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 to follow these links, especially in times of heat wave.
The areas of Spain most affected by the “urban heat island effect”
Cities, unlike rural environments, tend to accumulate heat. The asphalt of streets and roads, the construction materials of buildings, the metal of parked cars… All of these are factors that contribute to heat being accumulated during the day and released during the night, so that urban areas do not cool as much as they should after sunset.
There are studies in which it has been seen that in some large cities, such as New York, the temperature can be more than 4ºC above its surroundings. There can even be a difference of 11ºC between the center and the periphery of the same city.
This effect can be mitigated by planting trees and green areas. However, in most cases these are not large enough. It has also been seen that, in coastal cities, the heat wave effect intensifies at night, possibly due to the effect of water vapor which acts as a moderator of thermal variations and retains heat in the air.
The heat island effect has not been studied in absolutely all cities in Spain. but, of those analyzedamong those most affected by this phenomenon that causes extreme temperatures are Madrid, Barcelona, Valencia, Murcia and Palma de Mallorca.
Why current heat waves are different from those of decades ago
According to some studieshuman influence has doubled the probability of heat waves in many regions. This is clear. Greenhouse gas emissions are contributing to global warming; which, in turn, increases the probability of heat waves. In addition, humans have influenced by eliminating natural refrigerators through deforestation and increasing warming through urbanization and the aforementioned heat islands. For all this, although in the past temperature anomalies were expected, Nowadays they are more frequent, intense and long.
The impact of climate change on the frequency and intensity of the Spanish summer
The Spanish summer is no longer what it was. We have all realized it. For starters, we have more extreme temperatures for longer. Specifically, the average temperature has increased 1.75°C since 1961. If we take into account that the global average in that same period is around 1.1-1.2°C of warming, we see that Spain is among the countries that are warming the fastest. Furthermore, the 12 warmest years in the entire historical series in our country belong to the 21st century.
On the other hand, as we have been seeing, each time there are more heat waves, longer ones and with extreme temperatures that leave those of a few decades ago far behind.
Evolution of heat waves in Spain
If we take as an average the temperatures recorded between 1991 and 2020, we see that in Spain thermal anomalies in summer are increasing, with a clear peak between 2022 and 2025. This graph from the AEMET shows it perfectly.
How to survive an extreme heat wave
When there is a warning of intensely high temperatures, it is important to take measures to prevent possible heat stroke.
For a start, It is important to avoid going outside during the hottest hours of the day.. Some town councils are activating climate shelters. That is, properly air-conditioned buildings that you can go to at any time of the day to protect yourself from the heat, but it is not necessary to go to these places. If we can, it’s okay to stay home and use properly fans and air conditioning. During the day it is good to close the blinds to prevent the rooms from overheating. At night the ideal is to ventilate, although the minimum temperatures are increasingly more suffocating and it becomes more complicated. Of course, we should not exercise outdoors during those hottest hours.
On the other hand, hydration is essential. We should drink a lot of water, since in the heat sweating increases and it is easier for us to become dehydrated. It is also important that we wear cool clothes.
Regarding diet, foods rich in water are a great option, while alcohol will only increase the effects of alcohol. In fact, there are already countries like France that They are restricting their consumption during heat waves.
Pregnant women, children and people over 65 years of age are the most vulnerable to the effects of extreme temperatures. Therefore, more attention should be paid to them. In addition, there are drugs that enhance the effects of heat. If you are following any treatment, check the package insert to make sure this is not the case and, if it is, reinforce all these measures.
We must prevent heat waves by combating climate change; But, until this is possible, it is important to know how to deal with them. Because they are already here and, possibly, the situation will continue to get worse.
Images | Magnificent |AEMET
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