Wednesday, April 13, 2016, 7:40 AM -If you live near water, or are from the Great Lakes Region, you"re already familiar with hearing the following forecast in springtime and early summer: "Temperatures will be quite mild across the region, though much cooler toward the lake." But have you ever wondered why meteorologists and forecasters make this distinction?If not, don your science cap, and get ready to learn the science behind a lake-breeze.

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More often than not, it’s made to give you advance warning that a meteorological phenomenon known as a lake-breeze is likely to set-up. Simply stated, a lake-breeze is the movement of cool air from across the surface of the lake toward the land, and is notorious for drastically altering the temperature in regions close to the lake. Take the following two examples:

On May 18th, 2015, Toronto’s Pearson Airport (YYZ) recorded the highest temperature for the whole month: a toasty 29.4˚C. Meanwhile, Toronto’s City Centre Airport (YTZ) – located on an island in Lake Ontario a mere 25 km away from YYZ – recorded a maximum high temperature that day of only 19.3˚C – a full 10˚C cooler.

That same day, temperatures along the Niagara escarpment climbed to 32˚C, while a few kilometres away – along the QEW parallel to Lake Ontario – temperatures were a chilly 7-8˚C on average – a 25˚C difference!

While these examples highlight how impactful a strong lake-breeze can be, let’s now delve into the science behind their formation to better understand them.


Photo courtesy: Wikipedia

Lake-breeze formation:

To make sense of a lake-breeze, it is first very helpful to first understand the concept of heat capacity. Very simply, heat capacity represents the amount of energy it takes to raise the temperature of a substance (such as water, soil, or concrete) by 1˚C. The higher a substance’s heat capacity, the greater the energy required to heat it up.

As it were, water – and particularly a large body of it, like Lake Ontario – has a significantly higher heat capacity than that of soil, rocks, or concrete (roughly five times as much, in fact).

With this in mind, let’s now imagine a situation where the sun is shining uniformly over Lake Ontario and its surrounding shores. In this case, the same amount of energy is applied to heating both the land and the water, which in turn heats the layer of air directly above each surface.

Owing to its lower heat capacity, the surface temperature of the ground – usually consisting of rocks, concrete, or soil – can heat up by 1˚C on the order of minutes to an hour under direct solar radiation, reaching upwards of 30˚C or higher depending on the material. This causes the layer of air directly above the ground to heat up in turn – allowing it to rise at it becomes less dense – and creates a localized area of low pressure.

Meanwhile, because of its higher heat capacity, the surface temperature of Lake Ontario remains significantly cooler – averaging a mere 7˚C by mid-May. The layer of air above the lake’s surface does not heat up as much as over land, making it comparatively cooler and more dense. Instead of rising as it does over land, this cooler air over the lake has a tendency to sink, and creates a localized area of high pressure.

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Physics dictates that air must flow from high to low pressure, so the relatively cooler air over the lake (high pressure) necessarily flows toward the shore (low pressure). And thus, a lake-breeze is born.

Once this cooler, denser air over the water reaches the shore, it undercuts the warm air over the land – dropping the air temperature in the process – and forces the pre-existing warm air further up into the atmosphere. With rising motion over land and sinking motion over the water, a type of conveyor-belt circulation is induced, allowing the lake-breeze to continue throughout the day.

This circulation continues as long as solar radiation is being applied; shortly after the sun goes down, this circulation weakens, and can even reverse its direction during the night, causing the exact opposite phenomenon known as a land-breeze.

Additional information on lake-breezes:

Lake-breezes are strongest when there is a significant difference between the average air temperature over water and over land, such as in early or mid-spring. This is because a stronger pressure gradient exists between the two locations, which increases the speed at which the air flows from high to low pressure.

As well, lake-breezes are most common when there is an absence of strong synoptic-scale flow – meaning that no significant low-pressure systems are entering or leaving the region. They frequently occur when background winds are light – such as the case when high pressure is dominating – or when there is a strong ridge in the jet stream.

As a final note, you may be surprised to learn that lake-breezes can have quite the positive effect on the local growing season. In the Niagara region – Ontario’s primary fruit growing region – cool lake-breezes prevent fruit-bearing trees – such as peach, cherry, or apple – from blossoming until late in the spring, when temperatures are normally consistently warm and the risk of frost is at a minimum. These trees can then bear the fruit that we all love throughout the summer months.

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So as we continue into spring and then into summer, be sure to keep lake-breezes in mind as you plan your activities around the Great Lakes – the temperature differences can be surprising.