By Philip Eden
One of the most common fallacies in meteorology is that mountain ranges always provide shelter to the region on the lee side of the range. That protection from wind and rain often occurs is not contested, but there are special circumstances when the normal sheltering effect is suspended. On these occasions the wind may be even stronger leeward of the mountains than to windward.
If you lived in Sheffield 50 years ago this month you will certainly recall such an event. Westerly gales swept the entire country during 16 and 17 February 1962, and wind speeds recorded at exposed hilltop and coastal sites were exceptionally high. A gust of 104 knots was measured at Lowther Hill in Lanarkshire, and one of 154 knots at Saxa Vord - a clifftop location in Shetland - although this latter figure will not be found in the official record books since the site is non-standard.
Between six and seven o'clock in the morning on the 16th the wind in Sheffield averaged over 60 knots with a peak gust of 84 knots; such a strength of wind is very rare for a relatively low-lying inland site in the UK. In Sheffield alone over 150,000 houses - approaching two-thirds of the city's entire housing stock - were damaged to some degree. Almost 7500 suffered serious structural defects, and 98 was so badly damaged that they had to be demolished. Three people died and about 250 reported to local hospitals with injuries sustained during the gale. Substantial damage was also reported in other West Riding towns and cities on the eastern flank of the Pennines.
Sheffield's wind speeds on that occasion contrasted dramatically with those recorded at Manchester, on the windward side of the Pennine range. Here the highest mean hourly wind speed was 32 knots and the maximum gust was 56 knots.
The principal cause of this exceptional gale is known as the "resonant lee wave effect". The airstream covering the British Isles on February 16 1962 had travelled from the sub-tropical Atlantic, having originated some 1500 kilometres southwest of the Azores. It was therefore an unusually warm airflow although near the Earth's surface it had been cooled by several degrees during its journey across the ocean. This sort of temperature profile suppresses turbulence and the flow of the wind becomes strongly stratified. When such an airflow crosses a mountain range, standing waves are established downwind on the range. You can see similar standing waves in a river, downstream of an obstruction such as a weir. Sheffield lay under a wave-trough with the airflow "squeezed" - and therefore accelerated - between the base of the trough and the ground beneath.
The destruction of property and toll of casualties were even worse on the other side of the North Sea. The westerly gale combined with a high spring tide to create a huge storm surge in the German Bight during the early hours of February 17, and high tides were up to 4 metres above predicted values. Dykes were breached and 340 people were drowned in and around Hamburg.
By Philip Eden