The Heat Index (HI), a measurement of the effective temperature felt by the human body, is based on both 2 m air temperature and relative humidity (RH) values. This National Weather Service (NWS) index, however, is generally calculated by use of only airport data. It thus cannot account for urban heat island (UHI) effects, which would raise the temperature values used in its calculation and thus create greater HI values and thus a more accurate estimate of the danger to human populations.

The current study thus uses 12 mesoscale sites around Portland, Oregon to map the UHI and resulting HI fields during the heat wave of 20-24 July, 2006. Past studies have observed UHIs in the area, but temperatures during this heat wave were unusually high due to a combination of synoptic influences: high temperatures aloft and a surface high pressure region. The associated surface southerly flow of moist air also produced high RH values during both daytime (which raised HI values) and nighttime (which kept min temperatures high) periods.

Results showed two separate Portland mid-day UHI (of up to 16 F) centers, divided by the Willamette River that flows through the city. The UHI thus produced significant differences in the HI values across the city, with the highest variability during the 22nd of July. HI values from the airport NWS site were much lower (up to 20 F) than those from the center of the UHI. An urbanized HI needs to thus be considered (either from mesoscale observations, statistical extrapolation, or mesoscale modeling) when forecasting HI values during heat waves.