This weekend, close to 200 million Americans will face temperatures of 90F (32C) and higher. Add in humidity, and many cities across the East Coast and Midwest will be feeling more like 110F (43C).
Heat waves have killed more people on average than any other extreme weather event in the US, according to the Centers for Disease Control (CDC).
Officials define extreme heat as a period of two to three days of high heat and humidity with temperatures above 90F (32C).
On the heels of earth's hottest June on record, the US National Weather Service (NWS) estimates over 100 record-high minimum temperatures could be set as the heat lingers even past sunset.
Dangerous heat and humidity is expected from the Plains to the East Coast over the next few days.— National Weather Service (@NWS) July 18, 2019
Current heat watches, warnings and advisories can be seen here. See what's in effect for YOU at https://t.co/VyWINDk3xP pic.twitter.com/QWFHbXAtHx
Here's what that sort of heat can do.
1. Air conditioners make cities hotter
Air-conditioning is used in 87% of US homes, according to a 2018 report by the US Energy Information Administration (EIA). During heat waves, air conditioning use stresses power grids and can lead to city-wide outages.
In cities, that means millions of units - including those on cars and buses and trains - constantly pushing out heat into the atmosphere. Studies have found the extra heat from air-conditioning can raise temperatures by as much as 2C. And when it gets hotter, our thermostats turn lower and the cycle continues.
But it goes further than just an ever-hotter summer season - the emissions from air conditioners and their refrigerants is contributing to climate change. The man-made greenhouse gases used in air conditoners, called hydrofluorocarbons (HFCs), are thousands of times more potent than carbon dioxide in the atmosphere.
In cities, the cycle is also exacerbated by all the concrete, asphalt, steel and glass, creating an urban heat island.
2. Planes can't take off
When temperatures get too high, planes get grounded.
Extreme heat reduces air density and the amount of lift a plane can get to take-off. Temperatures of 120F (49C) saw dozens of flights cancelled in Phoenix during a 2017 heatwave.
Smaller planes are affected first, but larger Boeing or Airbus jets have maximum operating temperatures around 126F (52C), AZ Central reported.
3. Roads can melt - or explode
In a heat wave, concrete and asphalt don't fare well.
Asphalt warps and melts. Concrete, if water is involved, can sometimes explode or break open.
This week, local media in Kansas has reported several instances of cracked and buckled roads as temperatures rise.
In Iowa, one city mayor said old concrete roads soaked with floodwaters from this spring are now exploding and damaging sewer lines.
4. Biscuits bake in cars
The NWS outpost in Omaha, Nebraska, demonstrated just how hot vehicles can get in heat waves by cooking American biscuits on a dash.
If you are wondering if it's going to be hot today, we are attempting to bake biscuits using only the sun and a car in our parking lot. We will keep you posted with the progress. Stay cool! #newx #iawx pic.twitter.com/cXZgdRIgcK— NWS Omaha (@NWSOmaha) July 18, 2019
And after nearly 8 hours in the sun, the outside of the biscuit is actually edible. The middle is still pretty doughy though. The max temp on the pan was 185! Also we made festive biscuit hats 😂 Stay cool out there. #HeatSafety #LookBeforeYouLock pic.twitter.com/ptWP2jksrU— NWS Omaha (@NWSOmaha) July 18, 2019
The temperatures reached a high of 185F (85C) - not quite enough for a proper bake, but certainly too hot for humans or animals to survive in.
According to the National Weather Service, 21 young children have died in hot cars this year.
5. Metal gets even hotter
Unsurprisingly, as everything warms up in a heat wave, anything metal gets even hotter.
When it comes to power lines, this can cause dangerously low sagging. If lines droop low enough to touch the ground or trees, they can short out.
Kinks can form on train rails when the metal expands. In 2012, a "heat kink" caused a 32-car-long freight train to derail and fly off an overpass; a similar derailment happened in 2017. Experts told Fox 26 News more than 2,100 trains have been derailed in the last 40 years because of these heat warps.
Metal components in draw-bridges can expand and become inoperable - last year, Chicago firefighters had to hose down a downtown draw-bridge so it could be used.
6. Agriculture takes a hit
Crops can also feel the heat.
Farmers across the Midwest are warning that vegetables can wilt in the heat and farms will lose productivity - worsening the situation after a historic flooding season this spring.
Soybean farmers also say the dry conditions could cause an increase in spider mites and other plant diseases.
7. Smog worsens
Extreme heat makes pollution worse, which can be dangerous for the very young, the elderly and anyone with respiratory diseases.
In the nation's capital, where the mayor has declared a heat emergency, officials warned that air quality would reach unhealthy levels for these sensitive groups over the weekend.