Design Challenges

Hot Water Freezes Faster than Cold Water

Now that winter is here and we are getting snow on the ground, it is highly important to know this fact. This phenomenon, often called the Mpemba effect, is implemented on a regular basis in Ice Skating events, such as hockey and synchronize skating, via the Zamboni . Many of you may think that you can quickly melt that snow and bypass shoveling by boiling some water and spreading it across the white powder, yet many forget that water will freeze causing greater dangers than snow.

Black ice is the blame for many accidents in the winter. This is because ice blends very well with asphalt making it almost impossible to see once darkness sets in, which in the winter, occurs during the late hours of the afternoon.

There are several physical factors that cause hot water to freeze faster, a lengthily explanation can be found from the University of California, but here is a small summary:

1. Evaporation -- As the initially warmer water cools to the initial temperature of the initially cooler water, it may lose significant amounts of water to evaporation. The reduced mass will make it easier for the water to cool and freeze. Then the initially warmer water can freeze before the initially cooler water, but will make less ice.
2. Dissolved Gasses -- Hot water can hold less dissolved gas than cold water, and large amounts of gas escape upon boiling. So the initially warmer water may have less dissolved gas than the initially cooler water.
3. Convection -- As the water cools it will eventually develop convection currents and a non-uniform temperature distribution. At most temperatures, density decreases with increasing temperature, and so the surface of the water will be warmer than the bottom -- this has been called a "hot top." Now if the water loses heat primarily through the surface, then water with a "hot top" will lose heat faster than we would expect based on its average temperature. When the initially warmer water has cooled to an average temperature the same as the initial temperature of the initially cooler water, it will have a "hot top", and thus its rate of cooling will be faster than the rate of cooling of the initially cooler water at the same average temperature.
4. Surroundings -- A final difference between the cooling of the two temperature water relates not to the water itself, but to the surrounding environment. The initially warmer water may change the environment around it in some complex fashion, and thus affect the cooling process.

The bottom line is that there is no quick fix for getting rid of the snow and it is better to play it safe. Invest in a snow thrower which in the long run becomes a cheaper investment than paying for plowing and will save you a lot of pain and time in the short run.