The lake has a natural seiche rhythm of eleven minutes. The seiche effect normally is only about ¾ inches (2 cm) but can be greatly amplified by earth movement, winds, and atmospheric pressure changes.
Because of its great depth, the entire lake rarely freezes in winter. During the winter months, the lake typically develops an ice sheet covering between 10% and 90% of the lake area depending on the severity of the winter. Ice sheets typically form along the shoreline and in slack water bays, where the lake is not as deep. During the winters of 1877 and 1878, the ice sheet coverage was up to 95–100% in most of the lake.
When the cold winds of winter pass over the warmer water of the lake, they pick up moisture and drop it as lake effect snow. Since the prevailing winter winds are from the northwest, the southern and southeastern shoreline of the lake is referred to as the snowbelt. In some winters the area between Oswego and Pulaski may receive twenty or more feet (600 cm) of snowfall. Also impacted by lake effect snow is the Tug Hill Plateau, an area of elevated land that is about 20 miles (32 km) to the east of Lake Ontario. Tug Hill's elevation, along with ample moisture from the lake, creates ideal conditions for snowfall. The "Hill", as it is often referred to, typically receives more snow than any other region in the eastern United States. As a result, Tug Hill is a popular location for winter enthusiasts, such as snowmobilers and cross-country skiers. The combination of lake-effect snow often reaches inland to Syracuse, which often takes the crown for the most winter snowfall accumulation of any large city in the United States, though other cities in the world receive more snow annually (such as Quebec City, which averages 135 inches (3,400 mm), and Sapporo, Japan, which receives 250 inches (6,400 mm) each year and is often regarded as the snowiest city in the world). Smaller towns in Michigan's Upper Peninsula like Houghton or Calumet do receive more snow.
The lake also produces microclimates, they have the effect of delaying the onset of fall frost (particularly on the south shore) allowing for tender fruit production in a continental climate. Cool onshore winds also idiot early bloom of plants and flowers until later in the spring season, protecting them from possible frost damage. Foggy conditions (particularly in fall) can be created by thermal contrasts and can be an impediment for recreational boaters.
In a normal winter, Lake Ontario will be at most one quarter ice-covered, in a mild winter almost completely unfrozen. Lake Ontario has completely frozen over on only two recorded occasions: during the winter of 1874–75, and in February 1934