Running Water and Freezing
Does water freeze at a quicker rate when it is standing or running? For
instance would a pond (standing water) that is 5 feet deep freeze (to a point where kids could
ice skate on it) faster than a stream (running water) that is 6 inches deep (so that it would
support the weight of an average 18 year old)?
To freeze, water gives up its energy to its colder surrounding by conduction (through contact with
colder solids), convection (through contact with colder fluids such as air), or by radiation (into
colder surroundings). It starts freezing as soon it is cooled down, and its temperature approaches
freezing point of water.
If this water is contained in a glass, freezing occurs at the same speed whether or not the water
is stationary. To a first approximation, stirring the water has no effect on freezing time.
What happens in running water, for example, in a river, is that warmer waters may replace or mix with
colder water, or colder water comes into contact with surface warmer than air, and this mixing or
contact delays or postpones freezing.
There are some other minor effects involved but the main reason running water freezes "slower" is
that it picks up heat from parts of it surroundings at the same time it loses heat to other parts.
But if one keeps a thermally isolated volume of water in a cold environment, it will freeze at the
same speed whether it is standing still or moving at reasonable speeds.
Dr. Ali Khounsary
Argonne National Laboratory
This is not an easy question to answer because of the many factors that enter into the RATE at which
water freezes. Water has a maximum density at about 4 C. so water that is warmer or colder than 4 C.
will tend to rise to the surface. Of course, ice too has a lower density than liquid water (about
10% lighter) so it too will start to form on the surface of a pond. This is all complicated further
by how fast the air is moving above the surface of the water and its relative humidity. Dry air will
tend to evaporate water, and since the heat of vaporization of water is about 10 kcal/mole (and that
is only 18 ml. of water!!) air movement has an appreciable influence on the freezing of water. If
that is not enough, the temperature of the ground, with its large capacity to absorb/retain heat,
plays an important role too. As you can see the detailed heat balance will be rather complicated.
Gives new complexity to the statement "skating on thin ice".
Still water freezes faster than running water. An especially dangerous place for ice skating or ice
fishing is where a current flows in or out of an otherwise still pond or lake, so that most of the
ice is safe but areas where there is water flow are not. Always check carefully for ice thickness on
EVERY part of the ice before going on it.
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Update: June 2012