But there might be an even more animalistic explanation why a cloudless sky buoys us up: "When it's good weather there's much more flesh on show, which cheers most people up, " says Dr Workman.

BBC: Call that a summer?

But a clear, cloudless day-time sky is blue because molecules in the air scatter blue light from the sun more easily than they do red light.

BBC: rainbow

The sky is still a perfect cloudless blue and the forecasters have dropped their previous prediction of rain by the weekend.

BBC: US crops tell story of future world food prices