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No Flow Portal
Portals are sometimes helpful. Sometimes they're unreliable, or dangerous. Sometimes, though, they're just weirdly selective.

No Flow Portal is a strange effect where a portal immersed in whatever medium — a liquid, like a body of water, or a different kind of atmosphere — or exposed to conditions which would otherwise affect the other side of the portal, for some reason doesn't let it through even when the portal is working. The Hero can swim into the portal on one side and walk out of it the other side, yet the body of water he swam in remains as stolidly fixed as if it were up against glass. The hero could even cause an explosion next to a portal, and the people on the other side won't even feel a breeze.

This trope isn't restricted to water, though water is one of the easiest ways to show it in action in fiction. Different atmospheric conditions can be strangely shy about crossing the portal barrier. This is obvious enough with two completely different atmospheres, but for a few more observant viewers, this trope may manifest itself in a case where the atmospheres are the same but one is blowing across or even into the portal on one side and having no effect on the other side. What you'd expect to happen in the former is that the air would get sucked through the portal as the wind creates an area of lower pressure in the other world. Even sand may fall victim to this trope - Sand Is Water, for a given definition of 'water' - though in this case the portal is more likely to be just partly submerged in the sand rather than completely smothered by it.

The opposite of this trope, naturally, would be where the conditions do affect things on the other side of the portal. A portal which lead straight to a Lethal Lava Land, for instance, would be pretty toasty on the other side if Convection Schmonvection was done away with. If combined with Time Travel, this trope aversion is usually closely allied with San Dimas Time.

Sometimes, an inexplicably fierce whirlwind sucks in everything on one side of the portal. This is quite common in fantasy works where the portal in question leads to somewhere nefarious and supernatural, though it'll always be less about thermal currents and fluid dynamics and more about looking impressive - how else to make a portal look foreboding and scary?

For those of us who like to dwell on this sort of thing, it often leads to a Fridge Logic moment when you wonder why biological matter (which itself contains a lot of fluids, including water) can pass through but a body of water can't. MS T3k Mantra is usually enough to dismiss it.

May be justified by A Wizard Did It. Not to be confused with Portal Pool, which is where a body of water is the portal.


Examples:

Literature
  • Averted in David Weber and Linda Evans Hell's Gate series, if only in passing. At least one portal connects two points that are very disparate in terms of altitude. While the situation has mostly stabilized by the time of the story, the initial gale scoured the area down to the bedrock and the vegetation has yet to recover. There is also still a constant wind blowing from both sides of the portal.
    • It is mentioned that the almost constant cool breeze blowing through another portal is welcome to those living in the houses nearby.
    • And there is one storm and another instance of heavy cloud cover stated to be the result of the two air masses clashing.
      • Storms in general are said to be more common by portals for this reason.
  • In Spin, the enormous portal connecting the Indian Ocean to a distant alien planet not only doesn't let water pass through, it doesn't let anything pass through unless it contains sentient passengers. A manned boat will pass through; the same boat floating by itself won't. Justified in that the portal is to some extent, a sentient, or at least reasoning entity, and can thus decide what goes through and what doesn't.
    • Direction also matters. Only manmade objects with people inside traveling directly North at 90 degrees to the Arch will be transported. Anything else will simply pass through as if the Arch wasn't there. An identical-looking Arch is placed in a Martian desert and connects to a similar arid world.
    • The Spin membrane is also this, to an extent, even though there's no teleportation involved. The Spin blocks all radiation from in or out, creating a false Sun with a realistic day/night and yearly cycle. It also blocks any non-manmade object, including meteorites, from penetrating. Unlike the Arch, the Spin does allow unmanned satellites to pass both ways. In fact, since Earth exists in a Time Dilation field, any rocket attempting to reach the orbit will be accelerated by the Spin to match the universal time flow.

Live-Action TV
  • In the first series of Primeval, neatly averted: a portal which opened in the Cretaceous sea allowed water to flow into the present and flood a basement, resulting in the Hesperornis getting through.
    • Also averted in series two, when a Precambrian atmosphere (basically a dark mist) flowed into the present in an office block.
  • Stargate SG-1 and Stargate Atlantis, over and over again. Having one end of a Stargate immersed in water doesn't result in it flooding the other end (or having said water disintegrated if it's the destination end), nor does it suck the air out of the room if the gate is opened into the vacuum of space. Somehow, the gate can differentiate between a specific object entering it and ambient pressure, only allowing the former through.
    • Confusingly, the above rule doesn't seem to be absolute. In one episode where Teal'c is gated into an underground cavity he cannot be retrieved from due to him being in kawoosh range, Carter mentions that the gate shutting down due to reaching its activation time limit will cut off his air supply, implying that the gate is letting air through this time.
    • Even gravity appears to be partially filtered out, being relevant only in the episode where a gate connects to a black hole.
      • Perhaps whatever compensators the Gate is using can only do so much?
    • Ring transporters, however, do take water through. It looks fairly awesome.
  • One episode of Angel had the titular character black-mailed into entering a Hell Dimension to save someone. He gears up before entering only for all the weapons to drop to the floor when he vanishes.

Tabletop Games
  • This is generally played straight for portals in Dungeons & Dragonsnote , though Forgotten Realms plays with it by featuring portals that only lets the atmospheric conditions/water, etc through, and blocks creatures and constructs from going through. Don't want air elementals to disturb that fresh air you're bringing in from the Elemental Plane of Air, after all.

Video Games
  • In Spyro 2, the one side of a portal from Aquaria Towers to Summer Forest is underwater and yet the Ghibli Hills level isn't flooded at all.
    • In Spyro 3, the portals to a chinese fireworks factory, an abandoned ghost ship and a Slippy-Slidey Ice World are all accessed only by underwater lake, yet the respective worlds are still dry and the lake hasn't emptied its water through them at all.
  • In Crash Bandicoot 3: Warped, Crash can take a Time Twister portal from the Warp Zone straight to the underwater levels and no water escapes from there to the present.
  • Averted in Portal 2, as a portal on the moon causes Wheatley, the space core, and various junk to be sucked out into space.
    • Played straight however, with the fire pit at the end of Test Chamber 19 in the first game. If you put a portal as low as possible, and look through from the other end, the heat won't hurt you, even if you're right next to the other portal. Also, if you activate a cheat in the console that allows you to place a portal on any surface, it's possible to have one portal be half-submerged in Grimy Water and have it not come through.
    • Sound also has issues traveling through Portals, though this is difficult to experience in game due to the size of the test chambers and general lack of objects that make noise, though in both games there are test chambers with radios large enough to experience this effect with.

Web Comics
  • Averted in this Awkward Zombie comic.
  • Discussed in Concerned where two combine soldiers mention how their dead buddy used to say that they could place a teleporter at the bottom of the ocean and bring it with them off-world. Even though he did the math, they still didn't believe him.

Western Animation
  • In Transformers Prime the Autobots space bridge can open into space without sucking the air from the base into a vacuum.

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