Wednesday, January 28, 2015

Snow as a Roof Threat

FEB 23 UPDATE: News reports from across eastern New England are of more than 100 roof collapses of business, apartment buildings, barns, porches, garages, etc.

The rule of thumb is ten inches of snow melts down to one inch of water (so, 1.2 inches to a foot of snow). The reality is that most snow, in New England at least, is in the range of 9 to 15 inches of snow melting down to one inch of water.

The blizzard of January 26-28, 2015 was on the powdery side, and hence less water per inch of snow. Boston got a tad under two feet of snow and a melted measure of 1.4 inches. That works out to 0.7 inches of water per foot of snow, or 17 inches of snow melting to one inch of water.

Post storm, many people in New England were removing snow from flatter roofs. The issue is not so much the weight of the freshly fallen snow as it is the subsequent weight added when melt water from a higher roof gets trapped by the snow below. Rain on top of snow has the same effect, as does new snow on top of the old snow.

Methods are climb out a window, climb up a ladder, or use a very long handled roof rake to accomplish the task from the ground or on a ladder.

In our region of Massachusetts a flat or low-slope roof is supposed to be designed to support a snow load of at least 50 pounds per square foot. Freshly fallen snow averages six pounds per cubic foot, but dense, wet snow can be three times that. The powdery stuff this last storm dumped was under four pounds per cubic foot. Artificially made snow, because it is made from tiny droplets that freeze into round shapes, packs much denser than natural snow, reaching densities of 25-28 pounds per cubic foot.

THE SCIENCE OF COMPRESSION

Sintering describes the process of compression of snow. At one end of the scale this is packing down snow by walking on it - at the other, glacial ice that has formed over thousands of years of compression under the tons of weight of accumulating snow.

To quote one source: "Snowflakes are delicate structures with lots of thin points radiating from a central node. If they are disturbed, the points are broken off and the snowflakes compact together. The energy from this disturbance briefly melts the edges of the squashed flakes, which then freeze together to create a cohesive snow mass. This process, known as sintering, is what happens when you compress snow to make a snowball in your hands."

Or, for that matter, what happened to the dense, quickly solidifying snow of an avalanche once the snow has stopped moving. Not only is the snow three to five times as dense post avalanche, but all the newly sheared surface areas of the snow granules quickly freeze together.

ALL SNOW IS NOT EQUALLY WET

The percentages below are percent of volume, not percent of weight. As water has a higher density than the snow it is in, percent by weight would be higher. Standard definitions:

    DRY snow is at a temperature below freezing and will not form a snowball
       when squeezed in a gloved hand.
    MOIST snow is at freezing temperature and contains 0-3% of its volume as liquid water.
    WET snow is 3-8% liquid water but when squeezed into a snowball, will not drip water.
    VERY WET snow is 8-15% liquid water and will drip water when compressed.
    SOAKED snow is 15-20% liquid water, very drippy, but still 20 to 40% air.

When a snowball is made from moist or wet snow the air is partially squeezed out, but the water does not freeze until the snowball has time in sub-freezing air. Giving it time to freeze would not make it denser, but it would be harder.

THE SCIENCE OF MELTING

Simply put it takes only modest amounts of heat to warm ice up to 0C, but then huge amounts of heat to turn the ice at 0C to water at 0C. Then, once it is water, it takes only modest amounts of heat to warm the water to higher temperatures. Example: a kilogram of ice at one degree Centigrade below freezing will require 2.1 kilojoules (kJ) of energy to warm it one degree - to the freeze/thaw point. It will then take 334 kJ to melt it, although at the end of melting it will still be at 0C. Warming the now liquid water each degree above the freezing point will require only 4.2 kJ. In this example, warming it to just below boiling - to 100C - would require 422 kJ. Look at that! It takes almost as much heat energy to convert ice to water as it does to then heat that same amount of water to the boiling point!

