Heatwaves: What are they and why do they occur?

Heatwaves and high temperatures have struck with greater frequency and intensity in recent years. But why do heatwaves happen in the first place and what threat do they pose to our health?

The past few decades have marked a shift in how we experience extremes of heat. Not only have heatwaves grown in their frequency, with each year bringing more and more days surpassing the heatwave threshold, but their severity has grown alongside. With this brings a series of health and societal consequences, the extent of which we are only beginning to uncover.

So, what are heatwaves and how will their acceleration impact us now and in the future?

What is a Heatwave?

At its core, a heatwave is simply when a given region experiences an extended period of hot weather. Temperatures seen during heatwaves greatly exceed those usually seen in that region at that time of the year, with the threshold above which a heatwave is said to occur being dependent on the local climate and geography.

As such, there is no internationally agreed definition for what constitutes a heatwave.

For the UK, the definition as provided by the Met Office states that: “A UK heatwave threshold is met when a location records a period of at least three consecutive days with daily maximum temperatures meeting or exceeding the heatwave temperature threshold.”

As different portions of the UK experience varying averages in temperature, this threshold is defined on a county-by-county basis. These thresholds are based upon the average temperatures in each county over a given reference period, here 1991-2020.

Source: Met Office

Hence, if the same temperature were seen concurrently in Glasgow and in London, it is possible that one city might be in the grips of a heatwave while the other, definitionally speaking, is not.

Alongside heatwaves, we can often see what is known as ‘tropical nights’. This is when nighttime temperatures fail to drop below 20°C, resulting in discomfort when individuals are trying to sleep and often insomnia. Tropical nights can greatly contribute to the overall burden of heatwaves.

What causes Heatwaves?

Heatwaves are complex weather events with a series of underlying meteorological factors contributing to them. However, they are generally attributed to high pressure systems.

A high pressure system develops when air descends from the Earth’s upper atmosphere and accumulates at its surface. This air begins to compress as it sinks, which is what results in the area of higher pressure. As it does so, the amount of energy that can be stored by the air increases, leading to overall higher temperatures.

This process of heating also increases the amount of water that the air can hold, prohibiting the formation of clouds, which is why high pressure systems are typically associated with clearer skies. The knock-on effect of this is that there is nothing to impede the Sun’s rays, leading to further increases in temperature.

Source: BBC Website

Often, high pressure systems can become ‘blocked’ over a certain area, simply meaning that other weather patterns nearby are preventing it from becoming dislodged, which results in periods of persistent hot weather, driving more long-term extremes in heat.

What are the health impacts associated with Heatwaves?

Extremes of heat can present an extensive health risk. Heat-related illness (HRI) can present in a variety of different ways, yet manifest when the body is unable to reduce its temperature due to conditions in the external environment, namely high heat and humidity.

At the more benign end of the spectrum, we see that overheating can cause more mild symptoms like heat rashes, also known as ‘prickly heat’. These rashes present as red stinging patches on the skin that predominantly form in areas where sweat gets trapped, including inside your elbows and behind your knees. Alongside this, many people experience muscle cramps in high heat, particularly when exercising, but this symptom is seen more as a nuisance than anything life threatening.

However, more severe HRI includes heat exhaustion and heatstroke, both of which develop when the body’s mechanisms for cooling down, namely sweating, is insufficient to keep the body temperature within a healthy range. This leads to a number of physiological symptoms, ranging from dizziness and fatigue, to nausea and vomiting, to hallucinations and aggressive behaviour.

Heatstroke in particular is a medical emergency and, if left untreated, can easily result in death.

These examples of HRI all present in the short term. Yet the impacts of heat exposure can also exacerbate underlying and long-term health issues.

One major concern is the impact of heat, particularly chronic heat exposure, on our cardiovascular health. As temperatures rise, our bodies need to work harder to maintain an optimal core temperature, which puts greater strain on our hearts. Therefore, for individuals with underlying cardiac and respiratory issues, periods of hot weather can present a greater risk of heart attacks, arrhythmias, and heart failure.

This is much more common in certain individuals, namely those who are very old or very young, those who are overweight, and those who have unhealthy lifestyles, such as smokers and heavy drinkers. This explains why the majority of heat-related deaths occur, at least in the UK, in individuals over the age of 65.

