Niagara Falls, where they are located

In the Niagara Falls area, the climate is continental, with cold winters and warm summers. The daily average temperature ranges from -4 °C (25 °F) in January to 22.5 °C (72.5 °F) in July. Although we are on the 43rd parallel, in winter the weather is cold because the North American landmass cools down a lot, and outbreaks of polar air masses are frequent, to the point that on colder winters the waterfalls can even freeze.
Here are the average temperatures of the Niagara District Airport, at Niagara-on-the-Lake, Ontario, Canada.
Niagara District - Average temperatures
Min (°C)-8-7-3391418171371-4
Max (°C)-1051319242726211582
Min (°F)181927374857646355453425
Max (°F)303241556675817970594636

Precipitation amounts to about 950 millimeters (37.5 inches) per year and is well distributed over the seasons. In fact, weather fronts can affect this area in every month because of the continual clash between warm and cold air masses; in summer, when they are less frequent, thunderstorms can erupt in the afternoon, adding more rain, so much so that summer is as rainy as autumn, at least in quantity. Here is the average precipitation.
Niagara District - Average precipitation

In winter, from late November to March, precipitation occurs mainly in the form of snow (there are on average around 50 days with snowfall per year). Snow normally covers the ground from mid-December to early March. In a year, about 180 centimeters (70 in) of snow fall; sometimes a snowfall can be seen already in October, and in spring, still in April. In winter, real snow storms can occur, accompanied by gusts of blizzard. During the most intense outbreaks of cold air, the temperature can drop to around -25 °C (-13 °F).
However, given the unstable weather, the temperature can exceed freezing (0 °C or 32 °F) even in winter, in periods when snow can melt and it can even rain.

Spring, from mid-March to May, is an unstable season, characterized by sudden changes in temperature: initially cold in March, with frequent snowfalls and frosts, it advances gradually, with the first warm days but also the return of cold weather; it is still on average quite cold in April, and becomes mild in May, when the first afternoon thunderstorms begin to occur.

Summer, from June to August, is warm and quite sunny, although there is a moderate amount of cool and cloudy days, with some chance of rain, while when the weather is fine, thunderstorms can erupt in the afternoon. Sometimes hot periods can occur, with highs around 34/36 °C (93/97 °F).

Autumn, from September to mid-November, is initially pleasant, with many sunny days, when the temperature exceeds 25 °C (77 °F): these periods, called Indian summer, occur in September, but sometimes also in October. As the weeks pass, however, the temperature decreases rapidly, weather disturbances become more frequent, and there are the first cold days.

Niagara Falls

The amount of sunshine in the Niagara Falls is overall good in summer, and acceptable in May and September; it is not high in the other months, and it's quite scarce from November to February, when the sun shows up rarely.
Niagara District - Sunshine

The falls are formed by the confluence of Lake Erie and Lake Ontario, at the border between the US state of New York and the Canadian province of Ontario. There are two waterfalls: the Horseshoe Falls, the main ones, at the border between Canada and the United States, and a short distance the smaller American Falls, which are entirely in the U.S. territory, and are separated from the Horseshoe Falls by Goat Island. The American Falls are actually divided into two because beside them there are the smaller Bridal Veil Falls, which are separated from them by Luna Island.
The waters flow northwards, in fact Lake Erie, in the south, is located at about 170 meters (555 feet) above sea level, while Lake Ontario, in the north, is located around 100 meters (325 ft) below. However, the actual jump is about 30 meters (100 ft) in the American Falls (at least compared to the rocks below) and approximately 52 meters (170 ft) in the Horseshoe Falls.
The big cities closest to the falls are Buffalo, NY, Usa, and Toronto, Ontario, Canada.

Lake Ontario and Lake Erie are very cold in winter, when the waters approach the freezing point, while in summer they become almost acceptable for swimming, at least for those who do not suffer from the cold, especially in August, when they reach 21 °C (70 °F). Here are the average water temperatures.
Lake Erie - Lake temperature
Temp (°C)32237132021191495
Temp (°F)373636374555687066574841

Best Time

The best time to visit the Niagara Falls runs from mid-May to late September. In midsummer, in July and August, not only the waterfalls are very crowded, but also there may be hot and muggy days, so this period can be excluded if you cannot stand the heat. You can therefore choose May and September; in May and early June, it can still be a little cold, and in any case, you have to take into account the possibility of rain and temperature swings. In the first half of October, it can be cold, but you can admire the trees in the spectacular autumn colors, while in spring you can see flowers blooming in the nearby botanical garden.