People who live anywhere near Avonmouth are often puzzled by the alarm sound they hear at the start of each month.
On the 3rd of every month at 3pm, a siren sound which can be heard in Portishead, Avonmouth, Shirehampton, Pilning and Severn Beach.
It is the sound of the Severnside Sirens - warning people of possible incidents involving dangerous substances in Avonmouth.
But providing you hear it on the 3rd of the month, it is most likely just a test.
The test siren includes a three minute "Alert" tone, a one minute silence and a one minute "All-Clear" tone.
In an actual emergency, police will trigger the Severnside Sirens from Police Headquarters in Portishead.
What are the Severn sirens in Avonmouth?
The Severnside Sirens is an emergency warning system which alerts people to possible incidents involving dangerous substances in Avonmouth.
It is maintained by the Severnside Sirens Trust which is an independent charitable organisation.
What to do if you hear the air raid siren in Bristol?
If it is not a test on the 3rd of the month, people are told to go inside and close windows and doors. Local radio stations will provide instructions and updates.
Official advice from the Severnside Sirens Trust when hearing the sirens is to:
Go indoors and stay there. Do not collect children from school as they will be properlycared for
Close windows and doors
Turn off any gas appliances, ventilation systems and ensure there are nonaked flames.
Do not leave the area unless you urgently need help or are advised to do so by the emergency services or the Local Authority.
Do not use telephones unless you urgently need help. This will keep phone lines free foremergency use.
Stay indoors until you are advised otherwise or the all clear is sounded by theSevernside Sirens.