The Dorset and East Devon coast has a temperate climate, with occasional frosts and falls of snow. Temperatures range from below freezing in the coldest winters to over 30^oC in the warmest summers.

Land absorbs more solar radiation and retains more heat than water, which means that land warms quicker than water. This time-lag between sea and land temperatures and their difference in temperature helps to produce sea breezes, which are locally important for people (especially for sailors) and for sand dunes.

The impact of waves driven by Atlantic storms, the effects of frosts breaking into the cliffs or the long-term changes in sea level each play a role in changing the coast.  This coast has a very long history of not only these day-by-day changes but also the impact of rare but ferocious storms.

The dynamism of our coast is primarily a result of the constant variability of our climate.  The way in which the coast changes depends on the local climate, the climate of the North Atlantic, and on historical changes in climate.

The coast has been affected by colder climates during glacial periods.  The combination of lower sea levels and a much colder climate meant that the floor of Lyme, Weymouth and Poole Bays were affected by periglacial conditions, similar to that of Northern Canada or Siberia today.

Today, sea level is rising each year in the far southwest of England by about 3 mm (and in the eastern Solent by over 5 mm).  Along the central part of the Dorset coast, it was steadily rising by about 1.5 mm, but in recent decades this has accelerated to about 3.5mm.

Annual rainfall increases westwards and as you go further inland. For example, Lyme Regis (West Dorset coast) averages 1000 mm rainfall annually, but Swanage (East Dorset coast) only averages 780 mm. Portland Bill is even drier with 680mm annually, but Dorchester only 15 km inland has over 1000 mm, mainly because of its higher altitude. Runoff to the coast depends on the higher rainfalls on the south-facing slopes of the chalk downs.

This coast previously held the UK record for the largest total rainfall in a single 24 hour period from 0900 GMT: the storm of July 1955 when over 280 mm fell at Martinstown and an estimated 180 mm on the coast at Osmington. Here the stream cut down over 2m in a single day and night.

Annual rainfall amounts in western Dorset increased during the 20th century at the same time as they decreased in eastern Dorset. However, there are significant monthly variations in these trends, which may ultimately affect the behaviour of coastal landslides.

In 2012, mudslides and landslides affected many of the cliffs, especially around Lyme Regis, after several weeks of rainfall, culminating in heavy localised rainfalls on 7th July. During the 2013-14 winter, a series of storms caused major changes to many of the beaches as well as triggering rockfalls and landslides.

April 2020 saw the largest landslide in living memory between Eype and Seatown, as explained in this video.

Winds are mainly from the west, but there are occasional long periods of easterlies. The easterlies have the very important effect of eroding beaches which are normally sheltered, and their effects can mislead both casual observers and engineering consultants into believing that longshore transport of sediments is from east to west.

Local very strong winds are channelled from the land through the deep coastal valleys on the western part of this coast.  Sea breezes are common in summer and can play an important local role in beach dynamics.

Waves are produced by the action of wind on the sea surface.  Very large waves can occur along this coast. Waves generated in the North Atlantic reach the coast as swell or long period waves (i.e. there are long time intervals between wave crests). 10 to 12 second intervals between wave crests are common but occasionally occur with periods up to 20 seconds intervals.

The prevailing and dominant waves are south-westerly, commonly with interval periods of over 10 seconds. Whereas, waves from the southeast have shorter interval periods (typically 5 seconds) and, although smaller, can be very energetic in eroding beaches or moving sediment along shore.

The east Dorset coast is microtidal with generally weak current and a tidal range less than 2.0 m.  Tidal range increases to over 3.0 m at the western end of the World Heritage Site.

At St. Aldhelm’s Head (Headland 5 km southwest of Swanage) there is a double high tide to the East and, to its west, there is a double low tide.  This means that the length of time when waves can attack the beach or cliff is different.

West of Portland there is a simple single high and low tide twice daily (a semi-diurnal tide).

Tidal currents are slow: for example off Lyme Regis, mid-depth currents average only about 40 cm per second (0.8 knots).  They are much faster off Portland Bill (over 250 cm per second – about 5 knots), in the races off St Aldhelm’s Head, Durlston Head and Peveril Point.

The modern headlands mark the boundaries of past bays into which the sea has flooded, and, from which it has shrunk several times in recent geological time.

Storms

When low atmospheric pressure and onshore winds coincide with high tides, storm surges can occur, which causes flooding. Fortunately these large storm events occur infrequently.

Storminess is, however, becoming more frequent with climatic changes, and this threat of increased storminess is one of the most significant and least predictable consequences of human accelerated climate change. The power of the sea in a raging storm is quite frightening and it can transform sections of beach and cliff forever in a matter of hours.

For the World Heritage Site and geomorphologists, storms are an exciting event and a natural part of the evolution of the coastline. For communities, major storms can have destructive impacts, but their generally infrequent nature means there is usually plenty of time for communities to recover once the waves and wind have abated.

These kinds of events usually only occur once every few decades with the very worst coming only once every couple of centuries. As climate changes, significant storms will become more numerous and occur closer together. The very worst case scenario is when major storms cluster close together in a single season or even month. In 2014 two storms hit within a fortnight of each other and both were of a scale that is supposed to only occur once every fifty years. The result was that Chesil beach, the enormous 15m high pebble structure that runs for miles along the Jurassic Coast and makes the largest excavators look like dinky toys, was moved back by an average of ten metres along its entire length. The impacts of larger storms are bigger than that though, affecting entire regions, so whatever recovery work might be needed, it is likely that the cost would be spread across multiple coastal communities and assets. For that reason, many vulnerable communities have emergency plans in place to help them deal with just such an event.