This video was sent in by a good friend in the Midwest. Credit to: JMS, southern WI, snowbelt territory. I lived in Wisconsin for many years before moving to the sunnier climes of Arizona. And I definitely remember the phenomenon of “squeaking snow.” It didn’t always happen, so when it did it got your attention. But I never really thought about the science behind why snow squeaks. His video inspired me to delve deeper.
When snowflakes fall to the ground, what happens to them is a function of several environmental factors: air and ground temperature, humidity, wind speed, and snow depth. To form squeaky snow the individual snowflakes need to “weld” together in a rigid array, instead of stacking in a pile of individual flakes with air pockets between them. This starts to happen when a sufficient depth of snow begins to press the flakes together.
When two snowflakes come into physical contact, and if all the environmental factors are in the right range, small drops of liquid water can form on the surface of partly melted crystals. When two crystals touch, the drops will merge via cohesion and bind the crystals together. If the temperature continues to fall, as it usually does overnight, those drops can then freeze and become rigid connectors in a complex scaffolding of flakes. This process is known as sintering.
It’s the same thing that happens to individual ice cubes stored in a bucket or cooler. Over time, they weld to each other in large clumps that need to be broken apart for use.
Here’s a simplified (2D) diagram showing how flakes of different sizes and shapes can be in mutual contact and fuse into a rigid continuous sheet. Of course, in a real snowfall this connectivity occurs in 3D and binds the flakes in all directions:
The deeper the snow, the more the compression at ground level. Sintering increases with depth, where the flakes are more strongly pressed into physical contact. This gradient of sintering is crucial to producing the “squeak” sound audible in the video. Here’s a sonogram showing amplitude (loudness) vs. time for a single squeak from the audio track:
The sound from the snow is caused by the welds breaking — it’s like the cracking sound you hear when separating welded ice cubes. But in the case of snow, it involves millions of tiny welds breaking so their cumulative sound is audible. Note how the waveform oscillates rapidly at the start (when the foot is entering the upper softer layers of snow), and transitions to slower vibrations and louder sounds as the foot enters deeper more compressed layers. To our ears that waveform sounds like a “squeak.”
Under other environmental conditions the sound of stepped-on snow might be more like a “smoosh” (soft cold snow), or a “squish” (wet heavy snow).
A similar effect occurs between grains of sand and water. Silica is slightly soluble in water, so wet grains of sand can weld from the cohesion force of H2O. When those bonds break, sound will be generated. That’s why beach sand near the shore can also squeak.
Interestingly, several other distinct sounds can be made by sand. Check out the “singing sands of Wisconsin” in this video (sand sounds at 2:00):
https://www.pbs.org/video/in-wisconsin-singing-sands/
Sand can produce sounds described variously as “whistling” “barking” “roaring” and “booming.” The exact mechanism is still debated, but it requires sand grains of a certain size, shape, composition, and water content (sand is never 100% dry).
Next Week in Sky Lights ⇒ How They Make Helium