For a heat wave to be declared, a generally prolonged period of high temperatures and humidity must be established. One day or more constitutes a prolonged period, but it is common for heat waves to last for periods that go from several days to several weeks.
When a heat watch, warning or advisory is issued, temperatures have been 10 degrees F over normal for a prolonged period, with high humidity that may be enough for the human body to have difficulty tolerating the extra heat. Extreme heat and humidity prevent the body from being able to cool itself through the perspiration process, where there is sweat, then exposure cool enough air to reduce body temperature. In two days or so, heat related illness will occur, unless there is artificial air conditioning to cool the environment.
In urban environments, pollution becomes trapped in stagnant air, adding to the toxicity of the air in addition to the heat.
A Heat Index tells how hot it is in relation to increasing levels of humidity that are added to the air temperature.
The term, "Heat Storm" originated in California, where Americas most record breaking temperatures occurred. A Heat storm is declared when the temperature is 100 degrees Fahrenheit for three or more consecutive days over an area of ten thousand square miles or more.
Belgium, Luxemborg and the Netherlands, amazingly, use the temperature in De Bilt, a town and region in the Netherlands as a base for declaring a heat wave. In one way, the temperature must exceed 25 C/77F for five consecutive days, AND three of those five days must have temperatures in excess of 30C/86F.
The United States National Weather service uses the following criterion:
EXCESSIVE HEAT WARNING: Two consecutive days of heat that exceeds 105F/41C for three hours or more each day.
HEAT ADVISORY: 105F/41C to 115/43C for up to three hours a day, or 80F/24C at night for two consecutive nights.
It is clear that widely varying climates in America and in other larger countries, combined with differences in humidity, pollution, and other contributors to heat, such as the heat effects of built up urban areas, makes for a widely varying definition of a major heat incident in different parts of the world. Commonly in California, pollution advisories called "Spare the air days" accompany heat advisories and warnings. These advisories tell when certain activities are called for, such as sensitive groups staying indoors, stopping burning of waste and starting up outdoor barbecues.