A supernatural geological event occurred and created a huge caldera-like formation on the coast of a region similar to Scotland. Over time the caldera filled with water and became a bay surrounded by a rim of mountains and a chain of islands in the mouth of the bay.


In the center of the bay, something is producing enough heat to warm the water to mediterranean temperatures (86 °F or 30 °C) while the body of water outside the bay is similar in temperature to the North Sea (52 °F or 11 °C). How would the temperature difference between the area within the bay and the colder region outside of it interact and what sort of weather phenomenon would it cause?

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    – JBH
    Jun 29, 2019 at 2:38
  • $\begingroup$ Welcome. It would be helpful if you could give temperature ranges. I have a vague idea of what the North Sea vs the Mediterranean is like, but the actual temps will be important for weather calculations (not by me, as it's not my field, but by others). We try to make questions self sufficient. People often Google facts to help them write answers but we shouldn't need to Google to get the question right. Remember that people here hail from all over the world. Thanks for the graphic though! $\endgroup$
    – Cyn
    Jun 29, 2019 at 2:47

2 Answers 2


Heat expands things

Your first behavior is that you will not have a caldera filled with water that stands at Mediterranean temperatures with a cold "North Sea" like water on the outside. Heat causes things to expand, which means you'll have heated surface water spilling into the "North Sea" area and cold "North Sea" water coming in on the ocean floor. The results are:

  • An out-bound surface current spilling water into the "North Sea." What impact this will have depends on the currents along the coast. If we assume the current flows northward, then you'll have a moderate climate along the coast for some distance until it cools off on approach with the arctic. Given the Earth-example of Scotland, I doubt the moderating effect would last more than 100 km.

  • An in-bound sea-floor current bringing cold water from the "North Sea" into the caldera basin. I'm certain you've seen a toilet flush. That's a dramatic example of what happens when water enters a somewhat circular bowl. Because...

  • Your caldera will have a clockwise current, I suspect with pretty good speed (0.5-1.5 km/hr). If you have a northbound current along the coast in the "North Sea" it will exacerbate the rotation somewhat (kinda depends on how intense those breakers are). This would not be an easy place to use a boat and especially not an easy place to dive.

Cold air and warm water = fog

The whole area would be almost permanently blanketed in fog. I'm not convinced there's enough of an impact to significantly increase the amount of overall rainfall, but you'd pretty much always have fog save on the hottest of summer days.

Fall might be a problem

The transition from cold winter to hot summer would probably not be an issue, but the autumnal transition probably will be. The potential for local thunderstorms and even tornadoes.

The area around the caldera would be mild

Finally, around the caldera itself (maybe for a distance of 10 km) you'll experience milder temperatures and greater humidity than you would elsewhere in "Scotland." I suspect people would have settled this area very early in the nation's colonization.

How accurate are my descriptions?

You haven't provided a lot of details including the depth of the caldera, the diameter, the detailed topology of the surrounding area for a radius of 50 miles, the existing climate beyond citing "Scotland" (although that's likely specific enough in that regard). Therefore, I believe there's quite a bit of interpretation going on here.

Oh, BTW, it doesn't relate to climate, but if this warm caldera harbor has been around for a long time, you should expect some very unique species of bird and aquatic life. Kind of a sea-based Madagascar Island.


There would be a continuous outflow of hot water from the basin.

Hot water rises. Cold water sinks. Where the two meet, the cold water will flow in from the ocean underneath the hot water in your basin. This will push the overlying hot water up, and therefore out into the outer ocean.

Tides will periodically affect the strength of this current but I expect that there will be a continuous outflow of hot surface water and inflow of cold bottom water. If your heat source is continuous this acton would also limit the temperatures achieved in your basin - as it gets hotter in there the flow would increase and so serve to cap temperatures where you have them.


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