Back away from the coasts
After spending some time with the upper air charts this morning, I started writing out the forecast for our region. I noticed something very common at this time of the year, but sometimes overlooked: the impact of the ocean on our weather.
Truth be told, the list is long, but today and often in May, I find myself repeating the phrase, “but cooler at the coast.” Two things are at play here: ocean temperatures and wind direction.
I grew up on a farm in Ontario. There, strong southwest winds in May bring a taste of summer heat. Depending on where you live in relation to the water, that’s not always the case in Atlantic Canada.
Last Wednesday, while it was only 17° C in Yarmouth, it was 30° in parts of Nova Scotia’s Annapolis Valley. While it was 16° in Borden, it was 26° in Fortune Bridge, P.E.I.
There are significant thermal property differences between land and water:
• The sun is only able to warm a thin top layer of the land while it can penetrate many meters into the ocean.
• The ocean, unlike land, is subject to vertical mixing and convective movements.
• The thermal capacity of the ocean is much higher, because the water is considerably denser and has roughly four times the specific heat (the amount of heat required to warm a given volume by 1° C) as most land surfaces. For these reasons, land warms more and faster than the ocean in the spring, and cools down more and faster in the fall.
Back to the ocean.
The water temperature that concerns us most is the sea surface temperature (SST). Sea surface temperature is the water temperature close to the ocean’s surface. Air masses in the Earth’s atmosphere are highly modified by sea surface temperatures within a short distance of the shore.
Right now, the sea surface temperatures off our coasts range from 3° to 9° C. Not surprisingly, air temperatures by the water, when the wind blows onshore, will struggle to reach the mid-teens, even with the benefit of the May sun.
So, to answer a question I hear a lot in May, “Where is our spring?” It’s about 10 km inland.