5km Portal above a Hot and Cold Climates, What happens to the weather of the area?

Question

I'm writing a story where approximately ever 5 - 7 years, A 5km in radius portal opens up above 2 major metropolitan areas of the world. the height is about 750m - 1km up

This portal follows the rules from the Portal video game series where anything can pass through them but they stay fixed and can only be entered from one side. (From above the portal is invisible)

If these portals opened over a particularly Hot city in the southern hemisphere e.g.: Brisbane, or Santiago, and another opened up over a cooler city in the north. e.g. Oslo, Anchorage or Moscow.

What would the impact be to the weather and climate of the areas around these cities?

My first thought was polar blasts would burst every uninsulated water main in a unprepared city however any other situations would be appreciated.

Answer 1

Cold air would pour through

If air pressure were equal, and everything were perfectly still, then not much. In a normal environment you get perterbations and vortices that will make some of the cold air and warm air pass through the portal.

When warm air passes through the portal, it is less dense than the cold air on the other side, so it would rise back through the portal. This creates a natural barrier where the warm air passing through would push back.

Not so with the cold side. A bit of cold air that gets through the barrier will suddenly be much heavier than the surrounding warm air and start falling. This would create negative pressure at the portal interface that would suck more cold air through. Eventually you'd have a torrent of cold air pouring through.

The cold air pouring through might push some warm air in the other direction. That air would roll over the lip of the other side of the portal and create a wall-like updraft in the cold air.

All that presumes equal pressure on both sides.

Higher cold side pressure

If the cold side has a higher pressure, then you'd just get the instant heavy cold downdraft, like a bomb cyclone. On the warm side, it would increase air pressure, pushing any weather system away from the portal.

On the cold side, it would draw all of the surrounding air into the area, increasing rotational velocity. Oddly, that rotational velocity would reduce the air pressure around the portal, slowing the transfer of cold air. If it were open long enough, a cyclone would inevitably form.

On the warm side, you'd get a snap freeze on the ground. Weather patterns would be highly dependent upon geography, as the cold air would roll downhill. The downdraft would likely create strong winds and pull warm air with it until the two mixed.

Higher warm side pressure

A more interesting case happens if the warm side has more pressure. The immediate response would be a burst of warm air pushing through and stopping as the warm air on the other side countered the flow with its buoyancy. You'd get a roughly ring-shaped updraft around the edges. The fun starts when someone passes through the ring from the cold side.

Passing through from the warm side wouldn't have a meaningful effect. A burst of warm air would trickle back into the warm air bubble. In the other direction, the aircraft/person/projectile would pull a stream of cold air with it. That stream would create suction where it passed through the portal, perpetuating and strengthening the stream until you wound up with half of the portal pouring cold air while the other half poured warm air.

In this case, the rising warm air on the cold side would immediately create clouds and start precipitating downwind of the portal. Everything would be covered in sheets of ice in no time. On the warm side, you'd still get the cold downdraft, but it wouldn't be as much of a weather emergency.

Sounds like a fun disaster film.

Answer 2

I'm not an expert, but I do not believe there would be significant exchange through the portals. While warmer air might rise through the portal, the cold zone will be higher pressure and so probably resist those currents relatively well.
It would lead to very interesting migratory patterns for birds, though.

Answer 3

Not Much

You've stated that from above the portals are invisible so they are both facing down towards Earth.

Cold air falls and hot air rises, but because of the way they are facing the hot air will rise through the portal, potentially get stuck by the cold air, but even then as hot air rises through the portal to reach cold side, once it goes through it will rise straight back up to it's original hot place.

On the hot side you might get some extra wind current as air rises around the portal. On the cold side you might end up with shelf of snow or ice on the non-portal side of your portal.