Environment

Rainfall

The Annual average rainfall in the London region is 756 mm, ranging from a monthly average of 46 mm in July to 79 mm in December and January. The monthly average minimum temperature ranges from 0° C in January to 11°C in July and August. The monthly average maximum temperature ranges from 7° C from December to February to 22° C in July.

Temperature

The Greater London Assembly has published worrying predictions for the consequences of global climate change (GLA, 2002). There has been an increasing frequency of particularly dry summers since the mid-1970's. Over the same period, throughout the Thames region, winter rainfall has increased, resulting in a greater contrast between winter and summer cloud-cover and rainfall. Between now and the middle of the century, the predictions for the region are that winters will become warmer by 1 to 2° C and summers by 1.5 to 3.5° C, that winters will become wetter by 10 to 20 per cent and summers drier by 20 to 40 per cent, and that there will be more frequent severe winter storms, with mean winter wind speeds increasing by as much as 10 per cent. By the 2080's it is predicted that summer soil moisture will reduce by up to 50 per cent. At its most mundane, we can expect an even bigger problem of muddy paths in winter unless our pathways improvement project can be funded. Of much more serious concern is the impact that these changes will have on all the Commons' habitats.

Geology

The Commons cover the summit of a hill, the highest point of which is at around 90 metres above sea level. Much of the ground is a glacial head, with a disturbed layer of clay underlying the slightly acid or neutral, thin, sandy, gravel soils of the Blackheath beds which are typical of much of this south-eastern part of the Greater London conurbation. Being within the London basin, one of the underlying strata is chalk. In Chislehurst it lies around 40 metres below the surface, coming much closer to the surface at the bottom of Old Hill, where Chislehurst caves have been dug out. Immediately above the chalk there is a stratum of sand 10 to 15 metres below the surface, overlaid by a layer of London clay about 2 metres down. The superficial soil is sandy with rounded black flints of varying size. It is fast draining, quick to dry out and unproductive and this probably explains why there is little evidence that most of the ground covered by the Commons was ever cultivated. As a consequence of the underlying clay there are several springs around the crown of the hill and in the vicinity of the Commons, which before the building of houses and roads, would have drained south-westwards towards the Kyd Brook and eastwards towards the Cray. The ditches on the Commons are very largely not spring-fed but drain the Commons and convey run-off from the intersecting roads.