When Opening a Berlin Window Actually Dries the Room
Everyone in Berlin is told to air the flat three times a day, and for roughly four months of the year that advice moves almost no water at all. What decides it is how many grams of water Berlin’s outdoor air is already carrying compared with the air in your room. This board draws every month against every hour of the day from ten complete years of Deutscher Wetterdienst readings: dark green where a five minute airing strips five grams of water from each cubic metre, orange and red where it puts water back in. Pick the room you are standing in and the whole grid redraws around it.
The grid is built from the Deutscher Wetterdienst Climate Data Center hourly record for Berlin-Tempelhof, station 433, covering the ten complete calendar years 2016 to 2025, which is 87,116 measured hours of paired air temperature and relative humidity. Each month-and-hour cell is the mean absolute humidity for that slot, converted with the Magnus formula, and compared against the absolute humidity implied by the room you select. DWD data is published under GeoNutzV and free to reuse with attribution. The surface thresholds come from the Umweltbundesamt mould guide, which puts mould growth at 70 to 80 percent relative humidity at the material surface.
The first cold night of a Berlin autumn puts water on the inside of the glass, and the advice that follows is always the same: lüften, three times a day. In January that advice strips 5.5 grams of water out of every cubic metre of air you swap. In August the same window puts 0.7 grams back in. The number nobody quotes is the absolute one, grams per cubic metre, and it is the only one that decides whether the window is helping. This board holds all 288 month-and-hour combinations for five real Berlin rooms, so you can see the answer before you reach for the handle.
Why relative humidity misleads you
Ninety percent humidity at 3 degrees and sixty percent at 20 degrees are not comparable numbers. Cold air holds far less water in total, so a raw reading on a weather app tells you nothing about whether outdoor air can absorb what your flat is producing. The figure that matters is absolute: grams of water per cubic metre. A living room at 20 degrees and 60 percent holds 10.4 grams per cubic metre, and outdoor air only helps if it carries less than that.
What the ten year record shows
Averaged across 2016 to 2025 at Berlin-Tempelhof, outdoor air holds 4.9 grams per cubic metre in January, 8.3 in October, 9.6 in September and 11.1 in August. Against that same living room, January airing removes 5.5 grams from every cubic metre and August airing adds 0.7. Across the whole grid, 49 of the 288 month-and-hour cells make the room wetter and another 68 shift under half a gram.
In summer the afternoon beats the morning
The instinct to air early because the air feels fresh is wrong in a Berlin summer. Moisture collects near the ground overnight and drier air mixes down through the day, so the dew point falls while the thermometer climbs. In July the air at 06:00 carries 11.3 grams per cubic metre and at 16:00 only 10.2. In August it is 11.6 at 07:00 against 10.4 at 15:00. From November to March the hour barely matters: the whole of January sits inside a 0.2 gram band.
The wall temperature where mould starts
The board also prints two surface temperatures for the room you pick. The first is where the air touching a cold wall reaches 80 percent humidity, which the Umweltbundesamt gives as the threshold for mould growth. The second is the dew point, where water finally appears as droplets. In a 20 degree, 60 percent room those are 15.4 and 12.0 degrees, so there is a band of about three and a half degrees in which mould has what it needs and nothing is visible.
What this board cannot tell you
It shows a normal Berlin year, not today’s weather. A wet week sits worse than its square and a dry one better, and a single warm humid spell in October can look like August for a few days. It also assumes your room really is at the temperature and humidity you selected, which is what a ten euro hygrometer in the coldest room will tell you. Put the meter where the problem is, not in the living room.
Read next
Mould in Berlin Flats: When Airing Dries the Room and When It Soaks It
Ten complete years of Berlin hourly weather turned on the one question every Berlin lease answers badly. Airing takes 5.5 grams of water per cubic metre out of a room in January and puts 0.7 back in during August, the summer afternoon is drier than the summer dawn, and mould starts on a wall 3.4 degrees before any water shows on the glass.