The other night, many of the colder parts of Vermont had their first frost. Our garden didn’t quite freeze, sparing the beans for another week or two, but I found this frost at the rest stop in Randolph:
Before I went to bed Monday night I was looking at the RadarScope weather radar app (highly worth the money if you want a good weather radar app) and noticed an odd area of radar returns. I didn’t get a snapshot, but took one Tuesday night when the phenomena repeated.
I was curious. The night was completely clear a half hour earlier, so I knew it wasn’t raining or snowing. I pulled up the “Hydrometeor Classification” view, in which the new dual-polarization radar is able to determine the type of precipitation that is falling. This indicated neither rain nor snow, but instead mostly “biological” echoes – flying animals of some sort.
I fell asleep without thinking much more about this, but the next morning I saw this interesting paragraph in the technical forecast discussion:
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BURLINGTON VT
627 AM EDT TUE SEP 17 2013
INTERESTING THAT THE DOPPLER RADARS AROUND THE REGION CONTINUE TO
PICKUP RATHER HIGH RETURNS OF 20-30 DBZ NEAR THE FREEZING LEVEL
ALOFT AT 2-3K FT NEAR BTV AND DUALPOL ALGORITHMS INDICATING MOSTLY
BIOLOGICAL RETURNS THINKING MAINLY INSECTS. VAD WINDS RATHER
UNIFORM AT 25-45 KNOTS FROM 1K TO 6K FT.
For us, the possibility of an early frost means covering any still-living garden plants with an old curtain and taking the wood furnace for a test run. For many organisms, including many plants and insects, the first frost is the last day of life. Hopefully before the frost they have had the time and energy to create enough eggs and seeds to overwinter. An early frost potentially means the loss of several weeks of growing, mating, laying eggs, and the other things organisms frantically try to finish in the early fall. This week, for instance, highs are forecast to hit the mid to upper 70s for the next two days, and no temperatures below 40 are forecast for at least the next seven. If there were a way to survive one or two nights of early frost, insects would get at least a week of extra time…
Early and late frosts tend to form during clear, dry, mostly calm nights where cold air can sink into hollows. Montpelier and Barre did not freeze because river valley fog covered the valley like a blanket, but many areas above the fog such as the Montpelier Airport (actually in Berlin, Vermont) and the previously mentioned rest stop, did freeze. If you were to rise above the hollows and hills, however, you would remain above freezing until you reached the above mentioned freezing level of 2000 to 3000 feet.
I wonder if this is exactly what the insects were doing. It would take a lot of energy to spend hours hovered high above the ground, but if the alternative is death, it would be worth trying. If countless millions of insects were hovering above Vermont’s cold hollows on the last couple of chilly nights it seems likely that enough would survive to descend after morning light, and enjoy this last week of warm weather. These insects would lay more eggs than their deceased brethren, thus reinforcing the behavior in future generations.
Or maybe the flight of insects was unrelated. Maybe there was something wrong with the radar. I’m not sure, as I’ve never heard of insects flying above temperature inversions in early fall. If anyone does know, either way, I’d love to hear about it!