We’re used to hearing complaints about damp cold, but it’s definitely worse when humidity accompanies high temperatures. Just a few weeks ago we were talking about a heat wave that swept through India, where extreme heat combined with high humidity created a death trap. We’ve seen thermometers rise in Spain these days, so it’s worth asking if we’re likely to find ourselves in a situation where the humid heat can play tricks on us.

How is the heat index measured?
Wind chill is measured in different ways at cold (wind chill index) and warm (heat index) temperatures. It is measured in cold weather as a function of temperature and wind, and with heat begins to depend on temperature and relative humidity. This last measurement also corresponds to the heat index.

The State Meteorological Agency, AEMET, (which is the one who calculates the thermal sensation), explains that it is “a measure that helps to assess the greater difficulty the body has in lowering body temperature through the evaporation of sweat from the skin.” to the influence of ambient humidity. The heat index begins to be controlled from 26ºC and 40% relative humidity. The formula by which it is calculated is complex, but the effect is simple: at high humidity, the index adds a few points to the temperature.

AEMET table summarizing the equivalences between the heat index and its components (temperature and relative humidity).

How is this translated?
Thus, at 27ºC and 45% humidity, the index will be 27, but if the humidity rises to 80%, the index will be 30, so the thermal sensation will be 30ºC. Similarly, at 32ºC and 45% humidity the index will show 33, but again if the humidity rises to 80%, the wind chill will reach 44ºC.

Caution is recommended when the index reaches 27. If the index is between 33 and 40, AEMET advises you to be very careful anyway. We are talking about danger between 41 and 53, and from a heat index of 54 the extreme danger level is reached. There are other tools to calculate this index.

example of Spain.
It’s clear that hot and humid places like India can suffer from this problem, but what about Spain? The climate in most of the Iberian Peninsula is quite dry and even drier in summer. However, relative humidity between 45% and 50% is sufficient to pose a hazard during heat waves, as these levels mean that a temperature of 35ºC feels like a temperature of about 40ºC. If the mercury had reached 40º, we would already be on the verge of the extreme danger zone.

For reference, suffice it to say that the average relative humidity in Extremadura in July is around 45%, and in Madrid it remains at 40%, but averages in Andalusia and Murcia. It is around 60%.

Add to this the uncertainty created by climate change, which can have (not necessarily in the same direction) effects on both temperature and humidity.

It is not the same thing as heat waves.
The heat index is not directly related to heat waves. There is no precise definition of what constitutes a heatwave, but referring to the definition given by AEMET, a heatwave would be a situation in which higher temperatures are reached for at least three days”[al] 95% of the maximum daily temperature range for July and August for the period 1971-2000 (at least 10% of the weather stations.

The 95th percentile is calculated by sorting all the days of July and August of the reference years by temperature, and then the 95% group of the coldest days is taken. The hottest of those days will be the day marking the 95th percentile (alternatively, 5% of the hottest days can be taken as the coldest day, the result will be similar or the same).

Why is moist heat dangerous?
Humid heat, embarrassment, can be particularly dangerous to health. Our bodies have various mechanisms to control our body temperature, but moist heat makes it difficult for this internal “thermostat” to work.

The problem arises because one of these mechanisms we use to lower body temperature when it gets too high is sweat. Our body expels sweat, especially water, which absorbs this body heat. As it evaporates, this heat passes to the environment. But sweat’s ability to evaporate depends on the humidity of your environment: The more humidity there is, the harder it is for sweat to evaporate. Sweat that does not evaporate remains as a layer of hot water in our body and therefore does not perform the function of thermoregulation.

Heat is especially dangerous for older people, but there are many factors that influence this risk. The Department of Health cites personal factors such as age or suffering from certain diseases, social and work factors such as socioeconomic status and workplace heat exposure, and local factors such as the climate itself.

How to deal with damp heat.
When the heat comes on, the first thing we think about is trying to escape from it. Going to the beach or pool is the preferred option for many, especially when turning on the air conditioner is a luxury (more than in other years). Besides consuming current like air conditioners, fans can be useless and even backfire when the temperature rises above 32ºC. There are many other strategies for the rest of the day, including heeding the usual recommendations, such as avoiding sun exposure or physical activity during the hottest hours of the day.

Cover image | Daniel Gonzalez / GTRES