Temperature difference in Europe from the average during the European heat
wave of 2003
A heat wave is a prolonged period of excessively
hot weather, which may be accompanied by high humidity. There is no universal
definition of a heat wave; 
the term is relative to the usual weather in the area. Temperatures that
people from a hotter climate consider normal can be termed a heat wave in
a cooler area if they are outside the normal climate pattern for that area.
 The term is
applied both to routine weather variations and to extraordinary spells of
heat which may occur only once a century. Severe heat waves have caused catastrophic
crop failures, thousands of deaths from hyperthermia, and widespread power
outages due to increased use of air conditioning.
Definition of Heat Wave
In the Netherlands, a heat-wave is defined as period
of at least 5 consecutive days in which the maximum temperature in De Bilt
exceeds 25 °C (77 °F), provided that on at least 3 days in this period
the maximum temperature in De Bilt exceeds 30 °C (86 °F)..
This definition of a heatwave is also used in Belgium,
Denmark and Luxembourg
In the North America, a heat wave is usually defined
as a period of 3 or more consecutive days above 90 °F (32.2 °C).
Heat waves often occur during the Dog Days of summer;
indeed the French term canicule, denoting the general phenomenon of
a heat wave, derives from the Italian canicula (small bitch) applied
to the star Sirius, also known as the "Dog Star." 
Some regions of the globe are more susceptible to heat
waves than others, such as Mediterranean-type climates with a summer dry
spell which becomes much hotter than usual during certain years.
Effects on health
Severe heat waves can lead to deaths from hyperthermia,
also known as heat stroke. Older adults, very young children, and those who
are sick or overweight are at a higher risk for heat-related illness.  Heat waves are the most lethal
type of weather phenomenon, overall. Between 1992 and 2001, deaths from excessive
heat in the United States numbered 2,190, compared with 880 deaths from floods
and 150 from hurricanes.  One
public health measure taken during heat waves is the setting-up of air-conditioned
public cooling centers.
Mortality and the "harvesting" effect
Part of the mortality observed during a heat wave, however,
can be attributed to a so-called "harvesting effect", a term for a short-term
forward mortality displacement. It has been observed that for some heat
waves, there is a compensatory decrease in overall mortality during the subsequent
weeks after a heat wave. Such compensatory reduction in mortality suggests
that heat affects especially those so ill that they "would have died in the
short term anyway".
Heat waves often lead to electricity spikes due to increased
air conditioning use, which can create power outages, exacerbating the problem.
During the 2006 North American heat wave, thousands of homes and businesses
went without power, especially in California and the St. Louis, Missouri
area. In Los Angeles, electrical transformers failed, leaving thousands without
power for as long as five days. 
If a heat wave occurs during a drought, which dries
out vegetation, it can contribute to wildfires. During the disastrous heat
wave that struck Europe in 2003, fires raged through Portugal, destroying
over 3010 km² (740,000 acres) of forest and 440 km² (108,000 acres)
of agricultural land and causing an estimated €1 billion worth of damage.
 High end farmlands
have irrigation systems to back up crops with.
The European heat wave of 2003 killed tens of thousands.
Much of the heat was concentrated in France, where nearly 20,000 people died.
In early 2006, Adelaide, South Australia was hit by
a heat wave with temperatures ranging 40+ °C for five days in a row,
while Port Augusta experienced temperatures hovering around about mid 40s
°C with one day recorded at approx 48 °C.
In July 2006, the United States experienced a massive
heat wave, and almost all parts of the country had recorded temperatures
above the average temperature for that time of year. Temperatures in some
parts of South Dakota exceeded 115 °F (46 °C), causing
many problems for the residents. Also, California experienced temperatures
that were extraordinarily high, with records ranging from 100 to 130 °F
(38 to 54 °C). On July 22, the County of Los Angeles recorded its
highest temperature ever at 119 °F (48.33 °C).  Europe suffered from a massive heat wave as
well, with temperatures rising to 40 °C (104 °F) in Paris, with even
Ireland, which has a moderate maritime climate, reporting temperatures of
over 31 °C (88 °F). Temperatures of 35 °C (95 °F) were reached
in the Benelux, the United Kingdom and Germany. Many heat records have been
broken and many people who have experienced the heat wave of 1976 draw comparisons
Major heat waves
- 1901 United States midwest heat wave
- 1936 North American heat wave
- 1976 United Kingdom heat wave
- 1976 European heat wave
- 1980 United States heat wave
- 1983 European heat wave
- 1988 North American heat wave
- 1992 Venezuela heat wave
- 1993 South East heat wave
- 1995 Chicago heat wave
- 1995 European heat wave
- 1998 Indian heat wave
- 1998 Middle east heat wave
- 1998 Southern heat wave
- 1998 Mexican heat wave
- 1999 Northeast heat wave
- 2001 Eastern United States heat wave
- 2003 European heat wave
- 2003 Pacific Northwest heat wave
- 2005 Desert Southwest heat wave
- 2006 North American heat wave
- 2006 European heat wave