Hurricane Irma Just Made a Digital Walkie-Talkie the No. 1 App Online

As Hurricane Harvey dropped anchor over Southeast Texas last week, Zello became the go-to app for rescuers working to save thousands of people trapped by floodwaters. Within days of Harvey's arrival, the app saw a 20-fold increase in usage in Houston, according to Bill Moore, the Austin based startup's the chief executive. As Hurricane Irma hurtles across the Caribbean toward the coast of Florida, Zello continues to boom in popularity. The free Internet "walkie-talkie" app - which relies on cellphone data plans or WiFi and is designed to operate in places where signals are weak - became the top app on iTunes and Google Play Wednesday.

The latest influx began Tuesday and, at one point, Moore said, 120 people were registering for the app every second. In recent days, the app has also trended on Facebook and Twitter, offering another example of the pivotal role social media is playing in natural disasters. "The heat map of where the registrations are occurring looks like the hurricane's forecast path," he added. "It's very dense at the tip among the Caribbean islands and then fans out across Florida."

Moore said so many people registered for the app Tuesday that Zello suffered some performance issues, which have since been resolved. Forecasters are calling Irma "a potentially catastrophic Category 5" storm with sustained winds of 185 mph, according to the National Oceanic and Atmospheric Administration. Florida officials have begun evacuating portions of the eastern coastline, which is densely packed with cities, towns and suburbs...