What if cold wind blew over a warm lake?

Water stays warm for a long time after summer. Air cools down much faster.

What if very cold wind blew across a lake that was still warm? Let's find out.

By late fall, the air over the Great Lakes region can turn freezing cold, but the lakes are slow to cool and their water stays relatively warm. That difference sets up one of winter's most surprising kinds of weather.

What if cold wind blew across a warm lake? In this thought experiment, you can follow three paths: how the air picks up moisture, why the snow falls in narrow bands, and what happens around the Great Lakes, and across the world in Japan.

Water's high specific heat means lakes warm and cool more slowly than land or air. In late autumn and early winter, that leaves large lakes much warmer than the arctic air masses that begin to cross them.

This thought experiment follows a parcel of cold, dry air across a warm lake and asks what happens at each stage: the transfer of heat and moisture, the formation of clouds and snow bands, and where the snow lands. The physical sequence is well documented by the National Weather Service; specific imagined outcomes, such as what would change if a lake froze, are labeled as ideas.

Cold wind blows over the warm lake. The lake warms the air at the bottom and gives it lots of water.

Sometimes you can even see the lake steaming!

The National Weather Service explains that the cold air often comes from Canada. As it passes over the unfrozen, relatively warm water of the Great Lakes, warmth and moisture move from the lake into the bottom layer of air.

On very cold mornings you can sometimes see this happening: wisps called steam fog rise from the water, because very cold air is drifting across much warmer water. Warmed and moistened from below, the air begins to rise.

Cold, dry continental air crossing open water gains sensible heat and water vapor through its lowest layer. Warmed and moistened from below, the air rises and clouds form. The NWS describes this transfer of warmth and moisture into the lowest portion of the atmosphere as the first step of lake-effect snow.

The NWS glossary describes a visible sign of the same exchange: steam fog forms when water vapor is added to air much colder than the water supplying it, as when very cold air drifts across relatively warm water. Below about −20°F, it can freeze into ice fog called frost smoke. USGS notes that water's high specific heat keeps lakes warm long after the air cools.

Let's pretend we ride with the cold wind across the lake.

We get warmer and wetter. We rise up and make clouds. On the far shore, snow falls! This is a pretend ride.

Imagine riding along with a gust of freezing wind as it crosses a big lake. At the start, the air is cold and dry. Over the water it warms a little at the bottom and soaks up moisture. It starts to rise, and clouds bubble up.

By the far shore, the clouds might be heavy with snow. If the wind kept blowing the same way for hours, the snow might keep falling on the same towns. And if the lake froze over, the wind might pick up much less moisture. These are ideas, built from how lake-effect snow works.

Following one parcel: it starts cold and dry over land, gains heat and vapor over the water, rises, and builds clouds that grow into bands. Over a long fetch, it might accumulate enough moisture to snow heavily on landfall.

Two variations show what matters. A steady wind direction might pin a band over the same communities for hours, since the NWS calls wind direction a key factor. And because the NWS describes lake-effect snow forming over unfrozen water, a lake that froze over might cut off the supply of heat and moisture. These consequences are reasoned, not model results.

The clouds line up in long, thin stripes. Under a stripe, lots of snow falls.

Just a little way away, the Sun might be shining!

The rising air forms long, narrow bands of clouds lined up with the wind. Under a band, the National Weather Service says, snow can fall at 2 to 3 inches an hour or more. But just a mile or two to either side, the Sun may be shining.

Lake-effect storms are not like ordinary snowstorms, which come with big areas of low pressure. They come from the lake itself, and the snow usually lands to the south and east of the Great Lakes, where winter winds carry it.

Convection over the lake organizes into narrow bands aligned with the flow; the NWS reports snowfall rates of 2 to 3 inches an hour or more beneath them, with sunshine possible a mile or two away. Wind direction determines which shores receive snow, and the geography of land and water shapes the bands.

NSSL emphasizes that lake-effect storms are not low-pressure-system storms: a cold, dry air mass picks up moisture from the lakes and releases it as snow, generally to the south and east of them. Shoreline shape matters too: research around Lake Ontario found that bulges in the shoreline help create air-mass boundaries that organize the bands.

Tug Hill is a hilly place near a big lake. It gets huge amounts of snow.

In Japan, cold wind blows over a sea. It makes even more snow!

East of Lake Ontario in New York, the Tug Hill Plateau rises about 2,000 feet and gets some of the most intense snowstorms on Earth. During one three-week stretch, scientists measured 100 inches of snow there, including one storm that dropped 40 inches in 24 hours.

The same thing happens over seas. Cold air from Asia crosses the Sea of Japan, which is more than 50 times bigger than Lake Ontario. Sapporo, a city of more than a million people, averages 235 inches of snow a year.

The NSF-funded Ontario Winter Lake-effect Systems (OWLeS) project used Doppler on Wheels radars, the University of Wyoming King Air aircraft and mobile weather balloons around Lake Ontario. Over one three-week period it recorded 100 inches of snow on the Tug Hill Plateau, three storms with peak rates of 4 inches an hour and one storm of 40 inches in 24 hours.

Jim Steenburgh of the University of Utah calls Japan's sea-effect region the 'Greatest Snow Climate on Earth'. Cold outbreaks from interior Asia cross the Sea of Japan, more than 50 times Lake Ontario's area, into high mountains: Sapporo averages 235 inches a year, and Sukayu Onsen in northern Honshu averages 694 inches.