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 WiFi connection. Users can join channels and instantly send voice messages or photos, and the app even works over older 2G networks.

Zello is at the top of the App Store's free-apps chart, and downloads have spiked twice in the past two weeks — 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 would 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 WiFi goes down, the app will not work.

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

Zello is free to use and doesn't have ads. To start using the app, though, you'll need to make an account.

You can provide as much or as little information as you'd like, including a photo, description, and voice greeting.

Zello takes a no-frills approach to its interface. By clicking the menu button, you can check your volume level, change your status, view your contacts, and see available channels.

Zello lets you join existing channels or create your own. To check out some of the available channels, click Add Channel and then navigate to Trending Channels.

In the trending-channels area, you can see what topics are popular at any given time. Unsurprisingly, most of the trending channels lately relate to Hurricane Irma or Hurricane Harvey. When you click on a channel, you'll get an overview of what people are talking about before you join.

Once you join a channel, you'll hear audio from other members right away, much as if you were listening to a police scanner. You can hear everything posted in the channel. When I listened, I mostly heard people checking to see if the app worked for them as they prepared for the storm.

Once inside a channel, you can also see a list of users and listen to some of the voice messages people have posted.

If you don't want to listen to the audio transmissions, you can turn them off by tapping the power button. You'll still be a member of the channel, but the sound will be muted.

If you want to transmit a message, just press the record button. You can also send a photo by clicking the camera button on the left. Your message will post to the channel, where anyone can listen to it and respond.

Zello is available for free on the App Store and Google Play Store.