Here's how to use Zello, the walkie-talkie app people are downloading ahead of Hurricane Irma

As Hurricane Irma rips through the Caribbean and heads toward Florida, a free smartphone app is emerging as an important tool for search and rescue.

Called Zello, the app lets you use your phone as a walkie-talkie or two-way radio as long as you have a network or Wi-Fi connection. Users can join channels and instantly send voice messages or photos, and the app even works over older, 2G networks.

Zello is currently at the top of the App Store's free apps chart, and downloads of the app have spiked twice in the last week. The jumped during Hurricane Harvey and again on Wednesday as Irma became a Category 5 hurricane.

The app's popularity may have been influenced by viral Facebook and Twitter posts that said it will still work even without cell service, the Washington Post reported. But those posts were incorrect - if cell towers are wiped out in the storm and Wi-Fi goes down, the app will not work.

But if you do have cell service - even a 2G or 3G connection - the app could help in the event of an emergency. Here's how it works.