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WEATHER: U: Why some nights are so still

Night photography is tricky but Warren Hoeg can turn an evening photo into a masterpiece. He titled this one, “In the Still of the Night." As you can see, there’s wasn’t a breath of wind when he took this lovely photo at Sullivan's Pond in Dartmouth, N.S.
Night photography is tricky but Warren Hoeg can turn an evening photo into a masterpiece. He titled this one, “In the Still of the Night.

When I moved to Atlantic Canada more than 20 years ago, I was impressed with the average person’s weather knowledge. I wasn’t here long before I realized the wind was quite often the headliner in a weather conversation.

The other day, a man in a garden centre asked me why the wind seems to drop off at night. That’s a great observation; it often does, and here’s why:

The wind is one way the atmosphere moves excess heat around. Directly and indirectly, wind forms for the primary purpose of helping to transport excess heat either away from the surface of the Earth or from warmer regions to cooler regions.

The wind is caused by air flowing from high pressure to low pressure; nature is always attempting to reach a balance. If you think back to the weather unit you learned about in Grade 5, you might remember cold air is heavier than warm air. So, in keeping with that theory, it’s safe to say that air tends to flow from a cold spot to a warm one. The greater the temperature difference, the stronger the winds.

this time of the year, the water temperature around us ranges from 2 to 7 degrees; that means the air just above the water is also in that temperature range. On a clear day, the lovely May sunshine warms the earth. As the sun warms the land, it, in turn, warms the air just above it. Warm air rises; to balance things out, air from the colder region rushes towards the warmer area to fill the void. By mid-afternoon, the difference between the air temperature over land and air temperature over the water is at its greatest and the daytime heating winds are at their peak.

Later in the day, as the sun lowers in the western sky, the land begins to cool, the temperature difference is not as great and the winds begin to die down. After sunset, when the air has cooled enough to be close to the water temperature, the wind becomes quite light.

These winds are known as diurnal winds. In a nutshell, and with the absence of winds created by a nearby weather system, there is wind during the day because the sun warms the air and makes it rise; a process called convection. At night, the air doesn’t rise.

Cindy Day is the chief meteorologist for SaltWire Network

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