Cancún weather, month by month, for an owner

What is the weather in Cancún like through the year?

It is warm in every month and wet in most of them: monthly mean temperatures run from 24.4 °C in January to 30.1 °C in August, and the year brings 1,371.3 mm of rain across 125.3 days. The rain has two peaks — June, then a larger one in September and October — and does not stop in winter, which is the part most descriptions leave out.

For anyone asking what the weather in Cancún means for a building, the useful comparison is not with a northern city but with the other coast. Acapulco receives slightly less rain in a year and gets it on roughly seventy days; Cancún gets a little more on nearly twice as many. A house here is not tested by one long wet season and then left to dry out. It is wetted a little, often, across most of the year.

The figures below come from station 23155, Cancún, of the National Meteorological Service, nine metres above sea level, on the 1991-2020 normal, with 28 or 29 years of data in each column. The last column is evaporation from an open pan, and it is there because it is the one that matters most to a house: it says how much water the air can take back off a surface in a month, against how much the sky put on it.

Read the rainfall and evaporation columns side by side and the year divides differently from the way a travel calendar divides it. In January, June, September, October, November and December more water falls than the air takes back. In the other six months the balance runs the other way, and in March and April it runs hard: more than three times as much evaporates as falls.

Which months are dry, and which are the rainy season?

The dry window is February to April, which together carry 140.3 mm of the year's 1,371.3 — about a tenth — with April the month with the fewest days of rain at 4.2. The rainy season runs from June to October and carries 812.2 mm, close to six tenths of the year, with October the wettest month at 277.1 mm across 17.6 days.

Inside the rainy season there is a pause that owners on this coast learn to read. July drops to 67.0 mm, less than half of June, before August climbs again and September and October deliver the heaviest third of the whole year between them. A house that came through June without a mark has not come through the season.

The months on either side of the dry window are the ones that surprise people. November, December and January bring 331.8 mm across 35.5 days of rain — more rainy days than June, July and August put together. The rain in those months is lighter on each day, but the air is cooler, evaporation is at its lowest of the year, and a surface that gets wet stays wet for longer. For a building, that is the half of the year in which damp settles rather than drains.

The extremes in the same record are far larger than the averages. The wettest month the station logged was October 2005, at 1,188.9 mm, and the wettest single day was 770 mm, dated 21 October 2005 — the day Hurricane Wilma made landfall on Cozumel, before moving slowly across the north-eastern tip of the Yucatán Peninsula. No roof, terrace outlet or run of guttering is tested by the normal. It is tested by the day the normal does not describe.

What do humidity and salt air do to a house that stays closed?

Humidity keeps every soft and every cool surface in a closed house damp for longer than it can tolerate, and salt air adds a film that holds that damp in place on metal and paint. The threshold is short: the US Environmental Protection Agency's guidance is that wet materials dried within 24 to 48 hours will in most cases not grow mould, and a house nobody opens for a month cannot meet that on its own.

The same guidance puts the target for indoor air below 60 per cent relative humidity, ideally between 30 and 50. The station's normal does not publish relative humidity, so the honest way to give the local figure is not to give it; what the normal does show is the mechanism, in the six months when rainfall exceeds evaporation and the air has nowhere to put the water it is carrying.

In a closed house the soft things go first, because they hold the most water and show it last: mattresses, upholstery, curtains, anything in a shut wardrobe, anything leather or paper. Then the cool surfaces — the shaded side of an external wall, the back of a fitted cupboard, the inside of a glass door on a grey morning — where warm air condenses without anybody having spilt anything.

Salt is the part that does not look like weather. It arrives on the air from the sea, settles on every surface facing the water, and on aluminium tracks, door hardware, balcony rails, air-conditioning condensers and pool equipment it turns ordinary damp into corrosion. A house shut down with everything switched off at the board is not being protected. It is being left to the air.

Sargassum is a separate calendar on this coast, with its own published cycle and its own tools for following it from abroad, and it has a guide of its own. What the standing work of keeping the air moving and the equipment running actually consists of, act by act, is set out in what a property manager does.

