The samples below demonstrate METARs Downloader output that is produced when the conditions are right for the Heat Index to have an effect. This point is defined as when the Temperature is 80°F(26.6°C) and above and the Relative Humidity is 40% and above.
Visit Wikipedia for a good page on Heat Index: http://en.wikipedia.org/wiki/Heat_index
METARs Downloader only includes the Heat Index when the conditions described above are met and it has been enabled in Options.
The data used to create the samples is from ICAO station code KMIA, Miami Florida USA. and are not current.