I always thought that Snow in Summer was a plant.
The problem is that Poplars are wind-pollinated, and they also use the wind to spread their seed, so the female trees produce masses of fluffy white fibres to support the seeds.
Just like Dandelions. Only on a rather larger scale...
This white "fluff" is produced in June, and if it's not a particularly windy day, it can pile up and look just like snow. Where I live, there are a small group of large Poplars, and it's quite an impressive sight, watching the road collect ankle-deep drifts of the stuff!
All types of Poplar do this, as do Willows: but only the female trees produce seeds, so if you want to avoid it, buy male plants - although, of course, the trick is to find trees that have been accurately labelled, as you won't know for sure if your tree is male or female until it reaches maturity... and it's a bit late, then!
Apparently it's a big problem in Russia and China, both of which used Poplars as a cheap and quick way to "green up" their cities after the second World War, not realising what a problem they were creating.
So what is the "real" Snow in Summer?
Cerastium tomentosum is the proper name, also known as Dusty Miller, Jerusalem star, Snow Plant etc, and it's actually part of the carnation family.
As you can see, the name is well deserved!