Why don't water towers freeze solid in the winter?

The big freeze: 5 signs your pipes are frozen and how to thaw them

Do Water Towers Freeze? - SASS Wire Saloon - SASS Wire Forum

lessons « Galaxy Dreams

Frozen Water Tower In A Remote Village. Winter Landscape. An Old

Engineering, HowStuffWorks

Why water freezes from top down instead of bottom up

How to Keep Your Well Water System From Freezing in the Winter

lessons « Galaxy Dreams

Why don't water towers freeze solid in the winter?

Bolton water tank overflows after sensor freezes up

Structural Engineering, HowStuffWorks

Tough Winter Leaves Municipal Water Towers Frozen - CBS Minnesota

Water tower water tower frozen by the cold hi-res stock

Texas colonias don't have running water. Will they ever?

Frozen Water Tower on Route D - Boone Country Connection