Storm sirens or outdoor warning sirens are recommended to be used to alert of impending weather or attack. The Iowa Emergency Management Association, or IEMA, recommends sirens only be used when there is an immediate threat to human life including severe weather. They recommend that in the event of severe weather, sirens should be sounded for tornado warnings, thunderstorms that may produce winds to 70 mph or more, or storms that could produce golf ball sized hail or larger. The counties that follow this policy are Black Hawk, Chickasaw (2" hail), Dubuque, Linn, Johnson, Allamakee, Buchanan, Delaware, Iowa, and Winneshiek.
However, this policy is not mandatory or enforced. Benton, Cedar and Fayette counties will use outdoor sirens for tornado warnings and severe storms producing 70 mph winds or more. Bremer, Butler, Clayton, Grundy, Jones, Keokuk, Tama and Washington counties will only alert outdoor sirens for tornado warnings.
Unfortunately, there can also be discrepancies within counties themselves. Local fire departments typically operate the outdoor sirens and some may activate them outside of their counties policy.
No matter what policy a county follows, sirens should always be taken seriously. When a siren is activated, shelter should be taken indoors immediately.