In uk.sci.weather on Wed, 28 Dec 2005 at 08:35:33, Mike Tullett wrote :

Is there a rough formula which can be used to judge their effects -
based perhaps on population and/or city size?

The centre of Cheltenham tends to be a little warmer, but the effects
seem to be minimal more than 1 mile out.

TR Oke's work in the 70s and 80s is as good as any on this subject. He
showed a good link between the magnitude of the effect and ln population.
There seems to be quite a lot online e.g.

http://www.epa.gov/heatisland/resour...ngResource.pdf

http://www.actionbioscience.org/environment/voogt.html

http://64.233.183.104/search?q=cache...rshi p.org/co
oltoronto/pdf/uhis_oke.pdf+oke+heat+island+cloud+wind&hl=en

Most studies were conducted in the ideal conditions of light winds and
clear skies, when the effect is a maximum.

Alas, not common conditions in the UK...

I recall Oke also suggested a
few simple formulae incorporating wind speed and cloud cover.

Alas, I lack the means to measure wind speed.
--
Paul Hyett, Cheltenham