The days are getting shorter, the sun is lower in the sky, and yet the British climate is still capable of producing spells of great heat and humidity even this late in the summer - witness last week's brief heatwave.

The reason is not difficult to explain and is intimately related to the geography of the British Isles. Water has a much greater thermal capacity than most solid substances. This means that it takes much more heat energy to warm up a given area of ocean-surface than it does a similar area of land. The net result is that the seas around our coasts warm up very slowly during the spring and early summer, reach their warmest between mid-August and late-September, and then cool down slowly during the autumn and winter.

Thanks to our island location, whatever direction the wind blows from it has to cross a certain amount of sea, and the lowest layers of the airstream will be modified by that sea crossing. Warm winds will pick up moisture as well as being cooled by contact with the water's surface. Now, our hottest weather is normally delivered by a wind blowing from a southerly quarter. Southwesterlies originating over the sub-tropical Atlantic will clearly be much modified. Southeasterlies are only slightly modified by their passage across the English Channel or southern North Sea, but it should be borne in mind that a hot airflow of north African origin will also have had to cross the western Mediterranean.

The strength of the midsummer sun can counter the effect of a cooling sea-track and that is why we can get exceptional heatwaves in June. But in late-August - even in September - the warmth of the seas which surround the British Isles means that we can still experience great heat even though the sun may not be as strong. Moreover, it also explains why late-summer heat is often accompanied by uncomfortably high humidity levels.

This was certainly the case last week. On Monday there was very little to be seen of the sun, and yet the temperature climbed into the mid-20s C over a wide area. Even on Tuesday there were large amounts of cloud and sunshine was still rather limited, yet the mercury touched 30C in the London area. By Wednesday much of the cloud had dispersed and the temperature hit 32C in London, at Gravesend in Kent, and at RAF Marham in west Norfolk.