Auckland does not have a painting season. It has painting windows.

Read that as a booking constraint rather than as trivia. Between June and September, Auckland offers somewhere in the order of two to four usable days a month. Every exterior painter in the city is chasing the same days, which is why winter quotes are quick to get and winter work is slow to schedule — and why a crew that promises to start your exterior in July is either working inside, or working outside the limits.

Two counts are computed. Dry runs are three-day windows where every day stays under the rain threshold — useful, but not sufficient, because a dry July day is often too cold. Fully paintable runs require all three days to pass every limit.

The gap between the two numbers is the interesting part. In winter Auckland has plenty of dry runs and almost no paintable ones. That is a temperature and humidity problem, not a rain problem, and it is why "but it hasn’t rained all week" is not the argument it sounds like.

Dew point is the temperature at which the air currently in front of your wall becomes saturated. Cool any surface to that temperature and water condenses on it. It is not a measure of how wet the day feels — it is a fixed temperature you can calculate from air temperature and relative humidity, and it is the single number that tells you whether a surface is about to get wet.

The rule is that the surface must be at least 3 °C above dew point when the coating is applied, and stay there while the film forms. Apply inside that margin and moisture condenses into the curing film. On a waterborne coating you get blushing: a milky, cloudy, patchy haze, usually worst on the shaded elevation, and it is not a cosmetic problem — the film has cured with water in it and it is weaker and less adherent than it should be. On an enamel you get loss of gloss and a soft film that stays tacky.

Two things about it are counter-intuitive and both matter on an Auckland job. First, the relevant temperature is the surface, not the air. A shaded south wall on a clear night radiates heat to the sky and can sit several degrees below air temperature well into the morning, so the air can read a comfortable 16 °C while the wall is still inside the margin. Second, it bites again at the end of the day: as the sun goes off a wall the surface cools fast while the dew point stays put, so a coat applied at four o’clock in April can be inside the margin by six — which is how a wall that looked perfect at knock-off is blushed the next morning.

The practical consequence is that an Auckland exterior has a working day in the middle, not from dawn to dusk. An infrared surface thermometer and a hygrometer cost very little and settle the question in about ten seconds. Anyone spraying a shaded wall at 7.30am in May has not checked.

How the dew point was computed

Magnus-Tetens formula (Alduchov & Eskridge 1996, a=17.625, b=243.04) applied to the daily mean temperature ((min+max)/2) and the daily mean relative humidity.

How the runs were counted

dryWindows3PerMonth counts sliding 3-day windows starting in that month where every day has precipitation < 1 mm. paintableWindows3PerMonth is the stricter version: every one of the 3 days passes the full paintable-day test. Both are totalled over the 30 years and divided by 30, so they are an average per occurrence of that month. Windows are sliding, not exclusive, so consecutive good runs are counted once per start day.

Where this model is weak

• ERA5 is a ~9-31 km reanalysis grid cell, not a weather station. It represents central Auckland as a whole and will understate microclimate (coastal exposure, valley shade, inner-west humidity).
• Relative humidity is a DAILY MEAN. Afternoon RH is typically lower and dawn RH higher, so the humidity test is a day-level screen rather than an hour-level one.
• Air temperature is used as the proxy for SURFACE temperature. A shaded south wall runs colder than air and a north wall in sun runs hotter, so both temperature limits are optimistic on the shaded side of a house.
• The rain test uses a daily total, so it cannot distinguish an overnight shower from a downpour at 2pm.

• Book exterior work for the October-to-April half. Not because winter painting is impossible, but because winter painting is mostly waiting, and waiting on a scaffolded house costs money.
• Treat February and March as the prime window. They combine the paintable-day rate with the longest useful runs, and the wall has had a dry summer to dry out.
• Use winter for interiors. Inside, you control the temperature and the humidity, and the whole limit set becomes a non-issue. This is the sensible seasonal split, and any competent Auckland painter is already working to it.
• Expect a late start each day, all year. The dew-margin numbers above say the wall is not ready first thing on most mornings. A crew that starts spraying at 7.30am in May is not waiting for the surface.
• Ask what the stop rule is. A specification should say the conditions under which the crew will stand down. If there is no stop rule, the stop rule is the deadline.