When is it no longer ‘cooler by the lake’?

You’ve been there before: It’s a rare 80-degree spring day in Chicago — perfect for an impromptu trip to the beach. But by the time you get to the lake, fighting against the wind to lay down a flailing beach towel, you see the goosebumps on your arm and remember the weather forecast: “80 degrees and cooler by the lake.”

But how close to Lake Michigan do you need to be to feel that dip in the temperature?

Lake Michigan is a powerful force. It can be an air conditioner, a heater, even a fog machine. There’s a science to this force of nature and the role it plays in Windy City weather, from generating microclimates to dumping lake-effect snow.

Cooler by the lake

The phrase “cooler by the lake” normally comes up in the spring and early summer, when the land and air are warmer than the water.

“Lake Michigan, we’re really blessed with this resource, but it is a huge body of water,” said Illinois state climatologist Trent Ford. Such a large water mass doesn’t heat up as fast as the air and surfaces around it, like grass, soil and especially concrete.

The city’s hot concrete jungle gives off heat, while Lake Michigan — a vast, cool body of water — absorbs heat.

“It's essentially taking in some of that energy that the city is giving off and therefore cooling a narrow strip of land right along the lake,” Ford said.

How far west of Lake Michigan does a Chicagoan have to travel before it’s no longer “cooler by the lake”?

Ford said it’s about a mile, but simply crossing to the west side of Halsted won’t immediately make you warmer. “It can be a bit of a gradient,” he said.

Usually by late summer, the water heats up enough that it’s no longer cooler by the lake. Then, in the fall and early winter, the effect reverses: It takes the water longer than the air and land to cool down, making it warmer by the lake.

“That same strip of land that was cooler in May than the rest of the city is now slightly warmer than the rest of the city through December and and even into January,” Ford said.

But the warmth of the lake in the fall and early winter doesn’t make as big a difference as the “cooler by the lake” phenomenon. The days are shorter, so the lake gets less energy from the sun to stay warm.

Lake-effect snow

Another Lake Michigan phenomenon can happen as early as the late fall, when the lake is still warmer than the surrounding air.

Usually Northwest Indiana and Western Michigan bear the brunt of lake-effect snow because of the usual flow of winter air. It starts with extremely cold air coming from the Rocky Mountains, across the Great Plains, traveling southeast toward Chicago.

“That cold air is just losing energy, just because it's getting colder and colder and colder, and then it slams into Chicago from the west. It's going to be really cold in Chicago,” Ford said. “It keeps pushing and it hits Lake Michigan, which is relatively warm.”

When warm water meets cooler air, it evaporates. So the air over Lake Michigan is not only getting warmer because of the lake, it’s holding more moisture. Ford said that warmer, humid air pushes across the lake into a colder Indiana and Michigan.

“All of a sudden, that warm, wet air starts to cool down, and it has to lose some of that moisture, has to condense out some of that water, and it does so in the form of snow,” he said — hence, lake-effect snow.

Because the jet stream moves away from the Windy City toward Indiana and Michigan, Chicago doesn’t usually get lake-effect snow. But it has happened, perhaps most notably during the Groundhog Day blizzard of February 2011.

For lake-effect snow to happen in Chicago, the winds need to reverse. Instead of going east toward Indiana and Michigan, they need to come west toward the city. A low pressure system, also known as a storm system, must come from the south, around Champaign or Kankakee.

Then a lot of other things need to line up, including the right amount of moisture, cold air and the right lake conditions for winds to turn back across Lake Michigan and push into Chicago.

“It's that point where the winds are taking warmer, humid air off the lake and pushing it into Chicago, hitting the cold city, and it dumps snow,” Ford said.

It’s rare that all these stars actually align. The low pressure system may come from north of Chicago instead of south, or by the time the wind travels to Lake Michigan from the south, the wind is no longer cold enough.

Chicago has already seen record snowfall this winter, so the city is doing just fine without adding lake-effect snow to the mix.

Erin Allen is the host of Curious City.