06 May 2019

Understanding Heatwaves Before They Hit

Thermometer indicating high temperatures

“It says here in the weather report,
It's saying a front of warm air is moving in from Jamaica.
Moderately high barometric pressure will cover the North-East and the Deep South... 
Hot and humid nights can be expected.”

Official definitions of what constitutes and causes a heatwave may vary slightly, but it’s safe to say that Marilyn Monroe’s boast in There’s No Business Like Show Business that she “Started this heatwave, this tropical heatwave…” isn’t one of them.

But what she describes will sound familiar to anyone who lives in areas prone to heatwaves. Hot temperatures, high humidity, and generally miserable conditions.

What are Heatwaves and how are they created?

Put simply, a heatwave is distinctly and unusually hot summer weather in an area lasting for at least two days in a row. They occur across the globe, although they manifest differently depending on the location.

What Causes Heatwaves?

When a high atmospheric pressure system moves into an area, it pulls air from the upper levels of the atmosphere towards the ground. This compresses the air and increases its temperature. 

All this pressure concentrated on one spot blocks other weather systems from moving in. It also gets in the way of winds, reducing them down to nothing, and keeps out clouds, allowing the sun’s energy and radiant hear to become more intense in that area. Meanwhile, that area becomes hotter the longer the system remains stationary. This is why we talk about a heatwave; rather than a short spike in temperature, it can often last for weeks on end, depending on how long the system remains. During that time, the heat can become extreme and dangerous.

But why might a high-pressure atmospheric system move into an area in the first place, you may wonder?

This phenomenon is caused by a combination of factors. Let’s take a look in more detail. 

Jet Stream

As you know, the Earth is divided into two hemispheres. Most heat and energy is concentrated on the Equator, so air moves around continuously to spread this towards the poles. In each hemisphere, this moving air divides into three “cells” that circulate air into the troposphere - the lowest part of the atmosphere.

As this warmer air crashes up against the cold air nearer the poles, you get a jet stream: a narrow band of air, varying in width, that flows westwards and circles the globe with great strength. 

In the northern hemisphere, these jet streams run between the cold air to the north and warmer air coming up from the south. In the southern hemisphere, they’re between the cold air far south and the warmer air coming down from the north. You end up with two or three jet streams in each hemisphere.

The jet stream is several miles above the Earth, but as it flows high overhead it creates or changes winds and pressure systems. When it comes into contact with an area of low pressure it sucks out air like a vacuum cleaner, increasing the pressure in the process.
Over the course of the year, the jet stream shifts. When it flows fast and uninhibited, it often causes dramatic weather events along the way. When it buckles it slows down, which can allow high-pressure systems to set into an area.

In the summer, there’s a smaller temperature difference between the poles and the Equator. That means less cold air clashes with warm air and the jet stream calms down a bit. It also drifts further towards the pole in that hemisphere. As a result, high-pressure systems can move in behind it, increasing the risk of a heatwave.

The tilt of the Earth

Heatwaves typically occur during height of summer. The Earth is tilted on its axis at 23.5° and summer occurs when that hemisphere is tilted towards the sun. 

As a result, the sun’s rays hit the Earth’s surface more directly, causing it to heat up more intensely, while the longer days mean that there’s less nighttime for it to cool down again. This creates and exacerbates the conditions needed for a heatwave - and, as we’ve seen, changes the course of the jet stream.

Heatwaves in Cities

Urban areas suffer heatwaves far worse than rural ones, thanks to the Urban Island Heat Effect (UHI). A lack of vegetation means little water for evaporation, while dark, dense construction materials like concrete and asphalt absorb and hold onto heat energy. 

Cities tend to be a few degrees warmer than the countryside anyway - and whereas rural areas can release pent-up daytime heat during the night, urban ones hold onto it.

What Are the Effects?

Heatwaves can be extremely dangerous. 

The European heatwave of 2003 is estimated to have caused thousands of heat related deaths, mostly among the elderly, and billions of Euros in economic damage. Much of North America experiences heat waves annually. States and municipalities are often prepared for these circumstances, issuing warnings and providing additional emergency services like indoor shelters and cooling stations. Health impacts include heat fatigue, heat cramps, heat exhaustion and heat stroke, as well as sun burn and dehydration. 

For construction teams that need to undertake intense physical labor, these conditions pose an enormous challenge. In the US alone, every year thousands of construction workers suffer heat related injuries and illnesses. What’s more, these are often accumulative: people only feel the impact after several days in a row working out under the hot sun. 

It’s not the only potential interruption to work. Access to electricity is sometimes undermined by heat waves, as power plant performance and efficiency is hampered, and plants are under greater risk of breaking thermal pollution rules. Meanwhile, as millions of people crank up their A/C to cope with the heat, the surge in demand can overload the grid and cause brownouts and complete outages.

Even without this challenge, you may be forced to stop work simply because your materials and equipment can’t cope with the heat. Concrete and paint might not dry. Glue, sealant and other finishing materials may melt or soften. Humidity can damage wood and other materials. Certain units might break down or malfunction in the heat. 

How to Prepare Your Jobsite for a Heatwave

One serious challenge for construction companies who want to protect themselves against the effects of heatwaves is that governments lack a clear definition of what constitutes a heatwave and exactly when to deliver warnings.

That means you need to be proactive about the situation, covering all bases yourself. 

The most important consideration is, of course, your team’s well-being. In the summer months, insist that your team wear sunscreen and provide helmets and vests with built-in cooling technology like increased airflow, fans and/or space for ice packs. 

Ensure that everyone has plenty of water, schedule regular breaks and try to organize shifts away from the hottest times of the day. 

It’s well worth looking into comfort cooling and dehumidification options, as well as tenting off work areas to provide some respite from the sun’s glare. Since dark materials absorb heat, white tents and covers will help to reflect back some of the sun’s energy.

Recognizing that a heatwave could cause power outages and equipment failures, determining a backup plan long in advance can help mitigate any bottlenecks in your work schedule and progress.

How will you get access to power and other important utilities quickly in an emergency shutdown? Which process trains rely on each other? Talking to a temporary utilities provider to figure out what you’ll need in a crisis and how you’ll roll it out in a matter of hours could save you a fortune in downtime and delays if the weather gets too intense. 

Staying safe and productive in a heatwave takes careful preparation, risk assessment and careful consideration for the needs of your team. It’s a tricky challenge to overcome, but as Marilyn sang in “Heatwave”... you certainly, certainly, certainly can!