocks’ were an important community facility in agricultural villages across Europe.

John Ashby’s windmill was built in 1816, halfway up the north-facing Brixton Hill. This was not an ideal location to catch the prevailing south-westerly winds. A windmill at the top of the hill, or even in central Streatham looking south-west towards Sutton, might have been more successful. But this would have been further from the customers of Ashby’s Mill, and a suitable piece of land might not have been available to purchase.

In the event, Brixton Windmill was only powered by wind up until 1862, after which the development of the surrounding area was sheltering the windmill from its vital power source and making it unviable, until steam power was introduced in 1902.