Allowing users to communicate even when a natural disaster affects phone signal, the app is the most-downloaded software this week.
What is the app?
Developed by tech start-up Zello, based in Austin, Texas, Zello walkie-talkie was launched in 2011.
Transforming smartphones into modern-day walkie-talkies, it topped the download charts on Google Play and the Apple App Store Friday.
Zello says it has 100 million users -- although before the arrival of Hurricane Harvey, analysis by Sensor Tower showed it was only the 1400th most-downloaded app.
But over a million people, mostly in Puerto Rico and Florida, installed it earlier this week as Hurricane Irma approached.
How does it work?
"It's voice-focused," explained CEO Bill Moore. "Voice is our natural form of communication. Talking for a few seconds can reveal so much information about your gender, your level of education, your place of origin or the emotions you're feeling."
Users can join a channel and exchange short messages with others. It's as easy as recording a message with your phone and sending it to the group, as with other applications such as Viber and WhatsApp.
It's also possible to send pictures -- but the app does use up a lot of battery.
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Why is it so successful?
Contrary to some reports on social media, the app does require an internet connection.
To send a message using Zello, a 2G cellular data connection is enough - making it practical in the case of a natural disaster, when networks can be disrupted.
Zello is also free to download, and does not feature in-app adverts.
During Harvey, its users exchanged advice and organized rescue efforts, with several channels created to discuss the aftermath of the hurricane. On Friday, with Hurricane Irma looming, thousands of users in Florida took to the app to swap tips.
"Thank you very much, that's very convenient!" said one Floridian to another who'd shared details of numerous shelters near a highway.
But the company warns the app is not intended as a replacement for coordinated rescue efforts, and urged users to follow official instructions.
"While Zello has been helpful in Harvey relief efforts, it is not a hurricane rescue tool and is only as useful as the people who use it, and as reliable as the data network available," it said in a post on its Facebook page.
