London is regularly 3-4° warmer than the towns and village that border the M25 ring road. You can genuinely feel the difference when you take a train into the city and vice versa.

Is that allowing for the time difference between when you get on and get off the train affecting the temperature?

Absolutely. If you so wish you can take the train at noon and get off at 12:30 where any temperature difference due to the sun should be minimal.

Not terribly relevant to your point, but: Noon is actually a time in the day where it is still getting warmer. Temperature usually peaks around 16:00 or 17:00 in the summer.

Example: http://wetterstationen.meteomedia.de/?station=104870&wahl=vo... (the forecast diagram for my local weather station)

any idea why? The ground starts releasing accumulated energy or what?

It probably reaches saturation at that time of day where it can no longer absorb anymore heat.

Because the sun is still heating faster, than heat dissipates at 17:00.

From the article: 'If the urban heat island theory is correct then instruments should have recorded a bigger temperature rise for calm nights than for windy ones, because wind blows excess heat away from cities and away from the measuring instruments. There was no difference between the calm and windy nights, and one study said that "we show that, globally, temperatures over land have risen as much on windy nights as on calm nights, indicating that the observed overall warming is not a consequence of urban development."'

This is really surprising to me. Not least because, on a sunny day, SF can go from pleasant to brutal as soon as a breeze kicks up.

Not only in US. This is applicable to all the major cities in the world.

isn't this also (at least partly) why cities are less likely to be affected by tornadoes?

Quoting the link:

> Research has been done in a few areas suggesting that metropolitan areas are less susceptible to weak tornadoes due to the turbulent mixing caused by the warmth of the urban heat island.

So every big city pretty much

Sydney is one of those.

Aren't all cities one of those? Bigger=bigger impact.

I think it has quite an effect in the summer because the city is cooled by being near the ocean but the western suburbs get super hot, so a 10 degree difference might not be unusual at times, for two places about 30km away.