L-Day26

 A blogger's work's never done. Wanted to put up something interesting and clever

for today's blog; turns out my starting point was ...strickly speaking, off the deep end.

On labor Day, at that. Below, chatting with Claude AI:

Seasonal temperature changes; I am working from the assumption that the way colder temperatures manifest themselves in the Fall is through colder nights, as we experience longer dark periods. Is this a reasonable view. Is there specific data on this??

This is a genuinely interesting question, and it's worth checking against actual climate data rather than just reasoning it through. Let me look into this.

Searched the web

Your intuition is reasonable as a starting point, but the actual data complicates it in an interesting way — and in some respects points the opposite direction from what you'd expect.

The core issue: longer nights alone don't dominate the autumn cooling signal

The bigger driver of fall cooling is reduced solar insolation overall — shorter days and a lower sun angle, which cuts daytime heating — combined with increasing cloud cover as the season progresses. Differences in diurnal temperature range between regions and seasons are driven by seasonal variations in insolation, and are modulated by factors like cloud cover that have an asymmetric effect on daily max and min temperatures. 

What the data actually shows about fall nights specifically

This is the counterintuitive part. If "longer nights → more radiative cooling" were the dominant mechanism, you'd expect the diurnal temperature range (the gap between daily high and low) to be especially large in autumn. It isn't. Multiple studies find the opposite pattern:

Diurnal temperature range shows strong seasonal variability, with the highest values in summer and lowest in winter — and during the transitional seasons, DTR is higher in spring than in autumn, based on 1961–2019 data across 16 European cities. 

A high-latitude analysis found DTR has a minimum near 0°C, with spring showing a higher DTR than autumn for a similar average temperature — attributed to snow's low heat capacity and a higher solar zenith angle in spring than autumn. 

In the Northern Hemisphere, the relationship between cloud cover and DTR is particularly strong in September–November, with the U.S. showing the highest correlation between cloud cover and DTR in that period, and the greatest cloud-related reduction in DTR also occurring in September–November. 

The mechanism there: increased cloud cover cools daytime maximum temperatures by blocking solar radiation, but in the presence of clouds, nighttime minimums are actually warmed because clouds trap outgoing longwave radiation. So as autumn skies get cloudier, nights get relatively less cold than clear-sky radiative cooling alone would produce — the opposite of "longer dark period = colder night" in a simple sense. 

Where your instinct does hold up

Clear, calm autumn nights genuinely do cool efficiently — radiational cooling is strongest on windless, long, clear-sky nights, since the ground loses more energy than it gains and cools faster than the air above it. So on any given clear fall night, the longer dark period does allow more cooling than a short summer night would. Your assumption works well as a night-by-night mechanism — it just isn't the main reason autumn as a whole trends colder, because cloud cover and reduced solar input dominate the average. 

There's also a separate, longer-term signal worth mentioning: in a warming climate, nighttime lows have in some regions been rising faster than daytime highs specifically in autumn — one study of China's Loess Plateau found nighttime minimum temperatures rose 1.88 times faster than daytime maximums in autumn, the largest seasonal ratio of the four seasons. That's a trend-over-decades finding, not a description of the day-to-day autumn cooling mechanism, but it's the closest "specific data" tying nighttime autumn temperatures to something distinctive about the season. 

If you want, I can pull region-specific data (e.g., for your area) on how much of fall's average cooling comes from dropping highs vs. dropping lows.

I am in St-Jean-sur-Richelieu...

Good, I can put actual numbers to this for your area. Using 30-year climate normals for Saint-Jean-sur-Richelieu (nearby weather station data covers the Montréal region well):

                                                                                       

A few things jump out of your own local numbers:

Daytime highs and nighttime lows drop by very similar amounts each month. From August through November, highs fall roughly 6–8°C per month, and lows fall roughly 5–7°C per month — close to lockstep. Your intuition that nights specifically are "leading" the cooling doesn't really show up here: the day–night gap actually narrows slightly as autumn progresses (12° in August down to 8° in November), rather than widening the way you'd expect if longer nights were doing extra work.

Why the gap narrows instead of widens: cloud cover climbs steadily through fall — the region goes from clearer skies in August to noticeably cloudier by November and December. More cloud cover in autumn does two things at once: it knocks down daytime highs (blocking sun) and it props up nighttime lows (trapping outgoing heat), which is exactly why the gap compresses rather than expanding.

What's actually happening physically: the dominant driver is the loss of daylight and sun angle together. Your daylight shrinks from about 14 hours in August to under 11 by November, and the sun sits much lower in the sky, so daytime heating input drops fast. Nights get longer too, but they're increasingly buffered by cloud, so the "extra darkness = extra cooling" effect is partly offset.

So the honest answer: your assumption holds in the trivial sense that clear, calm autumn nights cool efficiently the longer they last — but as a description of why the season as a whole gets colder in this specific climate, it's really the drop in daytime solar heating (shorter days, weaker sun angle, more cloud) doing most of the work, not a disproportionate collapse of nighttime temperatures. If anything, your nights are cooling slightly less aggressively, relatively speaking, than your days are through the fall.

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Oh,well!!

HAPPY LABOR DAY TO ALL!