Map from Google - Niagara_Falls

In the Niagara Falls the climate is continental, with cold winters and warm summers. The average temperature of January is 24.5 °F, that of July is 72.5 °F. 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 in colder winters the waterfalls can even freeze.
Here are the average temperatures of Niagara District Airport, at Niagara-on-the-Lake, Ontario, Canada.
Niagara District Airport average temperatures
Niagara Airp. J F M A M J J A S O N D
Min (°F) 19 19 26 37 48 57 64 62 55 44 35 24
Max (°F) 31 32 42 55 67 76 81 79 70 59 47 36

Precipitation amounts to about 37.5 inches per year, and is well distributed over the seasons: in fact, weather fronts can pass in all months, due to the continual clashes between warm and cold air masses, and in the only season in which they are less frequent, that is summer, 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.
Average precipitation Niagara District Airport
Niagara Airp. J F M A M J J A S O N D Year
Prec. (in) 2.7 2.7 2.8 3.0 3.0 3.5 2.6 3.3 3.8 3.0 3.5 3.8 37.6
Days 13 12 12 12 12 11 9 10 10 12 13 14 140

In winter , from late November to March, precipitation occurs mainly in the form of snow (there are on average fifty days with snow per year). Snow normally covers the ground from mid-December to early March. In a year, about 6 feet 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 outbreaks of cold air from the north, the temperature can drop to around -15 °F.
However, given the unstable weather, the temperature can exceed freezing even in winter, so that snow can melt and it can even rain.

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

Summer, from June to August, is warm and quite sunny, though there's no shortage of cool and cloudy days with some chance of rain, while when the weather is fine, thunderstorms can erupt in the afternoon. Sometimes you may experience hot periods, with highs around 93/97 °F.

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

The amount of sunshine in the Niagara Falls is overall good in summer, and acceptable in May and September; it is not much in the other months, and it's scarce in winter, especially in November and December, when the sun shows up rarely.

Niagara Falls

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, in the border between Canada and the United States, and at a short distance the smaller American Falls, entirely in the U.S. territory, separated from the Horseshoe Falls by the Goat Island. The American Falls are actually divided into two, because at their side there are the smaller Bridal Veil Falls, separated from them by the Luna Island.
The waters flow north, because Lake Erie, in the south, is located at about 555 feet above sea level, while Lake Ontario, in the north, is located 300 feet below. The jump itself is, however, about 100 feet in the American Falls (at least compared to the rocks below), and approximately 188 feet in the Horseshoe Falls.
The closest big cities 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 70 °F. Here are the average water temperatures.
Water temperature Lake Erie
Lake Erie J F M A M J J A S O N D
Sea (°F) 37 36 36 37 45 55 68 70 66 57 48 41

When to go

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 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 must always take into account the possibility of rain and temperature changes. In the first half of October it can be cold, but you can admire the trees in the spectacular fall colors, while in spring you can see flowers blooming in the botanical garden.