Global warming is bringing more frequent and severe heat waves. It is well known that heat waves can have serious impacts on society including a rise in mortality, an increased strain on power, water and transport systems, and a possible rise in social disturbance. Critical infrastructures such as drinking water and electricity systems may fail under high demand, with power cuts greatly magnifying the number of people at risk. Emergency staff and health facilities may be overwhelmed by the scale of an unfolding crisis.

Heat waves, or periods of anomalous warmth, do not affect everyone with the same rate; it is the vulnerable individuals or sectors of society who will most experience their effects. The main factors of vulnerability is being elderly, living alone, having a pre-existing disease, being immobile or suffering from mental illness and being economically disadvantaged or homeless. Living in a city is another risk factor during heat waves, because of an urban heat island effect, that imply an increase of 3°C to 5°C in temperatures, making cities much warmer than their rural environs.

Since there are many factors behind the impact of a heat wave, its consequences are defined not only by the weather but also by the social and economic context of the city it strikes. Therefore there is not a unique definition of heat wave, but in turn, each city or district has its own according to the effects high temperature spells have on their citizens.

According to the World Meteorological Organization (WMO) a heat wave occurs when "the daily maximum temperature of more than five consecutive days exceeds the average maximum temperature by 5 °C (9 °F), the normal period being 1961–1990". In Holland and Belgium it is associated to a “period of at least 5 consecutive days in which the maximum temperature inDe Bilt exceeds25 °C (77 °F), provided that on at least 3 days in this period the maximum temperature in De Bilt exceeds 30 °C (86 °F)”. Australia has different definitions according to the state: In Adelaide for example, is defined as “five consecutive days at or above 35 °C (95 °F), or three consecutive days at or over 40 °C (104 °F)” while in Melbourne it is declared when the average between a day's high and the following day's low exceeds a certain threshold that depends on the district.

As we can see, there are plenty of definitions, but in general they require temperatures to go above a certain threshold for a number of days (usually 2 or 3).

In BerecoLabs we study the record of temperatures of the city of Buenos Aires and consider various definitions of "heat wave" to understand what frequency has presented this phenomena and also if that frequency is increasing. Figure 1 is interactive, we invite you to try it and select a minimum maximum temperature (which has been equalled or exceeded) and the duration on consecutive days for that temperature, and see when these "heat waves" in the city of Buenos Aires arose in recent years.

Figure 1. This interactive animation allows you to view the occurrence of "heat waves" in the city of Buenos Aires according to different definitions for data from 1998 until 2014. To use it you must select the maximum temperature and minimum (matched or exceeded) and the duration of "heat waves".

Figure 2. Frequency of the waves of heat to the city of Buenos Aires (1998-2014). As you can see there is an increase in the occurrence of "heat waves" in recent years.

Forecasting is a crucial part of providing a proper emergency response to heatwaves. While early forecasts can increase dramatically the effectiveness of the response, it is also true that precision is lost the further away we are from the event and in this situations the risk of calling a false alarm also increases.

Figure 3 compares the prediction of the occurrence of a "heat wave" with 3 days of anticipation, with what actually occurred.

Figure 3 comparison between the forecast and the actual occurrence of "heat waves" in the metropolitan area of the city of Buenos Aires.

The ability to understand and predict heat waves is also key to be able to generate policies that increase the resilience of the cities, and encourage and help the most vulnerable to extreme weather events. In BerecoLabs we are developing knowledge and approaches to achieve these goals.