Friday, March 9, 2012

Outdoor warning sirens - part 2: the role of outdoor warning sirens

In part 1 of this MWN Blog series on outdoor warning sirens, we examined the role of the Memphis/Shelby County Emergency Management Agency (MSCEMA) with respect to severe weather, specifically the process by which outdoor sirens are sounded.  In it, you learned that the EMA is responsible for sounding all sirens in Shelby County except those in Bartlett, Collierville, and Germantown, who activate their own sirens. In part 2, we will focus on the purpose of the sirens, what they are intended to do, and more importantly, what they are NOT intended to do.

To recap, outdoor warning sirens are sounded continuously countywide whenever any portion of the county is included in a Tornado Warning issued by the National Weather Service (NWS). These outdoor warning sirens are NOT referred to strictly as tornado sirens for a reason: it is not fully descriptive of the mission they serve. From the MSCEMA website ( - emphasis added): "[Outdoor warning] sirens are designed to be an early warning device primarily for persons who are outside away from the television and/or radio."

As stated above and repeated many times by, these are OUTDOOR sirens. They can be extremely useful if one is working or recreating outside and severe weather threatens. When sirens sound, you should seek out additional information on the threat.  This can be done in a number of ways, but the most common are to tune into local radio or television, check local weather websites or properly-equipped smartphone apps, and/or turn on NOAA Weather Radio (which should have also sounded an alarm when the sirens sounded).  If none of these additional information sources is available, you should take adequate cover until the sirens cease.

Photo credit: MWN
Smithville, MS warning siren
Sirens are NOT designed to A) wake you up at night, especially if storms are in progress with wind and rain creating additional noise or you are a heavy sleeper; B) alert you if you are indoors, especially in newer, more energy-efficient, and better-insulated homes with additional electronic noise (like air conditioners running, music playing, or television on); or C) reach every square mile of the county.  In fact, even outside, environmental factors such as the direction and speed of the wind, relative humidity, and air stability can affect where sirens are heard on any given day.

The bottom line is this: sirens are designed to alert those outdoors to the threat of severe weather and encourage them to seek additional information.  Do NOT call local authorities, including the EMA, if you aren't woken up by a siren or you didn't hear it inside, UNLESS you are fairly certain that a nearby siren that you have heard under those circumstances before is not operating properly (i.e., fails to sound).  Click here for a list of all sirens and the person to contact in case of siren failure ONLY.

In the third installment of this series, we will examine the future of warning technology - from sirens to smartphone apps - in an age of polygon warnings.

For weather information for Memphis and the Mid-South, where and when you need it, visit on the web, on your mobile phone, download our iPhone or Android apps, or visit us on Facebook or Twitter.


Brett said...

We just rebuilt our system in St Louis County, and one of the questions we were asked was "what would it take to have the sirens work everywhere indoors too?"

Typical indoor background noise is only 50 dBC, so the siren loudness needs to be at least 59 dBC to hear it inside (9 dBC above background is the key threshold for perception).

An uninsulated drywall and plywood exterior wall will attenuate 33 dB of sound. That means to hear the siren inside, the siren loudness needs to be 92 dBC on that exterior wall.

Our sirens are almost all Whelen WPS-2910-S sirens. Those are 129 dBC; loud enough to cover 99% of the county at 70 dBC with just under 200 sirens. Most sirens are not this loud.

To have one of those big sirens hit your house at 92 dBC, your house has to be within 1330'. If you have a plaster with lathe or modern masonry house that can attenuate 50+ dBC, your house needs to be within 400' of the siren to hear it inside!

So, the kicker... in order to cover the entire county at 92 dBC instead of 70 dBC, we would have to increase the number of sirens from 200 to 3800. Our new system price tag would jump from $8 million to $152 million. And I can tell you from experience that it is physically impossible to find regularly spaced locations for 3800 sirens in a 500 sq mi county. It was hard enough to get 200 locations that most residents found acceptable.

And that is why siren systems are not designed to be heard indoors.

Meteorologist Erik Proseus said...

Thanks for your in-depth example from St Louis Co., Brett! I was aware that St. Louis just re-did their siren system. In fact, saw a GIS application showing the siren configuration. Your detail highlights why we say what we do - they are outdoor sirens for a reason.

So what was the impetus for the re-built system in St. Louis? Progressive-minded individuals in leadership, complaints from citizens, deteriorating infrastructure, or perhaps a combination? Just curious... thanks again!


Brett said...

Ultimately it was deteriorating infrastructure. Most of our sirens were over 20 years old, and many were 40 years old. We could no longer find parts to repair them. Sirens would lose drivers, stop rotating, or go dead, and there was nothing we could do about it. As well about 20-25% of the county that was rural when we first built was now a large suburb with no coverage.

The bond issue went on the ballot twice before getting passed. We thought too little was budgeted at first, but with a lot of testing and design work we were able to both decrease the number of existing sirens while expanding coverage to the uncovered areas of the county.

We still have two big issues left. Even with all the design work, we have dead spots from high background noise, terrain, vegetation, and/or large buildings. High background noise from traffic is especially a problem. The second is that we moved most of the sirens. While a lot of people can still hear the sirens inside, it is not the same people that could hear them inside before. The people who could hear the sirens inside before are concerned that they cannot hear them inside now.

This has been a blessing in disguise though. Now that the pattern of siren coverage has changed, people are much more aware of the need for weather alert radios. The local news is promoting them heavily, and local stores are routinely selling out. So, updating and changing our outdoor coverage is leading to more people taking care of their indoor coverage.

(And we used GIS extensively for system design and to communicate system status with the vendor. It was very useful when the radio system was being setup.)

Meteorologist Erik Proseus said...

Thanks again for your in-depth commentary Brett! Very insightful.