Thursday, June 17, 2010

Urban Heat Islands are Not So Simple

Here's an interesting result from a study of the urban heat island (UHI) effect in Phoenix, Arizona: minimum temperatures increased as the city was built up, but maximum temperatures decreased during at least part of the year as a result of urbanization:
Minimum temperature was most influenced by urbanization with the mean minimum temperature during the urban period exceeding that of the pre-urban period by 4.4°C (2.4°C) in June (January). A significant urban heat sink in January maximum temperatures was evident as the mean maximum temperature during the pre-urban period exceeded that of the urban period by 1.5°C....

-- Svoma BM, Brazel A (2010) Urban effects on the diurnal temperature cycle in Phoenix, Arizona. Climate Research 41:21-29

I suspect this depends a lot on local geography, but it just shows that UHIs do not necessarily make the region warmer. (And, just to be complete, their influence on global temperature averages is very small.)

1 comment:

Dano said...

This topic is one of my minor specialties, and I wrote a conf paper on the UHI and listened to many others talk about their work at that conf.

Anyway, I suspect the reduced max temps are due to the addition of lawns and trees, causing increased evapotranspiration and thus cooling during the ET periods (j~sunup - ~sundown-ish).

My 2¢.