Tuesday, July 10, 2012

Quit Your Crying!

Source: zazzle.com
Yesterday's high at the Salt Lake City International Airport was 100ºF.  The National Weather Service forecast highs for today and tomorrow are both 104ºF.  Yup, that's hot, but the dewpoint each afternoon should be in the low-to-mid 40s.  Assuming a dewpoint of 45ºF, that gives us a heat index of only 100ºF.  The heat index is an estimate of what the temperature feels like to humans from the combined effects of temperature and moisture.

For Utahns, that is hot and uncomfortable, but as the shirt suggests, quit your crying.

Washington-Dulles International Airport hit 104ºF on June 29.  Their highest hourly temperature was 101ºF, when the dewpoint was 68ºF, which equates to a heat index of 107ºF.  At peak temperature, their heat index was probably about 110ºF.

It can be worse.  During the 1995 heatwave, which caused at least 830 deaths in the midwest and 525 deaths in Chicago (Changnon et al. 1996), the heat index reached 119ºF at O'Hare airport and 125ºF at Midway Airport.  A lesson to be learned from this and other heat waves is that mortality is influenced not only by the maximum temperature, but also the humidity, minimum temperature, and air quality.   Extended periods with high humidity, elevated overnight temperatures, and poor air quality create serious physiological stress.  Societal factors also play a critical role.  As noted by Changnon et al. (1996):
"The primary victims of the 1995 heat wave, as in past heat waves, were older persons in large cities within the heart of the urban heat island. Urban areas, particularly older U.S. cities, are particularly vulnerable to heat waves. Many older citizens in low-income areas have no air conditioning or cannot afford to operate systems, and they fear use of open-window ventilation at night because of high crime rates in their neighborhoods. Many people have also forgotten how to "live and function" with high temperatures and need continuing education and reminders when heat waves approach. This is particularly true in more northern cities of the nation where extreme heat is not common."
This is perhaps a good reminder that even though things aren't as uncomfortable in Salt Lake this week, it might be worth checking on elderly neighbors to make sure they are comfortable.


  1. My roommates were asking about this so I did a quick wiki search on how the heat index was estimated. I enjoyed reading on the 172 degree F day.

    "However, in Dhahran, Saudi Arabia on July 8, 2003, the dewpoint was 35 °C (95 °F) while the temperature was 42 °C (108 °F), resulting in a heat index of 78 °C (172 °F). This is comparable to the temperatures that are recommended to kill bacteria in many meat products, and it is common in a sauna."

  2. It also seems like the heat index does not account well for high levels of physical activity. According to the heat index calculator on the NOAA site, (http://www.hpc.ncep.noaa.gov/html/heatindex.shtml) a temperature of 77F with a dew point of 75 yields a heat index of 79, as does a temperature of 83F with a dew point of 0F (extremely dry). However, if you have gone outside for a run in both of these scenarios, you can recognize right away that one of these situations (the dry one) is far more tolerable than the other. On the other hand, if you are sitting in the shade, your comfort level might be nearly identical between these two scenarios.

  3. Perhaps we need a heat index that includes solar intensity on the body? My experience is that the sun here in Utah feels scorching relative to back east.

  4. Admittedly, the heat index is a fairly crude measure of the effects of atmospheric conditions on the body. There's some background at http://en.wikipedia.org/wiki/Heat_index, which is hopefully correct.

    There is something called the "wet bulb globe temperature" that was developed by the military in an attempt to integrate temperature, wind, humidity, and solar radiation effects on humans. More information is here http://en.wikipedia.org/wiki/Wet_Bulb_Globe_Temperature