Numerous you despatched me emails concerning the snowflakes over the weekend.
Why had been the flakes so SMALL Saturday morning? Tiny flakes!
And then they obtained a lot larger Saturday afternoon and on Sunday. Why?
To see what they imply, right here is a picture of the snow falling Sunday morning. BIG FLAKES! Similar on Saturday afternoon
But a picture Saturday morning had snow so small you can barely see it! But snow was piling up.
There is a purpose for this distinction. The measurement, form, and construction of ice and snow crystals range with temperature, humidity and vertical movement within the ambiance.
So let’s do some ice crystal 101!
Snow begins with ice crystals that kind when water vapor freezes on to small ice nuclei (small particles) within the ambiance. This is a course of known as deposition.
Because of its molecular construction, water vapor produces six-sided ice crystals (hexagonal if you wish to impress your mates).
It seems the construction of the ice crystals (additionally known as ice crystal behavior) will depend on temperature and humidity (see determine).
Check out the determine under. On one axis is temperature. On the opposite is one thing known as supersaturation. When air rises, it will probably cool and may turn into saturated (100% relative humidity). But consider or not, with robust upward movement, the relative humidity can improve ABOVE 100%. Amazing. Relative humidity above 100% is known as supersaturation.
Keep in thoughts that ice crystals kind excessive within the ambiance, the place circumstances are cooler than on the floor.
Back to the determine. For comparatively heat temperatures (-10 to 0°C), the ice crystals are usually small and within the type of plates and cylinders.
But if temperatures aloft are in a magical zone from roughly -12 to -18C and supersaturation is excessive (which suggests robust upward vertical movement), big dendrites can kind. This is understood within the climate enterprise because the Dendritic Growth Zone (DGZ). For these circumstances, the ambiance is at most effectivity for forming ice crystals and snow. Cooler temperatures lead to extra of the smaller columns and plates.
And there’s something else: when huge snowflakes forming in chilly air aloft fall into warmer–near freezing or above freezing air within the decrease ambiance they will begin melting or turn into “stickier”. They can hit and be part of collectively into big snowflakes in a course of name aggregation
So now you’re prepared to grasp what occurred over the weekend!
We can begin with the simulated mannequin sounding from the UW forecasting system, exhibiting how temperatures and moisture (dew level) assorted with top at four AM Saturday morning, when the small snow crystals had been falling. Temperature is on the x-axis (°C) and top is in stress (700 is round 10,000 ft). Temperature is proven by crimson line and dew level by blue. When temperature and dew level overlap, the air is saturated (you want that to supply snow!). I’ve indicated with two dashed strains the temperatures good for giant dendritic crystals to kind (the Dendritic Growth Zone).
The robust forcing and saturation at the moment (Saturday morning) had been BELOW the DGZ (see arrow). In reality the air was not even saturated on the altitude of the DGZ. Good for the expansion of small crystals like we noticed.
And temperatures had been under freezing close to the floor, so not good for snow to stay collectively (aggregation).
But now let’s try the vertical sounding predicted for that afternoon when huge snowflakes had been falling. The air had turn into saturated within the crucial temperature zone of the DGZ…so extra huge snow crystals! And there was plenty of warming close to the surface–even above freezing. So the falling huge crystals would get stick and combination collectively in huge snowflakes….similar to we noticed! No marvel the snowflakes had been larger.
The backside line is that the kind of snowflakes reaching the floor will depend on temperature and humidity aloft, that are managed by many issues, together with the energy of upward movement within the ambiance and the adjustments in temperature and moisture produced by the winds aloft.
There are positively some little people which can be disenchanted that the snow is now melting….
______________________________________Public Radio Station KNKX and Cancel Culture
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