Haze Squall
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The Great Lakes waters and shorelines are renown for their rough weather. During the summer, Lake boaters and sailors fear fast-moving thunder squalls, particularly the sudden surprise strike of a haze squall.

Hi, I'm Bryan Yeaton and this is The Weather Notebook.

Thunderstorms and squalls moving over the Great Lakes usually give advanced visual warnings to boaters and sailors. In the late afternoon, their towering cumulonimbus tops reaching to 50,000 feet throw a long shadow eastward across the waters that can be seen for miles, giving those with a weather eye a warning of about 20 minutes.

But often, a line of thunderstorms moves over hazy Lake waters around midday or earlier, striking without warning. This is the notorious haze squall.

Here's what happens: the day may begin sunny with a southwesterly breeze, a great day for boating. Visibility on the lake, however, is limited to about a mile by a dense surface haze formed when moist warm air is cooled by contact with the much colder lake waters.

If around noon, a thunderstorm or squall line forms, its black clouds can swiftly cross the lake from a westerly or southerly quadrant at speeds of 25 to 40 miles per hour. But because the sun is high in the southern sky, the shadow of the towering cumulonimbus is short, pointing toward the north and thus hiding the storm in the haze as it advances.

There is little warning time before the squall strikes, usually it is announced by a sudden chill, the cold outflow from the thunderhead. Then quickly, high wind gusts, lightning and rain strike. Any small vessel caught in its fury is given a rough and dangerous ride.

For Great Lake boaters, marine weather radio can save your life. What you can't see, can hurt you.

Thanks to contributing writer Keith Heidorn, of Victoria, British Columbia. The Weather Notebook is a production of the Mount Washington Observatory. For more on haze, go to our website at weathernotebook.org.