Frost days, hot days, tropical nights
Climate indicators count how many days per year cross a given threshold — frost, strong heat, rain, a warm night — and so describe the distribution of a climate where an annual average merely summarises it.
Why count days rather than average
Two cities can share exactly the same annual mean and have nothing in common. One stays within a narrow band all year; the other alternates hard winters and scorching summers that, averaged, give the same figure.
A day-count restores what the average destroys. It answers concrete questions — how many sleepless nights, how many days of clearing snow — and it compares between places without anyone having to interpret a distribution.
It is also the form in which climate change reads most clearly. One and a half degrees of warming cannot be felt in an annual mean; it is very visible when a place goes from sixty frost days to thirty-five.
Frost day — minimum temperature below 0 °C
A day counts as a frost day if its minimum temperature drops below 0 °C. The threshold applies to the daily minimum, not the mean: a day can be well above zero in the afternoon and still have frozen at dawn.
It is the indicator most sensitive to warming, and by far the most legible. It matters to agriculture — late frosts on crops — as well as to buildings and road maintenance.
Hot day — maximum temperature of at least 30 °C
A day counts as a hot day if its maximum temperature reaches or exceeds 30 °C. The 30 °C threshold is common practice in Europe; other regions count at 35 °C, so an international comparison requires checking that the same threshold was applied on both sides.
A hot day is not a heatwave: a heatwave is defined by duration and by nights that fail to cool, not by crossing a threshold on a single day.
Tropical night — minimum temperature of at least 20 °C
A night is called tropical when the temperature does not fall below 20 °C. It is the most important of the four for health, and the most underrated.
The reason is physiological: the body tolerates a very hot day if it can recover at night. It is the accumulation of nights without relief that makes heatwaves lethal, far more than the daytime peak. It is also the indicator where the urban heat island effect is strongest — a dense city releases at night the heat it stored during the day.
Rain day — at least 1 mm of precipitation
A day counts as a rain day from 1 mm of accumulated precipitation. The threshold excludes traces and drizzle that wet nothing, and that would otherwise inflate the total.
This indicator is always read alongside the annual total, never alone. Many rain days and a low total describe an oceanic drizzle climate; few rain days and a high total describe violent, rare downpours, which is an entirely different regime — and an entirely different risk.
What a threshold implies
A threshold count is discontinuous: a day at 29.9 °C does not count, a day at 30.1 °C counts in full. In a climate whose distribution is dense around the threshold, a small shift in the mean can therefore produce a spectacular change in the count.
That is not a flaw in the measurement, it is a property to know about: threshold indicators amplify the signal, which makes them very legible but forbids reading them as proportional quantities.
A resolution reminder: these counts come from ERA5 cells about 28 km across. They describe a grid cell, not a garden — and not a city centre either, whose heat island is smaller than the cell.