Why the same droplet can freeze in 1.7 seconds or 68 seconds
The surrounding environment matters enormously. In one experimental study, a 5-microliter water droplet—about the size of a small raindrop—took 68 seconds to freeze into an ice particle at an air temperature of about −20 °C, but only 1.7 seconds when plunged into liquid nitrogen [3]. That is a forty-fold difference for the same droplet volume, showing that the cold medium's ability to pull heat away is at least as important as the temperature number alone.
Temperature within the same medium also shifts the time substantially. When the air temperature dropped from about −10.7 °C to −24.7 °C, the freezing time of the same 5-microliter droplet fell by 45.5% [3]. So a droplet falling through a winter cloud at −10 °C and one falling through colder air at −25 °C are on very different freezing schedules.
What actually happens inside a freezing droplet
Freezing is not a single instant. In cold gas, a droplet passes through four observable stages: pre-cooling (chilling down to the freezing point), recalescence (a quick partial freeze), solidification (the bulk turning to ice), and deep cooling (further chilling of the ice) [3]. The solidification stage is especially sensitive to droplet volume because water releases a large amount of latent heat as it turns to ice—the bigger the droplet, the more heat must be removed, and the longer that stage takes [3].
The geometry of freezing also matters. In the experiments, droplets formed an ice shell on the outside and froze inward [3]. On a cold surface, the frozen front—the boundary between ice and liquid—grows from the bottom upward, and the shape of that front depends on how the droplet sits on the surface [5]. A freely falling droplet has no surface underneath it, so its freezing geometry will differ from these surface experiments, but the shell-and-core pattern is a useful picture of how ice advances through a droplet.
Why salty water is a different problem
Seawater droplets do not freeze like pure water. As ice crystals grow, they reject salt, which concentrates in a brine film that emerges on top of the frozen portion [2]. The researchers propose a universal definition of freezing duration to compare droplets with different salt concentrations, precisely because the usual start-and-end markers of freezing do not transfer cleanly from pure water to salty water [2].
They also describe a growth pattern they call ice sprouting: ice crystals grow from the bottom of the brine film until they pierce the free surface, a process they say is controlled by condensation at the brine film's free interface [2]. This is a proposed mechanism, supported by molecular dynamics simulations, not an established universal rule. The practical point for a curious reader is that a salty falling droplet's freezing time is not simply the pure-water time plus a correction—the salt changes the sequence of events.
How small is small enough to freeze fast
Size is the other major lever. In a numerical model of micro-sized supercooled droplets, a 10-micrometer droplet and a 1-micrometer droplet froze by different patterns: in the 10-micrometer case, heat conduction inside the droplet slowed the advance of porous ice, while in the 1-micrometer case, strong heat exchange with the cold air stream allowed compact ice to propagate consecutively [4]. The model also found that for the 10-micrometer droplet, the freezing time was independent of the number of nucleation sites, whereas for the 1-micrometer droplet, multiple nucleation sites could accelerate freezing [4].
These are simulation results for idealized micro-droplets, not measurements of raindrops falling through real clouds, so they should be read as a guide to how size changes the physics rather than as a timetable. What they establish is that below a certain size, the limiting factor shifts from heat removal inside the droplet to heat exchange with the surrounding air [4].
Sources used in this answer
Simulation of freezing a sea water droplet moving in a cold air
A numerical simulation models the freezing of a sea water droplet falling under gravity in cold air, describing cooling, solidification, and further freezing stages.
Interfacial ice sprouting during salty water droplet freezing
Salty droplet icing is governed by salt rejection accompanied by ice crystal growth, producing a brine film and an ice-sprouting pattern, and the authors propose a universal definition of freezing duration for droplets of varying salt concentration.
Experimental study on the freezing process of water droplets for ice air jet technology
A 5-microliter water droplet freezes in 68 seconds at about −20 °C in cold gas but in 1.7 seconds in liquid nitrogen, and lowering the air temperature from −10.67 °C to −24.68 °C cuts freezing time by 45.5%.
Freezing dynamics of supercooled micro-sized water droplets
In a 3D model, 10-micrometer and 1-micrometer supercooled droplets freeze by different patterns, with the 10-micrometer droplet's freezing time independent of nucleation-site number while multiple sites can accelerate freezing of the 1-micrometer droplet.
Experimental and numerical study on freezing process of water droplets under surfaces with different wettability
On surfaces of different wettability, droplet freezing time decreases faster with surface temperature on a superhydrophobic surface than on a hydrophilic one, and the frozen front grows from the bottom upward.