Why does the sea feel warmer at night?

10 July 2018

Question

Why does the sea feel warmer at night?

Answer

Chris Smith put this coastal question to Weather Quest's Jim Bacon...

Jim - I would say, strictly speaking, there's quite often no difference at all in the sea temperature. Water is very good at spreading energy through great volume, so sea temperatures when you look at the deep ocean temperatures or say the North Sea or the English Channel they might be within point five of a degree for days at a time. So you won't get a diurnal change. I think we should be more talking about what the beach is like because one of the things about being at the seaside of course is that most beaches that are popular are sand and sand is very good at retaining the energy from the sun doing the day in the top few millimeters of sand grains and sand grains also have big air gaps between them so that's a good insulator. So it's very hard for energy from sunlight to penetrate deep into sand. So you know from your holidays in any sort of climate but particularly heat waves like the one we're having in the British Isles this summer you walk on a sandy beach and it's really hot to your bare feet and sandy soils are particularly good at gaining heat quickly and losing heat quickly.

So, let's say for example your going to the seaside at dawn, you've got out of your tent pulled back the door and you think yes let's go and have a swim before breakfast. The sandy beach will have cooled overnight so it'll be at its lowest temperature. You go into the sea and your feet are cold and it feels quite chilly. But let's say the tide is not in yet but when the tide comes in across the cold it's just coming in comes in across the cold sand it's not warmed up. You compare that to the second half of the day when you've had the whole day's sunshine heating the sand and the tide comes in over. It's like moving it in over a hotplate. So the water, the shallow layer of water, that you’re splashing about in is immediately being warmed from below. And so you will feel warmer and actually it is warmer but the deep well-mixed water off-shore isn't changing much at all. So I think he's absolutely right with the observation

Chris - I did quite a bit of sailing in my youth on estuarine waters and used to find the water on the outgoing tide at the end of a day was a lot warmer because the tides come in over very very hot mudflats and absorbed all the energy from the mud exactly as you're saying Jim. And then when that water goes back out to sea again at the end of the tide it's a lot warmer.

Jim - It's something that happens particularly in some parts of the country where you have very very shallow estuaries like the wash in East Anglia is one and that affects temperatures of the land nearby. So there will be many places around the world where you have deltas and suchlike where you've got these shallow seas and the temperature on the land will be affected greatly by whether the tide is in or out and that can also affect weather when you get mist and fog from the sea rolling in shore, that warm that's given over the over the mudflats will determine whether the fog evaporates before it gets in land or not. And you can have a very different day at the seaside with the tide in or out.


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