Sunny spring days are lovely, but they often lead to clear spring nights and they can be very cool!
Though many believe the coldest time of the day is in the middle of the night or some time before the sun rises, that’s not often the case: the coldest part of the day is usually just after the sun rises.
Here’s why - Most of the heat the Earth receives comes from the sun. During the day, heat is gained; at night heat is lost. As the sun rises, the portion of the Earth that is just beginning to receive the sun's rays is still in the process of losing heat. Because the sun is so low to the horizon at sunrise, most of the energy is absorbed by the distant atmosphere.
There are always exceptions. In some cases, a cold front could move across the region, leading to cold temperatures at any time of the day. In that case, the coldest time would likely be in the hours after the front passes through. This cold front scenario is not that common: it might only happen a few times a year. Overall, it’s safe to say that the coldest time of the day is usually just after the dawn.
On that note, gardeners are keeping a close watch on cloud cover and wind at sunset these days. Clouds act like a giant blanket. They can hold in quite bit of heat after the sun sets. A clear night will bring on a much more rapid temperature drop.
Check the average last frost dates for your area: Has Jack Frost left the region?
The wind plays a part in the overnight cooling, too. If the wind is calm or very light, the heat will radiate back into the atmosphere more easily than if the wind is blowing. The night wind is often referred to as a mixing wind: it stirs the warmer air with the cooler air and usually keeps the frost away!
A few things to watch for if you’ve already visited your local garden centre.
Today in weather history
It’s easy to be lured to the garden centres when the midday sun is beating down on your head, but it’s still pretty early for delicate annuals. While the average low temperature for St. John’s is 1 C on May 8, on this date in 1978 it was -6. The average low for Charlottetown is 2; on this date in 1950, it was -3. And the average temperature for Halifax for today’s date is 3, but back in 1966 it was -4!
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Cindy Day is the chief meteorologist for SaltWire Network.