Science Snapshots: No, Philadelphia did not get July snow
Manorites, I'm briefly coming out of parental leave from writing for the Manor to express my outrage at the headlines.
"Snow reported at Philadelphia International Airport in the middle of July: NWS"
"It Snowed in Philadelphia Sunday: National Weather Service"
"Philadelphia International Airport sees snow in July"
"Philadelphia sets record for snow in July"
Did the US National Weather Service report a trace of snow on July 14, 2024? Yes. Does that technically set a record? Yes. Did it actually snow? NOOOOOOOOO.
What you'll see in all these click-baity articles is that once you get past the headlines, it HAILED.
HAIL, MY HORS.
This all comes down to a peculiarity with reporting criteria that is frankly, dumb.
Thunderstorms that came through Philadelphia on Sunday hailed enough to leave a trace (that's <0.4 in or less depth - 10 mm for the smarter 98% of the planet) of frozen precipitation, i.e., hail, on the surface. And frozen precipitation gets counted as snow in climate reports. So that means Philadelphia technically set a record for snowfall on July 14, because the previous record was 0 inches (0 mm).
Because the previous record was zero, the National Weather Service is mandated to issue a Record Event Report. That's what the news picked up on. My local office actually defies this and refuses because it's pretty dumb to have a trace of snow reported on a day where temps were 100 F or higher (37 C).
They're both frozen, but why are hail and snow different?
Snow forms when water vapor (it took me like 2 minutes to remember that "water in the form of a gas" is called water vapor) deposits (goes from gas to solid) on ice nuclei - dust particles, etc., up in a cloud. Sometimes (always?) that water vapor comes from supercooled liquid water droplets...water droplets that exist in liquid form below the freezing temperature (32 F/0 C). Each little ice particle grows at the expense of the liquid particles until it becomes big and heavy enough to fall to the surface in the form of snow. The manner in which it comes into being and grows is so complicated that I never did understand it when I took cloud microphysics in grad school, so I won't bore you with that. Just trust me.
Fun fact: if you live far enough away from the tropics, all your rain starts off as snow that melts on its way down. It's called the cold rain process, or Bergeron–Findeisen process. Bergeron–Findeisen process is much more fun to say. If you're in the tropics, some rain can start as liquid water in the cloud. That's a warm rain process. Because we can come up with acronyms like EVIL and SNOWPLOW for field campaigns and advancements, but the basic process is called cold and warm rain.
Helpful graphic from StratusDeck
Hail is different.
Hail starts from a core of a frozen raindrop or graupel. A strong updraft carries it high into the thunderstorm, until it gets heavy enough with additional frozen water coating it to fall back down. They it gets lofted again, rinse and repeat. Here's a helpful discussion from the National Severe Storms Laboratory (NSSL): https://www.nssl.noaa.gov/education/svrwx101/hail/ . These multiple coatings of water freezing on the surface to grow the hailstone is why you see layers if you cut one in half.
The stronger a storm, the stronger the updraft (rising motion). The stronger the updraft, the bigger the hail.
A metric shit ton of hail (the proper SI unit) can accumulate, and then the NWS decides to tell everyone a trace of snow fell during a July heatwave.
Another fun fact: they have to get out the snowplows to clear hail sometimes.
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