Remember, they melt!
Ice shot glasses made with a mold
will chill but not freeze a drink  
Same in the opposite direction - modest amounts of cooling are needed to drop water to a temperature of 0C, but then huge amounts needed to freeze it. If water is frozen into the shape of a shot glass (see Cool Shooters photo) a beverage can be poured into the 'glass' and it will quickly become ice cold but never freeze.

THE ICICLE TRICK
When icicles form during the day and are actively dripping, the temperature of the ice is very close to 0C.  However, after a very cold night, the icicles will be well below freezing. During the following cold morning it is possible to break off an icicle, dip it briefly in very cold water, and then, by pressing it against a cold surface, have it freeze in place.

Thursday, January 8, 2015

Fire Alarm Call Boxes soon to be History

Trademark is clenched fist and lightning bolts
over the company name, registered 1879
Maynard's outdoor fire alarm call box system will be replaced with new technology. The existing boxes are painted red and white (with one exception), and either attached to a telephone pole, the side of a building or mounted on a pedestal, such as the one facing Nason Street in Memorial Park. In Maynard, the top of the face of each call box (with one exception) displays GAMEWELL below a clenched hand holding lightning bolts. This symbol was registered as a trademark in 1879. A few of Maynard's call boxes are topped with red lights. Sides have decals - often severely faded - reading "FIRE" in red letters on a white background.

The exceptions: On the wall of Suburban Glass and Mirror, on Powder Mill Road, the call box is branded SAFA. The initials stand for Superior American Fire Alarm & Signal Co., Meriden, Connecticut. SAFA was competing with Gamewell in the 1950s, but no longer exists by that name. How Maynard ended up with one SAFA box is a mystery. [UPDATE: The SAFA call box has been removed.] The other exception is that the box at Reo Park is painted white with blue trim.

January 29, 1921: Maynard Hotel lost to fire despite
timely efforts of the fire department
Gamewell, named after the owner, John N. Gamewell, was the New York and Massachusetts based company that had provided Maynard with its first box, in 1892, and is still in business today as a division of Honeywell. Because of a peaked roof design their outdoor call boxes are referred to as cottage style.

From a website: "Boxes were installed on buildings such as churches, schools, movie theaters and major factories. This was to provide a reliable method of sending a fire alarm from the protected facility to the fire department. These boxes were placed to provide for a rapid response to incidents where there was a large life-loss potential, schools for instance, or the potential for a large economic impact to the community such as a large factory."

Older boxes were made of cast iron and weighed close to 75 pounds. Manufacturing was located in Newton, MA, on the Charles River, a long-time iron works site. Decades later, cast aluminum replaced iron, so the newer boxes weighed a tad under 28 pounds. Up until some time in the 1970's the lower front read "Newton, Massachusetts," changed to "Medway, Massachusetts." Most (all?) of Maynard's call boxes read "Newton."

Gamewell call box on pedestal, Memorial Park
Click on any photo to enlarge
As noted above, Maynard acquired its first alarm box in 1892. Expansion was slow. The count reached 8 by 1903, 22 by 1938 and 30 by 1953. Maynard currently maintains more than 70 call boxes, plus a handful that are still out there but not operative, so wrapped in plastic.

Back when the town's annual reports used to have a list of call boxes, there was a subset described as phantom boxes. This term referred to sites throughout town without real boxes yet assigned box numbers. There was a filing system at the fire station with all the box numbers - real and phantom - and when a call came by telephone, the desk man pulled the card and then tapped out the box number (and thus location) to the responding crew.

However, in this era of 9-1-1 and cell phones, the use of outdoor-mounted call boxes to report fires has dramatically declined while the cost of maintaining the system has increased. The fire department's  intent, over the next two years, is to ship Maynard's boxes to other towns. Maynard's system will be modernized to radio call boxes akin to cell phone technology.  

One of the old Gamewell boxes - from the site of the now demolished Oriental Delight restaurant, formerly Russo's - has been donated to the Maynard Historical Society. It was recently on display at the Maynard Public Library as part of an exhibit on the history of the fire department.