Warming is not distributed evenly across the day. Overnight lows have climbed more sharply than afternoon highs across much of the country, and the reason lies in how the surface sheds heat after dark.
How the surface cools at night
During the day the sun adds energy directly. At night that input stops, and the ground cools by radiating heat upward toward space.
Some of that radiation escapes and some is absorbed by the air above and returned downward. The balance between the two sets how far the temperature falls before dawn.
Anything that increases the downward return slows the fall, which raises the minimum without touching the afternoon maximum.
Why humidity is the dominant control
Water vapour is the most effective absorber of outgoing heat in the lower atmosphere, and a warmer atmosphere holds more of it.
Humid nights therefore cool less than dry nights, which is familiar to anyone who has compared a desert evening with a coastal one.
As the air holds more moisture on average, the nightly floor rises, and the daily range between high and low narrows.
How cloud cover reinforces the pattern
Clouds behave like a lid at night, absorbing heat leaving the ground and radiating much of it back down.
By day the same clouds reflect incoming sunlight and hold the maximum down, so a cloudier climate compresses the range from both ends.
Changes in cloud frequency therefore leave a distinct fingerprint that shows up more in minimum temperatures than in maximum ones.
Why cities amplify the effect
Paved and built surfaces store heat through the day and release it slowly through the evening, so urban air starts the night with a reservoir the surrounding countryside lacks.
Buildings also block the open sky view that lets the ground radiate freely, trapping heat in streets and courtyards.
The urban and the broader signal add together, which is why the sharpest increases in overnight minimums show up in built-up areas.
Why the night matters more than the peak
The human body relies on cooler night hours to shed accumulated heat, and buildings without cooling depend on the same relief.
When the minimum stays high for several nights in a row, that recovery does not happen, and heat stress accumulates instead of resetting.
Warning systems have shifted toward overnight thresholds for exactly that reason, because the daytime peak alone misses the part of the cycle that does the damage.