Lakes affect climate in a lot of different ways. They're a heat sink, so it takes them a long time to heat up to air temperature or cool down enough to freeze. Lake breezes can warm the climate in winter or cool it in summer. Lakes also bring storms, a lot of them, and some of them can be severe. In winter, they bring winter snow squalls. You haven't seen real snow until you've shovelled out from under a major snow squall!
A lake's role in moderating temperature
Lake water stores a lot of heat energy. It takes a lot of warm air for a lake to heat up! That's why a deep lake is still nice and cool, even when it's broiling outside. Guess why everyone who's got a free weekend in the summer heads straight to the lake?
Then they hang onto that heat for a long time after the air temperature's dropped. An evening lake is nice and warm and perfect for a refreshing swim before the late evening campfire. Of course, by morning, it's chilly all over again.
The same kind of thing happens during winter and summer, only on a larger scale. In the summer, it takes a lot of heat for the lake to warm up and stay warmed up. In the winter, the air has to stay pretty cold for a long time before the lake cools down enough to freeze over.
Some of the deepest and biggest lakes, never do freeze over, no matter how cold it gets. That's true for most of the Great Lakes. They've just got too much heat energy in them to tip over into freezing.
As a bonus, areas which are adjacent to lakes often get moderating breezes during the summer. Next to the ocean, those breezes can get pretty strong, but at a lake they're usually just perfect. If it's moving towards fall in a lakeside city like Toronto, those breezes can get downright chilly!
Between the breezes and the heat-sink effect, lakefront areas can have a temperature as much as 10 degrees Celsius more moderate than places which are more inland.
This is why Southwestern Ontario in Canada, which is a peninsula with three Great Lakes surrounding it, is often called the banana belt of Canada. It's a little too cold to actually grow bananas, but there are kiwi farms and lots of world-class vinyards.
When it's a big lake like the Great Lakes, sometimes those lake breezes collide with unstable air during summer afternoons. That's when you'll get pop-up thunderstorms all along the lake breeze front.
If there's any wind sheer in the upper atmosphere, those pop-up thunderstorms can produce a lake-breeze tornado. These kinds of tornadoes aren't usually very strong, only about up to an F2, but still bad enough if you're underneath one. It's a good thing they don't usually last long.
However, if there's a major front coming over the lakes, the incoming unstable air can collide with lake-breeze fronts to create some really nasty storms. That's when you get the F4s. So far, the Great Lakes region hasn't had an F5, but it's probably only a matter of time.
Snow squalls coming off the Great Lakes are hard to imagine until you've experienced one. Imagine being dumped on by half a metre of snow, but just a short drive to either side of you there's been bright blue sky all along. That's what a snow streamer's like.
Lake-effect snow is caused when cold Arctic air sweeps over the open water of a big lake and picks up some of its moisture. It hangs onto that moisture until it hits dry land. Then all that moisture lets go and falls as narrow streamers of snow, a lot of snow!
The Great Lakes snow machine doesn't turn off until the nearby part of the lake freezes over. It happens faster for Lake Erie, but Lake Huron snowsqualls can last right through until January!