A walkie-talkie app called Zello is No. 1 in the App Store because of Hurricane Irma

A free walkie talkie app has shot to the top of the App Store as the Caribbean and southern US prepare for Hurricane Irma.

The app, called Zello, lets you use your phone as a walkie talkie or two-way radio as long as you have a network or WiFi connection. Users can join channels and instantly send voice messages or photos.

Zello

Zello lets you join a channel and instantly send voice messages back and forth.

Zello launched in 2011, and it's been used by rescue workers in the past: A volunteer rescue group called the Cajun Navy that formed after Hurricane Katrina frequently uses Zello, according to USA Today. And since Hurricane Harvey hit last week, new channels have been popping up on the app for groups like Texas Search and Rescue and Houston-area midwives, USA Today reports.

Downloads of the app spiked during Hurricane Harvey, and now, as Hurricane Irma descends on the Caribbean and the southern US, Zello has reached the top of the App Store's free apps chart.

According to app intelligence firm Sensor Tower, Zello has seen a massive increase in downloads in the last 24 hours since Irma became stronger than Hurricane Katrina was at its peak. Here's a look at how the app's hourly downloads since August 28:

Sensor Tower

As of Tuesday morning, Irma became a Category 5 hurricane and the National Oceanic and Atmospheric Administration detected sustained winds of 185 mph. On Tuesday night, the storm began hitting the Leeward Islands, the chain of islands separating the Caribbean Sea from the Atlantic Ocean. The National Hurricane Center predicts Irma could hit Florida early Monday morning.