Instant Snow! (and Why to Start with Very Hot Water Instead of Room Temperature or Cold Water)

There's a persistent myth that cold water will boil faster than hot water (I guarantee it doesn't) and that hot water freezes fast than cold water (under some limited conditions it can). But in this video we see hot water instantly converted to snow in very frigid air. Cold water would not do this. Why? I'll let the tumblr Fuck Yeah Fluid Dynamics -- which is where I go this -- fully explain after the break, with a short amendment, but here's the short answer: the thrown hot water forms into small ice crystals as it freezes; cold water would stay more clumped together and become ice balls and hail.

As you can see in the video (and in many similar examples on YouTube), tossing near boiling water into extremely cold air results in an instant snowstorm. Several effects are going on here. The first thing to understand is how heat is transferred between objects or fluids of differing temperatures. The rate at which heat is transferred depends on the temperature difference between the air and the water; the larger that temperature difference is the faster heat is transferred. However, as that temperature difference decreases, so does the rate of heat transfer. So even though hot water will initially lose heat very quickly to its surroundings, water that is initially cold will still reach equilibrium with the cold air faster. Therefore, all things being equal [ed: but they are not always equal], hot water does not freeze faster than cold water, as one might suspect from the video.

The key to the hot water’s fast-freeze here is not just the large temperature difference, though. It’s the fact that the water is being tossed. When the water leaves the pot, it tends to break up into droplets, which quickly increases the surface area exposed to the cold air, and the rate of heat transfer depends on surface area as well! A smaller droplet will also freeze much more quickly than a larger droplet.

What would happen if room temperature water were used instead of boiling water? In all likelihood, a big cold bunch of water would hit the ground. Why? It turns out that both the viscosity and the surface tension of water decrease with increasing temperature. This means that a pot of hot water will tend to break into smaller droplets when tossed than the cold water would. Smaller droplets means less mass to freeze per droplet and a larger surface area (adding up all the surface area of all the droplets) exposed. Hence, faster freezing!