If you’ve ever found yourself struggling to stay comfortably cool on a hot day or warm night, it’s very likely had more to do with humidity than air temperature.
When the relative humidity is high, even a warm (say, low 20s °C) day can feel unpleasantly ‘sticky’, and it can be easy to become overheated doing even moderate exercise.
The interplay between these two ultimately determines how difficult it is to stay cool and maintain good health.
It’s combinations of high heat and humidity that are responsible for most heatwave-related deaths. In the US was on average 153 per year across 2013-2022, more than the combined total of all other weather-related disasters.
Within an airmass of near-static moisture content, as air temperatures rise, relative humidity falls, increasing the amount of ‘room’ in the air for moisture to evaporate into. This increases how efficiently sweating cools our bodies, offsetting some of the effect of the increase in air temperature.
Its why, on rainless days with a lot of sunshine, conditions tend to feel less ‘sticky’ in the afternoon compared to overnight.
So, given a long-term upward trend in air temperatures, if there was no change in typical atmospheric moisture content, relative humidity would reduce worldwide, helping us to stay cool.
Unfortunately, the typical moisture content is increasing.
The graphs below show the summertime means of specific humidity for the northern and southern hemispheres from left to right. This is a measure of the mass of water vapour per kg of (moist) air.
The northern hemisphere shows a sharp upward trend since the late 1970s, while the southern hemisphere has a gentler upward trend since 1950. Notably, the most recent decade is by far the highest in the northern hemisphere.
Plotting this on a map reveals the extensive nature of this increase, affecting all but a few small regions of the northern hemisphere, and almost the entire southern hemisphere (outside Antarctica).
Quite simply, there’s more evaporation from water bodies or wet surfaces, and more transpiration from plants (collectively, evapotranspiration).
It’s a response to higher air temperatures that we’re all very familiar with; things tend to dry out quicker when its hotter.
Importantly, as air warms, its capacity to hold moisture also increases, so it doesn’t uniformly ‘fill up’, so to speak. Not that it would help us if it did, as that would raise relative humidity to 100%, at which point sweat doesn’t evaporate at all!
Ultimately, the warmer the global climate, the larger a proportion of Earth’s water is stored in the atmosphere instead of water features.
The Heat Index is a measure designed to represent the danger to health posed by certain combinations of air temperature and relative humidity.
The long-term upward trend in specific humidity means that for a given air temperature, the relative humidity now tends to be higher.
Consider, for example, what just a 5% increase in relative humidity does to the Heat Index at an air temperature of 34°C (a hot day by the standards of most): A 2-3°C increase depending on the start point.
Once up at 40°C – a very hot day – the effect is even stronger; a 2-4°C increase per 5% relative humidity.
In short, by reducing the effectiveness of sweating for cooling down our bodies, increasing atmospheric moisture content is adding to the health stress posed by high temperature events.
For us humans, this is a particularly big deal when the air temperature nears or exceeds human average body temperature (37°C). Then, radiating heat to cool down becomes ineffective, and we depend entirely on sweating (panting doesn’t really work for us).
Well established climate models, drawing upon the might of some of the world’s most powerful supercomputers, paint a concerning picture of the future in a warming climate.
They predict that if global mean temperatures reach 2°C above the pre-industrial mean (the target set by the Paris agreement), there will be a 3-10 times increase in exposure of human populations in the mid-latitudes (e.g. USA; Europe; South Africa; southern Australia) to dangerously high heat index conditions.
Without drastic measures to limit warming, it’s considered likely that by 2100, dangerously high heat index conditions will occur almost daily in inhabited tropical regions.
This all amounts to a substantial increase in the frequency and typical duration of heatwaves that pose a high mortality risk to anyone without reliable access to cooling measures such as air conditioning.
For every tenth of a °C that we can limit climate warming in the decades ahead, these unpleasant escalations will be reduced, saving countless lives.
James Peacock MSc
Head Meteorologist at MetSwift
Featured photo by Chris Weiher on Unsplash
