Why Puddles Freeze at different Temperaturs

At what temperature do puddles freeze?  It sounds like a simple question and should be, but is actually more complex than most realize.  There are several other questions that need answering first.  Water freezes at 0 degrees Celsius or 32 degrees Fahrenheit, so the first question is what thermometer one is using!

Now we start getting complex.  Is one referring to the temperature of the puddle, or of the air?  The puddle will be the ground temperature, but the temperature give by the weather service, unless it is an agricultural report, is given for the average person and taken at 4 feet about the ground!  This is where most people will “feel” the temperature.  There can be a difference of several degrees in those few feet.  If you then factor in shade verses sun and throw in wind chill you can now see how things start to get complicated!

To further confuse the issue, one needs to think about mineral content of the puddle.  Some minerals lower the freezing point of water and some raise it!  Salk is put on roads not for the grit it provides, but because it melts ice into water.  Thus areas where there is high salt concentrations, such as the coast, or later in the winter when salt has been used a lot, the puddles all freeze at colder temperatures. 

In some cases it may not just be the soil that effects the freezing temperature, but also pollution and plant matter.  Oils. antifreeze and gasoline can collect on the surface of puddles, it is what makes those bright, rainbow colored patterns.  Oils can not only come from automobiles, but also from plants.  They alter the freezing point!

The same apples to animal waste.  Urine and manure both contain salts and acids that will alter the freezing point of a puddle.  This may not be as much of a factor in a large city, except for dogs, cats and pigeons, but on a farm is a definite consideration!

Back to the general question of at what temperature does a puddle freeze.  There is no specific answer!  If using Celsius, most puddles outside will be frozen by the time the thermometer reads below minus 5 and if dealing in Fahrenheit that is by 23 degrees.  But they can freeze earlier, or much lower!