By Stephen Burt
It often comes as a great surprise to residents of Reading and the surrounding areas to find that sea breezes occur this far inland. In fact, they are not uncommon: in an average year we see around half a dozen, more in warmer summers. There have been several in the recent warm weather, some very pronounced and usually providing welcome evening relief to the day’s heat.
Most of our sea breezes originate from the south coast, although occasionally one can be traced back to the Thames estuary. They often arrive as a sudden gust of cooler, damper air accompanied by a freshening breeze and sometimes an increase in cloud, typically between about 6 p.m. and 8 p.m. on warm, sunny evenings between May and August – very occasionally as early as late March or as late as early September. Onset can be very sudden: an excellent example occurred just a few days ago. The top plot on Figure 1 below shows the rapid jump in humidity and fall in temperature resulting from the passage of the sea breeze over the Observatory instruments at 1905 GMT (8.05 p.m. BST) on Wednesday last, 5 July 2017; the middle plot shows wind direction (1 second values each minute, in degrees) and the bottom plot shows the 10 m wind speed (maximum and minimum 1 second values in each minute, the 1 minute mean given by the red line). The change in wind direction and speed, from a light easterly to a fresher south-south-westerly, is very pronounced.
Figure 1. Air temperature, relative humidity, wind direction and speed recorded at the University of Reading Atmospheric Observatory 1800-2030 GMT on Wednesday 5 July 2017, illustrating the passage of a marked sea breeze front. Data are plotted at 1 minute intervals: air temperatures, relative humidity and mean wind speeds are 1 minute means, wind direction is 1 s spot values and wind extremes are maximum and minimum 1 s values, all ending at the time shown.
This particular sea breeze reached Chilbolton (50 km south-west of the Observatory) at 1530 GMT (humidity record, Figure 2), while its passage was recorded on instruments at 1826 GMT at Stratfield Mortimer (10 km south-west), 40 minutes earlier than at the University – a typical forward speed of around 11 km/h (6-7 mph).
Figure 1. Relative humidity recorded at Chilbolton, Hampshire, Wednesday 5 July 2017: the sea breeze front reached Chilbolton just after 1530 GMT, as indicated by the abrupt rise in humidity.
Most sea breezes are entirely benign, indeed welcome at the end of a hot day. Sometimes, the convergence of one or more inbound sea breezes and warm air inland can kick off significant convective activity. On 7 May 2000 a series of evening thunderstorms developed across Berkshire along the line of an earlier sea breeze front: only 5 mm of rain fell at the university, but 87 mm was recorded at an unofficial raingauge in Bracknell, most of this falling in little more than an hour.
Much important theoretical and practical work on sea breezes was undertaken within our Department by John Simpson and James Milford in the 1970s, using a mixture of numerical and physical models together with observational campaigns involving surface instruments and a powered glider. A lasting legacy of this work was the book Sea Breeze and Local Wind by John Simpson (Cambridge University Press, 1994) – there is a copy in the Department library, and it is an excellent summary of this aspect of local meteorology.