the AEMET rule to declare a heat wave in Spain
We spent the summer talking about heat waves and extreme temperaturesbut sometimes it is difficult for us to differentiate exactly what they are. If we look back to the summer of 2025, we remember that there was constant talk of heat waves. We may have the feeling that July and August were a huge heat wave. However, if we look at the data from the State Meteorological Agency (AEMET)we will see that only 3 heat waves were recorded in the Peninsula and the Balearic Islands and 2 in the Canary Islands. The key is that they were long heat waves. Without going any further, the first of them extended from June 18 to July 4. The next one arrived on July 15, so we didn’t even have two weeks of respite. Be that as it may, the concept of a heat wave is somewhat diffuse. It is not described the same in some countries as in others, and even has variations in the same country. Of course, no matter what definition we stick to, it is clear that they are becoming more and more intense. That is why it is so important to take measures against global warming. What exactly is a heat wave: the scientific definition Actually, There is no single definition of what a heat wave is.. Broadly speaking, it can be considered a prolonged period of extreme temperatures for a specific region. As we have already seen, each country has its own definition, which usually follows historically selected criteria. Even institutions can have very specific definitions. For example, the World Health Organization (WHO) and the World Meteorological Organization (WMO) define a heat wave as an unusually hot and dry or hot and humid period in a specific place, with visible effects on nature and people’s health. Contrary to what usually happens with the definitions of different countries, these two institutions include the health of people and the effects on nature as important factors in the definition of a heat wave. The three requirements of the AEMET to declare a heat wave in Spain In the case of Spain, the criteria to describe a heat wave They are provided by the AEMET. According to this, three requirements must be met: Duration of at least 3 consecutive days Detection of extremely high temperatures in at least 10% of the reference observatories Maximums located above the 95th percentile of temperatures measured between July and August from 1971 to 2000. These criteria refer to mainland Spain and the Balearic Islands.. With the Canary Islands there is an exception, since they only have 6 observatories. If we obeyed the 10% rule, it would be enough to detect extreme temperatures above the guidelines in only one of them. It is too little, so there is a special requirement that at least two observatories detect temperatures above the 95th percentile. How long does a heat wave last? There is no defined duration for a heat wave. As we have seen, in Spain it must last at least 3 consecutive days to be considered as such. And from there to heaven. At the moment, the longest that has been recorded in Spain lasted 26 days and It was produced in 2015. It should be noted that in 2022 the heat waves were shorter, but it stands out for being the year with the most heat wave days added together. A total of 41, to which 2025 came dangerously close, with 33 days under a heat wave. Other countries, according to their criteria, have experienced heat waves that may be shorter, but very surprising due to their location. For example, in 2025, the subarctic regions of Norway, Sweden and Finland recorded their worst heat wave since records exist, with 21 consecutive days and temperatures that exceeded 30°C within the Arctic Circle itself. Why does the temperature threshold change depending on the province? As we have already seen, for the AEMET, one of its criteria when defining a heat wave is that the reference stations register temperatures above the 95th percentile measured in July and August from 1971 to 2000. Logically, each province will have different records. The records of Seville will not be the same as those of Oviedo. The 95th percentile temperature will be much higher in the Andalusian capital than in the Asturian capital. Be that as it may, it should be noted that there is no threshold per province, but rather per reference station. Each province may have several of these seasons, with slightly different temperatures. To give an example based on AEMET datain Jaén there are three seasons, whose thresholds are 39ºC, 40ºC and 42ºC. If we go to colder areas, in A Coruña there are three seasons and their thresholds are 29.2ºC, 30ºC and 31.6ºC. How to know if there is a heat wave alert in your autonomous community The best way to know if there is a heat wave in our autonomous community or an alert for intensely high temperatures is to stay up to date with AEMET updates. In any case, it should be noted that, individually, we are more interested in being up to date with the notices. We may not have a heat wave for a few days, because there are not enough stations in the region that meet the criteria, but there may be temperatures high enough in specific points to put people’s health at risk. Therefore, we must make a monitoring unusually high temperature alerts; which, as with rain or wind, follow a color code (green, yellow, orange or red) to indicate the level of risk and the measures to take. Real-time weather warning map: from yellow alert to red warning Unusually high temperature alerts are defined with three colors. Yellow refers to significant risk, orange to high risk and red to extreme risk.. If the area is colored green (or gray in the case of maps), there is no risk. Every day, the AEMET updates its color warning maps, so it is important … Read more