When does hurricane season start and end on the Caribbean coast?

The Atlantic hurricane season, which includes the Caribbean Sea, officially runs from 1 June to 30 November. Its climatological peak is 10 September, with most activity between mid-August and mid-October — the same weeks in which Cancún's normal reaches its wettest months.

Owners with a house on each coast should keep two calendars, not one. The Pacific season opens on 15 May and peaks in late August, as set out in the guide to Acapulco weather; the Caribbean season opens more than two weeks later and peaks later, and October, which on the Pacific side is already drying out, is on this side the wettest month of the year.

For a house, the practical consequence is that the preparation for the storm season and the preparation for the rainy season are the same work, and both have to be finished by the end of May. Drains, outlets, shutters, fixings, anything loose on a terrace: what is not done before June is done in a hurry under a warning, or not at all.

What is the weather in Cancún in December?

Cancún, Mexico, weather in December is warm by day and mild at night — a normal high of 29.4 °C and low of 21.0 °C — but not dry: the month brings 96.6 mm across 11.7 days of rain. January is cooler and wetter, at 28.7 °C and 20.0 °C, with 103.8 mm across 10.9 days.

Cool here is relative. The lowest daily minimum the station recorded across the whole normal was 9.5 °C, on 12 March 1996, and the normal minimum never falls below 20 °C in any month. A house on this coast is never heated, and it never gets the long dry cold that dries a northern building out from the inside once a year.

December matters for a reason beyond the temperature. It is the month in which evaporation reaches its lowest point of the year, 73.6 mm, against nearly a hundred millimetres of rain: whatever is damp in the house on arrival in December has been damp, in all likelihood, since October.

Which months is a house most likely to be occupied, and which is it most likely to be shut?

That is set by the owner's own year rather than by the climate, and for an owner who lives farther north it tends to mean a house opened in the northern winter and shut through the hot, wet half of the year. The climate's part is to decide what each of those choices costs the building: the months the house is most likely to stand empty, June to October, are exactly the months in which it most needs somebody inside it.

That mismatch is the whole case for planning a coastal house by the weather rather than by arrivals. The weeks when the owner is present take care of themselves; it is the long closed stretch, with its two rainfall peaks and the cyclone season on top of them, that decides what condition the house is in when the owner next opens the door.

There is one more useful overlap. The dry window, February to April, falls inside the months an owner from the north is most likely to be there, which makes it the natural time to see exterior work being done — painting, sealing, ironwork, roofs — rather than to hear about it afterwards.

What should be checked on the house before and after each season?

Three visits fixed by the calendar rather than by anybody's flight: one in May before the rains and the storm season, one in November after the heaviest months, and one in the February-to-April window to do the exterior work only that window allows. Between them, the standing work is to get inside the house often enough that the air moves and somebody notices damp before it becomes a ceiling.

The May visit is drainage and storm readiness: roofs and terrace outlets cleared and run with water, downpipes proved to discharge away from the walls, sealant runs and window seals checked along their length, sliding-door tracks cleaned and their drain holes opened, shutters and fixings tested, pumps and gate motors serviced. It is the plain content of preventive maintenance, and it is scheduled work rather than a response to anything.

The November visit is the same list read backwards, and it is the one that produces information: where staining appeared that was not there in May, which seal moved, what the soft furnishings smell like after five months, which hinge, lock or condenser has started to bloom. What it finds goes straight into the dry window three months later.

The February-to-April visit is the one that spends money well, because it is the only stretch of the year in which paint, render and sealant can be applied and left to cure dry. What to do in it — which elevation this year, which ironwork, which roof — is a decision taken before the window opens, as part of an annual plan, not a quotation sought after it has closed.

None of this helps an owner two thousand miles away unless it arrives in a form they can check without asking — dated, itemised, with photographs and the spending against them. That is what the monthly owner statement is for, and for a house on this coast the months that weigh most in it are June to January. How the rest of the work looks in this city, tower by tower and street by street, is on the page for Cancún.