Weather Almanac for May 2000

GREAT LAKES BREEZES

As many of my readers know, I was born in Chicago within ten miles (16 km) of Lake Michigan and until my relocation to Vancouver Island, I had lived 45 years within 50 miles (80 km) of at least one of the Great Lakes. For most of those years I was within a few hours drive of two or three of the Lakes.

Only the shining, big sea waters of Gitche Gumee -- Lake Superior -- have never been within easy reach of my home. Lakes Michigan, Huron and its siamese twin Georgian Bay (that some consider the sixth Great Lake), Erie and Ontario have all had direct influences on the weather and climate around me through most of my life. And many an Autumn storm born of Mother Superior has slipped southward to bring rain, wind and snow into my weather picture.

Each season brings slightly different lake influences on weather patterns within the Great Lakes basin. The basin lies under one of the major North American storm tracks and the interaction between weather systems and the Lakes' waters add a further dimension to life here. I believe the richest variety of weather found anywhere on Earth occurs in this region.

In Autumn, the Lakes moderate the cold air that sweeps south from the arctic, thus extending the frost-free period further into the season and allowing many areas south of the Lakes to be prime fruit growing sectors. The Lakes often give birth or new life to storm systems during this season, and a few such storms have blown with hurricane force winds. [See The Winds of November in 1998 Almanac.] Late in the season, outbreaks of cold arctic air generate bands of clouds that sweep onto the shoreline, pelting inland regions with cold rain showers, ice and snow pellets, and finally snow showers and squalls.

Winter brings an increased frequency of snow squalls as lake-effect snows develop within the howling cold air streams crossing the relatively warm lake waters. Under extreme conditions these squalls can last for days and bury areas under huge snow drifts. [See Lake-Effect Snowfall.] But in exchange, the moderation of the cold air mass often spares the lee shores from extreme cold temperatures.

Lake influences in Spring and Summer can often knock much of the heat and humidity from tropical air moving northward from the Gulf of Mexico as it passes over the relatively colder waters. The Lakes' shorelines become popular recreation areas as the cool and drier lake air moves inland on lake breezes bringing refreshing relief from the heat of summer to the millions residing in the region.

For a number of years, the influence of the lake breezes, which form along the Great Lakes' shorelines during Spring and Summer, were prime focuses of my professional attention. Lake breezes and their associated circulation systems can have a profound negative effect on air quality. During my years as a meteorologist with Environment Ontario, the lake breeze regime would often cause air pollution alerts in several Ontario locations by trapping industrial emissions within the stable lake air.

The Lake Breeze and Lake Breeze Front

Basically, a lake breeze is a wind flowing off a lake onto the shore. Most meteorologists, however, use the term to define a special, local air flow situation known as the lake breeze circulation or lake breeze regime. (The lake breeze is similar to the sea breeze found along sea coasts. See Sea-Land Breezes.)

The key conditions for the development of a lake-breeze circulation are light regional winds and a strong temperature contrast between air over the cold lake and that over the warm land. These conditions generally occur on Spring and Summer days when the sun shines strongly and winds from larger scale weather systems blow lightly. (Such a flow regime is considered a mesoscale flow, or perhaps a microscale flow on a small lake.)

Temperature Contrasts Form Lake Breezes

When the air is warmer over the land than the surrounding lake, slightly lower air pressure develops relative to the pressure over the cold lake. Since wind flows from high to low pressure, a breeze will blow inland across the shore. Typically, the lake breeze will commence in late morning after the sun has had sufficient time to warm the land, but the timing can vary from day to day depending on the land and lake air temperatures and regional cloud cover and wind speed. Late May and early June are the prime months for strong lake breezes because the lake/land temperature contrast is usually greatest during these months. As the lake waters warm and the lake/land temperature contrast lessens, lake breezes become less common in late August and September.

[When the temperature situation reverses and cold air is found over land and relatively warm air over the water, a flow known as the land breeze may occur, blowing from off the land and over the lake.]

Lake Breeze Front Moves in from Lake

When the lake breeze moves inland, it pushes cold lake air in like a cold front. This lake breeze front, as it is called, is narrow in width and can be slow moving, particularly later in the day when it approaches its greatest inland penetration from the shore. It is often possible to walk between the lake air mass and the land air mass across the frontal boundary and feel the differences between them.

