The go-to app during rescue efforts in Houston following Hurricane Harvey is rising in popularity as Hurricane Irma speeds toward Florida.

Zello, a walkie-talkie app where users can push a button to talk to any one through a cellular or Wi-Fi connection, is the most popular free app on both Android and iOS app stores.

According to mobile app research firm App Annie, Zello moved to the top spot on Apple's store Tuesday, then reached the top of Google Play on Wednesday.

Zello was among the key tools used by rescuers to help victims of Hurricane Harvey in Houston. Last week, Zello CEO Bill Moore said 20 times as many new users in Houston were on the app compared to the previous week.

"It’s centered on live voice," said Moore. "Our voice is how we most naturally communicate. A few seconds of voice has so much information on emotion, education, gender, what part of the country are they from."

How it works

Once users create an account, they explore a variety of channels to join and chat. In the case of Harvey rescue efforts, channels such as the Cajun Navy Dispatch and Harvey Animal Rescue appeared where users could quickly push a button to talk and seek help.

There is some potential for confusion as users parse which channels are linked to official rescue groups. Recently, a channel for Texas Search and Rescue was pulled after the official group claimed their logo and name were misused, said Moore.

The technology for Zello dates back to 2007, and saw boosts in use during global uprisings, including ones in Egypt and the Ukraine, as well as protests in Venezuela. Zello was blocked during several of these events, most recently in Russia in April.

Follow Brett Molina on Twitter: @brettmolina23.