Heat index raised to 50 degree Celcius in Islamabad

Few days ago, heat index in Islamabad was raised to 50 degrees Celcius, as reported by a news item in The News, a local daily:

The severity of the heat wave that resulted in the ‘heat index’ soaring to record-breaking 50 degrees in most parts of the Punjab, including Islamabad.

So what exactly is "Heat Index"??

The heat index (HI) or humidex and carries is an index that combines air temperature and relative humidity to determine an apparent temperature — how hot it actually feels. The human body normally cools itself by perspiration, or sweating, in which the water in the sweat evaporates heat away from the body. However, when the relative humidity is high, the evaporation rate of the water is reduced. This means heat is removed from the body at a lower rate, causing it to retain more heat than it would in dry air. Measurements have been taken based on subjective descriptions of how hot subjects feel for a given temperature and humidity, allowing an index to be made which corresponds a temperature and humidity combination to a higher temperature in dry air.

Source: http://en.wikipedia.org/wiki/Heat_index