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One poster's two data points say everything: "a day in late March of 2012 when it was 26°C. Last year, it barely cracked 5°C." March–April is called a transition from winter to spring where "temperatures swing wildly from day to day," and the forum's blunt view of forecasting it is that "predicting weather is a fool's gamble this far out." Some Canadians have shorts out by early April and golf courses can open; late March can still be winter, and it very much still is further east in Montreal and Quebec City. The advice is the only advice that works here — bring warm clothes but layer, and bring variety so you have options. Our record puts March at 4.9°C days, -2.4°C nights and 14.9cm of snow, which is the cold end of the forum's range, not the middle of it.

Regulars are honest this can swing from very nice, shorts-and-t-shirt days to stupidly hot and humid stretches where the main plan becomes sitting in an air-conditioned room, especially mid-to-late month. Our row backs the hot, humid read, 28.2C reading as a hot 30C, over 18.1C nights, with 99.5mm of rain across 8.8 days. Regulars specifically call the mist thrown off the falls themselves genuinely refreshing on the hottest days, a real, checkable relief rather than a gimmick. Long sleeves for evenings are still standard advice even after a hot day, since nights cool down more than the daytime heat suggests. Regulars are consistent this is peak season, with July and August the busiest months of the year here. The record: humidity 73 percent, 12 hours of sun, a 17 kph wind and AQI 70.

August here is as busy as July, because the Ontario school holidays run through both, and regulars say the cooling only starts around the middle of the month. Our row gives 27.3C reading as 29C at 74 percent humidity over 17.6C nights, with 82.6mm of rain across 8.7 days. The useful local knowledge is about water rather than weather. The Canadian side carries a permanent heavy mist and the American falls are easier to look at; and the attractions sort themselves by how wet they leave you - misted at the top of the gorge, damp behind the falls, wet on the boat, soaked in the jets and the Cave of the Winds. Dress for the one you booked. The record: 11.2 hours of sun and AQI 55.

Regulars make a correction to the forecast rather than repeating it: anywhere near the river, the falls or the lakes it will be cooler than the number says, and close in the mist chills you quickly and drops the temperature fast after dark. Take a jacket whatever the app promises. Mid-May is their pick - the crowds are not yet at summer level and the days are long enough to walk - with the caveat that Victoria Day, around the 24th, fills the hotels and lifts the prices without reaching mid-summer levels. Their 70F average is our 20.2C, over 8.8C nights feeling like 6C. The record: 76mm across 9.8 days, 10.5 hours of sun inside 14.7 of daylight and AQI 62.

September at Niagara is the month regulars recommend for the trade it makes: the weather is still good - average highs in the low 70s Fahrenheit, lows in the low 50s, which our row puts at 23.5C and 13.5C - and everything is still operating, but the July and August peak is over and the hotel rates come down with it. The crowds go in stages. They do not begin to thin until after Labour Day, and the weekend crowds hold on into October, so the advice attached is to come on a weekday; late September into early October is the part they name as the best combination of the two. The water in the river and Lake Erie is still around 70F. The record: 6.7 precipitation days carrying 68mm, humidity 74 percent and 9.6 hours of sun.