You should be able to detect the temperature difference and the wind shift at the frontal boundary without the aid of instruments. On the land side of the front, the air is warm, and winds are generally light and usually variable. On the other side, the air is distinctly cooler with wind blowing off the water, often being much gustier. Later in the day, the differences may become less apparent with the wind shifting to parallel the coastline rather than coming in perpendicular to it. (This is due to the influence of the Coriolis effect. Thus, with the lake to your right, the wind should be at your back.) The front may also be defined by a line of cumulus clouds along it, which can grow into thunderstorms under certain conditions.

Lake Breeze Circulation

Cold Air over lake (1) has higher pressure H than Warm Air (2)over land where pressure is relatively low L. This causes air to flow inland forming the Lake Breeze (3) and Lake Breeze Front (4). Warm air forced to ascend the Front may form cumulus clouds above it. Above the lake breeze, (5), a return breeze blows out towards the lake, closing the circulation loop.

I had the opportunity to frequently experience the contrasts on opposite sides of the lake breeze front during a field study in 1970. We were investigating the lake breeze character and its distance of penetration along the western shores of Lake Erie for a climate study of air pollution dispersion characteristics. We had positioned our monitoring equipment such that the front would travel inland past several of the stations but infrequently go past all of them. Thus, as I moved inland from the lakeshore, I was able to feel the difference between the two air masses. On several occasions, I was present at a station when the front passed by and could feel the changing conditions and see its distinct signature being written on the charts.

Lake Breeze Thunderstorms

The lake breeze front can also trigger thunderstorms as the cold lake air pushes the warmer land air upward along the frontal boundary. Such storms can become severe, particularly when separate lake breeze fronts approach from opposite directions and produce a strong convergence zone between them. Converging air masses can only go upward, and such strong upward motions are very conducive for thundershower and thunderstorm formation.

The region of southwestern Ontario along a line from Detroit to Toronto is bounded by Lake Huron to the north and Lake Erie to the south. Here lake breezes frequently move off each lake during hot weather, and the breeze fronts converge somewhere over the region. Recently, meteorologists participating in a field study dubbed ELBOW (Effects of Lake Breezes on Weather) documented a severe storm event in July 1997 that was triggered by lake breezes flowing off Lakes Erie and Huron.

Thunderstorm forming in convergence between lake breeze fronts. GOES-8 image for 14 July 1997, 2115 UTC

The researchers closely observed one particular thunderstorm cell develop between the converging lake breeze fronts and grow into a severe storm in about 20 minutes. A spotter team chasing the storm experienced very heavy rainfall and observed downed trees and flooded roads and fields. From local radar measurements, they estimated that 200 mm (8 inches) of rain fell at the small crossroads community of Punkeydoodle Corners over a five-hour period due to this stationary storm cell formed between the lake-breeze fronts.

The preliminary results from Project ELBOW suggest that thunderstorm formation and severity associated with lake breeze fronts may be more significant than previously believed. This gives forecasters for the region another aspect for consideration during the warm season.

[Further Information on Project ELBOW is available at: http://www.yorku.ca/research/blayer/ELBOW/cmosbull.htm and thunderstorm pictures from ELBOW can be found at: http://www.yorku.ca/research/blayer/ELBOW/elbowpix.html. ]

While lake breezes can increase pollution levels and bred thunderstorms, they are usually welcomed for the relief from the intense heat and humidity of summer days they bring to residents of the Great Lakes basin.

Learn More About Winds From These Relevant Books
Chosen by The Weather Doctor

• Williams, Jack: The Weather Book, 1997, Vintage Books, ISBN 0-679-77665-6.
• Deblieu, Jan: Wind : How the Flow of Air has Shaped Life, Myth, and the Land, 1998, Houghton Mifflin Company, ISBN 0-395-78033-0.

Written by
Keith C. Heidorn, PhD, THE WEATHER DOCTOR,
May 10, 2000

The Weather Doctor's Weather Almanac Great Lakes Breezes
©2000, Keith C. Heidorn, PhD. All Rights Reserved.
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