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The Santiago forum's spring thread is a corrective for anyone reading October as southern-hemisphere summer: it is when the weather is only starting to turn, and the days are not mild yet. November and December are when it settles into warm days that are never too hot, around 80 to 85F, with a light sweater still wanted after dark. The coast is its own case - the Humboldt current keeps it cool once the sun goes down, jacket weather at any time of year. On smog, regulars are clear that it is a winter problem, worst around the airport on the west side and not normally bad in the eastern parts of the city. The record: 23.9C days but 6.9C nights, 5.2 precipitation days, 11.2 hours of sun, AQI 84.

Regulars are specific this counts as the highest tourist season of the year across the whole country, a real booking-and-pricing concern rather than just a weather note - rooms and flights genuinely scarcer without planning ahead. Our row backs that hot, genuinely dry read, 30.7C reading as a hot 32C, over 13.2C nights, with rain essentially absent, 2.1mm across under a day, and humidity down to 54 percent. Regulars are consistent evenings cool noticeably after a hot day, a light coverup or sweater worth having even mid-summer. Nearby coastal towns are a popular escape from the heat, sea temperatures cool enough to refresh without being warm enough for a long swim. The record: 12.3 hours of sun, a 12.4 kph wind and AQI 102.

December is summer here and the board describes it as one long dry day with a cold end: around 30C in the afternoon, low twenties by ten at night, cooler again before dawn. Our row is that swing written as two numbers, 30C over 11.6C - an eighteen-degree range inside a single date. At 53.6 percent humidity the heat is dry, so a step into the shade cools you immediately, but the sun is strong enough that finding shade is the whole trick. Rain is 6.5mm across not quite one day. Most evenings want a sweater, and hotels are air conditioned where apartments often are not. The smog the board argues about belongs to winter: AQI 75 here against 138 in July. The record: 13.1 hours of sun.

March is autumn here and regulars rate it highly - exceptional weather without the summer crowds and without summer prices, still warm, with a wide gap between day and night. Our row is that gap exactly: 28.8C down to 11.1C, at 53 percent humidity and 3.1mm of rain across a single day. On the city's other reputation they are specific rather than dismissive: the smog is not usually bad in the eastern districts, is worst out west towards the airport, gets better on wet days, and turns on the weather rather than the season. Our AQI of 102 is a city-wide average and hides all of that. The record: 11.3 hours of sun and an 11.3 kph wind.

January in Santiago is hot and dry - most days high-20s to 30C, our row saying 30.4C at 54 percent humidity with 0.5 precipitation days - and regulars stress that the heat is a dry one that always drops away in the evening, so a light top for after dark is worth carrying. Their practical advice is to put the tours in the morning and look for air conditioning in the hotel. The unexpected bonus is the air: January and February are the Chilean holiday months, the city empties, and the smog that defines Santiago's winter falls to its lowest of the year. Our row's AQI 83 is the annual low rather than a good absolute number. The record: 13.5C nights, 7.3mm of rain and 13 hours of sun.