Walkie talkie app Zello has reached the top of the US Apple app store charts after it added over 6 millions new users since last Monday. The increased number of downloads is a direct response to Hurricane Irma, which has been making its way toward Florida.

Originally a Russian app called Loudtalks, Zello Inc. acquired the technology and launched its rebranded application in 2012. It simulates traditional two-way radios through the use of WiFi or cellular data, and now boasts over 100 million users worldwide.

As Zello allows between two and one thousand people to communicate live with each other from dispersed locations, it can be especially useful in search and rescue situations. As Category 4 hurricane Irma brings 130 mph winds, tornadoes, and flooding to Florida, it's hoped the app, which also lets users add screenshots and important information, can be put to good use.

One of Zello's new Irma relief groups - users can start their own channels - includes the South Florida Hurricane Irma channel, which has over 1800 members. "With the crush of new users and emergency situations, most of the Zello team is working long days either maintaining capacity or helping with customer support," said company CEO Bill Moore.

Zello made headlines in 2013 when it was used by Turkish protesters to avoid government censors. It's also been used during protests in Venezuela and was popular with relief workers in the aftermath of Texas' Hurricane Harvey.

In a recent blog post, Zello CTO and developer of Loudtalks, Alexey Gavrilov, warned that the app "is not intended as a replacement for instructions from government emergency agencies or sanctioned rescue organizations." He emphasized that Zello won't work if there is no WiFi and cellular data service, but added that mobile data networks often remain at least partially operational after natural disasters.