Can water freeze at an outside temperature of +1 degree Celsius with a wind chill of -18 degrees Celsius?

Answer

Wind chill is a subjective quantity that takes into account the effect of the wind on the cooling of the human body. At a temperature of zero degrees and a wind speed of 10 m/s, our body loses (on average) as much heat as if there were no wind and -7 degrees. So for a human it seems to be -7 degrees. Wind chill has something to do with the rate of cooling rather than the temperature itself and thus has nothing to do with whether or not liquids freeze or not.

Water does not freeze even at zero degrees. At zero degrees, the conditions for freezing have just been met, but the freezing has yet to start. That freezing (solidification) of water often starts around impurities or air bubbles in the water. As a result, the temperature often has to drop to a few degrees below zero before the water starts to freeze. We call this effect hypothermia. The purer the water, the stronger the effect of hypothermia will be. As soon as the water begins to freeze, the latent heat is released and the temperature rises to…exactly zero degrees. That is why it is often said that the temperature of freezing water is zero degrees (but the hypothermia was essential to start the freezing process).

Something similar occurs with the snow that you see at a temperature of 4 degrees. The environment constantly supplies heat to the snow, but that heat is not used to raise the temperature of the snow, but to melt snow. Stick your thermometer in the snow and you measure… zero degrees. Of course, this can only continue as long as there is snow.