Denver’s 86 emergency sirens were tested Wednesday morning with the warning signal sounding throughout the city and at the airport. The Denver Office of Emergency Management announced the annual test over the past several days, but still caught some residents off guard.

The OEM said in a tweet Tuesday that “[r]esidents can expect to hear the siren for at least three minutes and is solely meant to ensure Denver’s outdoor public siren system is working effectively.”

Tornado activity earlier in May caused a postponement of the tests that were scheduled for May 10th, the office announced at the time. Clear skies (not including the Canadian fires haze) on Wednesday allowed the alert test to occur as planned.

Residents took to Twitter to share what they heard with their reactions.

Testing of the city’s “Reverse Emergency Notifications” system, which involves calls to registered phones, was conducted during the week of May 8th throughout more than 50 neighborhoods.

Registration for the REN service is availableble at www.denvergov.org/emergencynotifications, the office announced in an email.

Denver also uses “Wireless Emergency Alerts” to send notifications “relevant to your location” about emergencies. These alerts don’t require registration, but government alerts — like Amber Alerts and others — need to be enabled on an individual’s phone to see the message sent. A “Smart-911” system is available for people who need disability-related accommodations.