Alongside these physical symptoms of HRI, more and more research is emerging showing a link between periods of extreme heat and an increase in or exacerbation of mental health issues.

What are the societal costs of Heatwaves?

Heatwaves also present a number of challenges at a societal level, having the potential to take an extensive economic and infrastructural toll on human systems and, as a result, impact the way we live our lives.

Some of the more obvious ways in which heat can affect human systems is their direct impact on our lived environment - during periods of extreme heat, wildfires can easily take hold and cause extensive damage. These are caused when the hot, dry conditions usually associated with high pressure systems persist for several weeks, baking the ground and dehydrating vegetation. This can turn wide areas into a tinderbox, which can be lit with the smallest spark or flame.

The immediate impacts of this include the destruction of any housing or property in the vicinity, the loss of potentially extensive tracts of habitat, and cause widespread air pollution. Each of these, in turn, have broad economic and environmental impacts that are still being defined.

More broadly, the economic impacts of heatwaves can be profound. Not only do we need to contend with impacts on our built infrastructure and the local environment, but they can impact us in more subtle ways.

The impact of extreme heat on our ability to work could be huge; high heat can greatly impact worker productivity across broad sectors of the economy, through reducing one’s ability to concentrate or operate as effectively. Whilst the overall impact is challenging to quantify, research has shown that this effect alone already contributes to reducing GDP by billions of dollars annually.

Are Heatwaves linked to the climate crisis?

It should come as no surprise that, in a world where the climate seems to break a new temperature record every year, global warming is intrinsically linked with the incidence and severity of heatwaves across the world.

Generally speaking, higher global temperatures will lead to us facing hotter, more frequent, and longer-lasting heatwaves.

As climate systems have more energy within them, owing to hotter temperatures, weather events will more frequently tip over the heatwave threshold and, when they do, they will likely come with more extreme temperatures.

Research is also starting to show that, as the climate has warmed over the past few decades, the high pressure systems that drive heatwaves - often referred to as ‘heat domes’ - are now slower moving than they were previously. This means that their effects will last much longer, exacerbating the risks associated with them.

What happened in the UK Heatwaves of 2022?

Much of the work that we conduct here at Shade the UK started directly following the deadly UK heatwaves of 2022. Here, the UK suffered through three separate heatwaves - the first in June, the second in July, and the last in August.

During the heatwave in July, the UK saw unprecedented high heat. On the 19th of that month, the UK heat record was smashed as temperatures of 40.3°C was recorded in Lincolnshire, breaking the previous record by 1.6°C.

This heatwave was incredibly representative of how they are changing for the worst and underscores the importance of our work. According to an analysis conducted the year following, this heatwave also broke records in terms of how deadly it was; in England alone, the number of excess deaths over this period exceeded 4,500, for a heatwave that lasted just 3 days.

What can be done about Heatwaves?

We at Shade the UK have committed ourselves to, one day, achieving zero heat-related deaths in the UK. Through a combination of raising awareness, working with industry to build a safer future, and driving advocacy around extreme heat, we hope to protect the health and wellbeing of the whole UK population in the face of this rising threat.

In particular, in unison with our sister company, Love Design Studio, we seek to promote a future whereby our built infrastructure is made more resilient to extremes of heat. This is known as climate adaptation. Whilst limiting carbon emissions and thereby preventing climate change in the first place - otherwise known as climate mitigation - is the best way to effect this change, the lack of leadership in this arena has necessitated a new way of thinking.

Climate change has already taken place, with many billions of people around the world now subject to the worst of its effects. In the absence of the change necessary to prevent climate breakdown, the next best option is to protect ourselves against its impacts.

Many of the projects that we work on are alongside partners that see the value in adaptive approaches to climate change and overheating. For instance, we have recently completed some work alongside the British Red Cross to define how individuals, such as yourselves, can modify their homes to be more robust as the mercury soars. We have also initiated a project with Holborn City Council, to identify how we can reshape the local neighbourhood to prevent overheating in key hotspots, thereby protecting local residents.

Our vision can be summarised in our ‘Theory of Change’, which can be found here.

To learn more about our work at Shade the UK, please email us at info@shadetheuk.co.uk.