After spending some time with the upper air charts this morning, I started writing out the forecast for the Bell Media radio stations.  I noticed something that is very common at this time of the year, but sometimes overlooked: the impact the ocean has on our weather.  Truth be told, the list is long, but today and often in April and May, I find myself repeating: “but cooler at the coast”.

Two things are at play here:  the ocean temperatures and wind direction. 

Remember that record breaking day, 2 weeks ago yesterday when it was 23 in Halifax?   Well, the wind was from the north.  That sounds cold but not as cold as the mighty North Atlantic.   

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 1 degree Celsius) as most land surfaces.

For these reasons, land warms more and faster than the ocean in the spring and cools more and faster in the fall.

Back to the ocean…

The water temperature that concerns us most is the sea surface temperature. Sea surface temperature (SST) is the water temperature close to the ocean's surface. The exact meaning of surface varies according to the measurement method used, but it is between 1 millimetre and 20 metres  below the sea surface. Air masses in the Earth’s atmosphere are highly modified by sea surface temperatures within a short distance of the shore. Localized areas of heavy snow can form in bands downwind of warm water bodies within an otherwise cold air mass. Remember the snow squalls?   Ok, we can move on…

Right now, the SST off our coasts ranges from 2 to 4 degrees. Not surprisingly, air temperatures at the coasts, where the wind blows onshore, will be near 5 all day and overnight too. 

How are these temperatures measured?  Weather satellites have been available to determine sea surface temperature information since 1967, with the first global composites created during 1970.

But, did you know:  SST was one of the very first oceanographic variables to be measured?  Benjamin Franklin suspended a mercury thermometer from a ship while travelling between the United States and Europe in his survey of the Gulf Stream in the late eighteenth century.


Chief Meteorologist

Cindy Day