Article - July 13, 2018

What Constitutes a Heat Wave Around the World?

Fred Schmude

Fred Schmude

Senior Scientist for Long Range Weather Forecasting, Houston

Cooling down in heat

Defining a heat wave can vary depending on season, place, and what typical conditions are in both. For example, from Texas to Florida on the Gulf Coast, temperatures above 90°F (32.2 °C) are quite common during summer, so temperatures would need to be much higher and last longer to be considered a heat wave, whereas 90°F (32.2 °C) in the north, where hot temperatures are rarer, would easily fit the bill. Makes sense, right? Not so fast, it gets a bit more complicated than that.

The U.S. National Weather Service defines a heat wave as: A period of abnormally and uncomfortably hot and unusually humid weather, typically lasting two or more days. Note the word, “uncomfortably.” This makes things relative and relies greatly on the availability of one of the greatest contributions to mankind: air conditioning.

The last factor when defining a heat wave is the heat index, which measures what the temperature feels like rather than what it actually is by combining the temperature with the level of humidity. This is why, in places of high humidity, the heat index must be taken into account before declaring a heat wave. Although the temperature may only be 25°C (77.0°F), it may feel much hotter, or more uncomfortable. This hotter feel is due to the fact that humidity makes evaporation (in this case, sweat) less efficient, making it more difficult to naturally cool down your body.

heat index

In an attempt to understand what is considered ‘uncomfortable’, here is the general criteria that varying places in the U.S. and Europe use to define a heat wave.

  • In the Netherlands, a heat wave is defined as a period of at least five consecutive days in which the maximum temperature in the city of De Bilt exceeds 25°C (77°F), provided that on at least three days in this period the maximum temperature here exceeds 30°C (86°F). This temperatures are also used as the definition in Belgium and Luxembourg.
  • In Denmark, a national heat wave is defined as least three consecutive days during which the average maximum temperature across more than 50 percent of the country exceeds 28°C (82.4°F). In addition, the Danish Meteorological Institute proclaims a "warmth wave" when the same criteria are met with slightly a lower temperature of 25°C (77.0°F).
  • In Sweden, the definition is a bit simpler: at least five consecutive days with a daily high that exceeds 25°C (77.0°F).
  • In the Northeast United States, a heat wave is typically defined as three consecutive days where the temperature reaches or exceeds 90°F (32.2 °C). However, this temperature is often combined with humidity levels to determine the heat index threshold.