Denver, CO tests 11 a.m., 2nd Wednesday each month

[Robert Gift]

Denver, Colorado tests their variety of sirens at 11 a.m. 2nd Wednesdays.

I know of two Thunderbolts on high schools, a 2001 on a pole next to a school, and another siren on a school roof whose big yellow cap and nothing else can be seen from the ground.
I hope it is not a vent cap. If so, why CD yellow?
Also there is a typical rocket-shaped vertical siren east of downtown.