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That's an interesting point about water towers. What *does* keep them
from
freezing in prolonged cold snaps?

I've wondered that myself, particularly in small towns.

OK - time to consult the oracle, Google:

http://science.howstuffworks.com/eng...wer-freeze.htm

Malcolm Jackson is the Business Development Manager for the tank
maintenance company Utility Service Co., Inc. The company has home offices
in Perry, GA, and has thousands of elevated tanks under full-service
maintenance contracts in 17 states.
Here is Malcolm's answer to the question:

They do freeze. They just don't normally freeze solid. In the more extreme
climates, like North Dakota, engineers incorporate heating systems into the
tank design. Specifically, the central pipe that runs from ground level up
into the bottom of the tank is called a riser. Water is pumped into the base
of the riser and allowed to rise into the tank. Many tank risers are
wrapped with heat tape, covered with insulation and capped by an aluminum
jacket. In addition, a heating system may be installed in an adjacent
building or into the valve pit underneath the riser. The heater introduces
heated water or steam into the base of the riser. In either case, the heat
rises into the tank to retard freezing. Ice forms on the surface of the
contents, in many cases several feet thick. Normally, this ice layer floats
on the surface as the water level rises and falls. Many times the ice
freezes to the roof or upper walls of the tank and remains there as the
level fluctuates underneath under daily usage. In more temperate climates,
steel ladders are installed inside the tank, and riser and water level
control probes hang from the roof. In colder climates, all inside fixtures
must be left out since the ice layer tends to rip them out.

--
Bobby G.