What is a Heat Wave and What Are Its Effects? (Heat Wave Part One)

An informative article detailing what constitutes a heat wave, and what the detrimental effects may arrise during a heat wave.

For those of us who live in countries with distinct seasonal weather changes, summer is often a long-awaited relief from the biting frost or gloomy dampness of winter.  But now that summer is here, and we have languished in the growing warmth and longer daylight hours, we might also be facing hotter than normal temperatures, and perhaps even a heat wave.  

What, exactly, constitutes a heat wave?  There is no universal definition of a heat wave; rather, it is subjective to what your region's "normal" is for that time of year.  However, a good guideline for determining what constitutes a heat wave is one recommended by the World Meteorological Organization :  when the daily maximum temperature of more than five consecutive days exceeds the average maximum temperature by 5 Celsius degrees or 9 Fahrenheit degrees (the normal period calculated from 1961–1990).  The weather patterns that are responsible for this are a combination of things: 

  • a high pressure system with little or no cloud cover or rain;
  • jet streams positioned so that hotter air is directed toward already over-heated areas;
  • hot air above tropical seas are moved inland by turbulent winds;
  • high density urban centres, or "concrete cities", which lack green spaces for cooling overnight;
  • lack of air movement;
  • a combination of high heat and high humidity-- referred to as the Heat Index (HI) or the Humidex (in Canada)--makes the heat feel even hotter than the actual thermometer reads. An example provided by Campbell Scientific, Inc. is "if the actual temperature is 100°F with 40 percent relative humidity, the heat index is 110°F meaning the apparent temperature feels like 110°F to the body."   In Canada, the Humidex is calculated using the dew point, rather than relative humidity. An example found at Wikipedia is "when the temperature is 30 °C (86 °F) and the dew point is 15 °C (59 °F), the humidex is 34 (note that humidex is a dimensionless number, but that the number indicates an approximate temperature in °C). If the temperature remains 30 °C and the dew point rises to 25 °C (77 °F), the humidex rises to 42."  The heat index and humidex figures are based on temperature measurements taken in the shade and not the sun, so extra care must be taken while in the sun.
Heat Index Temperature

Effects (shade values)

27–32 °C / 80–90 °F Caution — fatigue is possible with prolonged exposure and activity. Continuing activity could result in heat cramps
32–41 °C / 90–105 °F Extreme caution — heat cramps, and heat exhaustion are possible. Continuing activity could result in heat stroke
41–54 °C / 105–130 °F Danger — heat cramps, and heat exhaustion are likely; heat stroke is probable with continued activity
over 54 °C / 130°F Extreme danger — heat stroke is imminent

(Chart information courtesy Wikipedia)

This is an example of what can happen during a 5 day heat wave, when people are unprepared:

Eric Klinenberg, author of the book Heat Wave: A Social Autopsy of Disaster in Chicago, documents from his research of the July,1995 Chicago heat wave, (one of the worst in US history) the following:

  • On the first day of the heat wave, the temperature hit 106 degrees, and the heat index—a combination of heat and humidity that measures the temperature a typical person would feel—rose above 120.
  • Approximately 600 heat-related deaths occurred over a period of five days.
  • Children riding in school buses became so dehydrated and nauseous that they had to be hosed down by the Fire Department.
  • The city set new records for energy use, which then led to the failure of some power grids—at one point, 49,000 households had no electricity.
  • The heat made the city's roads buckle. Train rails warped, causing long commuter and freight delays.

In fact, in the United States, summer heat waves alone cause more deaths than all other weather events combined, including lightning, rain, floods, hurricanes, and tornadoes. (Wikipedia)  Health problems that can arise from prolonged heat exposure range from mild to severe:  heat exhaustion, muscle cramps, fainting (syncope), dehydration, heat stroke, and extra stress and strain on internal organs, especially the heart.  Heat stroke is a severe condition when the body's core temperature rises from 37 °C (98.6°F) to 40°C (104°F) or above.  As your work load and body heat increases, so does the need to lose that heat through sweating or external cooling.  A core temperature of 40 or 41°C (104+ °F) is considered life-threatening if not caused by a fever.  Symptoms (courtesy of Health Canada) that you or someone may be suffering from heat stroke are:

  • a throbbing headache
  • dizziness
  • confusion
  • nausea
  • red, hot and dry skin
  • lack of perspiration

It is vital to seek immediate medical attention if it is suspected you or another person is experienceing a heat stroke.  While awaiting help, be sure to sponge him/her/yourself off with cool water, offer water to drink, and provide shade from direct sun.

In my next article, (Heat Wave Part Two) , I will provide many ideas and help to survive a heat wave, while staying healthy, safe, and happy.

Copyright July 2010 by Sharla Smith

Photo source courtesy stock.xchng; Information sources have been given credit with the body of the text.

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