Carriers’ 5G networks are delivering super-fast speeds where service is available, but the summer heat forced new 5G smartphones to revert back to 4G connections, according to recent speed tests conducted across the country by the Wall Street Journal.

When testing millimeter wave (mmWave) 5G networks using the new Galaxy S10 5G, WSJ journalist Joanna Stern reported that normal summer temperatures in Atlanta (90 degrees), Chicago (90s) and New York (83 degrees) caused the device to overheat, resulting in reduced modem or processor functionality and a drop of 5G signals in favor of 4G.

“With 5G, data is transmitted at higher quantities and speeds, which causes the processor to consume more energy,” a Samsung spokeswoman told the WSJ.

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When the Samsung phone hits a certain temperature threshold, it moves to a 4G connection to optimize battery and minimize energy use, the spokesperson explained. 5G functionality will only get better as 5G technology and the ecosystem evolve, she added.

When the 5G connection was severed, Stern’s infrared thermometer showed the device surface temperature was more than 100 degrees. Ice packs and car air conditioning were employed to cool the device, switching the 5G signal back on within a couple of minutes.

The WSJ noted overheating wasn’t an issue when testing Sprint’s mid-band 5G network, which also included the LG V50 ThinQ 5G.

Super-fast speeds, limited coverage

Like results from other tech journalists’ 5G testing, networks delivered blazing fast speeds, particularly Verizon and AT&T’s, but mmWave 5G connections were only found outdoors and within roughly 100 feet of nodes.

Testing was conducted in Denver, Atlanta, Chicago, and Manhattan’s Lower East Side.

Stern reported one speed test clocked 1.8 Gbps on Verizon’s network in downtown Denver, and it took 33.6 seconds to download "Stranger Things" Season 3, which is 2.1GB of data at 1.27 Gbps.

AT&T’s 5G network in Atlanta clocked similar speeds, with "Stranger Things" downloading in 37.9 seconds at 1.1 Gbps. Testing never hit gigabit speeds on Sprint or T-Mobile’s network, but the WSJ noted their speeds were still about eight-to-10 times faster than 4G.

On T-Mobile’s 5G network in New York, it took 2 minutes and 57 seconds for the "Stranger Things" download, with speeds of 381 Mbps. On Sprint’s 5G network in Chicago, the video content downloaded in 3 minutes and 20 seconds, notching speeds of 301 Mbps. On average, it took more than an hour to download the same video over a 4G connection, according to the WSJ.

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Verizon, AT&T, and T-Mobile are all using high-band mmWave spectrum for their respective early 5G launches, which delivers increased speeds but can’t travel far distances or penetrate walls. The WSJ said it needed to ask carriers for landmarks and street corners to find 5G coverage. While gigabit speeds were clocked close to a 20-foot pole broadcasting a 5G signal on Verizon’s Denver network, the 5G connection disappeared as soon as Stern entered a bakery right next to it.

Unsurprisingly, Sprint’s network, which is using mid-band 2.5 GHz spectrum, did not have these coverage issues, though speeds were slower. The WSJ reported both the LG and Samsung device showed the 5G connectivity icon throughout testing in Chicago, even indoors including a hotel lobby, and at CVS, with speeds ranging between 100 Mbps and 300 Mbps.

Verizon has launched mobile 5G service in select parts of five cities, most recently in St. Paul, Minnesota, and has plans for 30 5G markets this year. AT&T has deployed 5G in 20 cities, most recently in Las Vegas. AT&T currently only offers 5G service to business customers and invite-only developers. T-Mobile has turned on 5G mmWave service in six cities, while Sprint’s flavor of 5G is available in five markets.