No-Flow Portal

Portals are sometimes helpful. Sometimes they're unreliable, or dangerous. Sometimes, though, they're just weirdly selective.

A 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. MST3K 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.

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 and while the situation stabilises eventually you can still see where the initial gale scoured part of the area down to the bedrock and circumstances still conspire to produce rather impressive winds under certain conditions.
 * 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.

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.
 * Except when plot-convenient, of course. For example, gravity comes through the gate, but only in the black hole episode.
 * This is mentioned in the first episode to feature a submerged gate, where they notice that the gate is actively preventing water from getting in by using some sort of density filter.
 * 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 & Dragons - A Wizard Did It, often literally.
 * 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.
 * The Tritons in Serôs (Sea of Fallen Stars) don't like planar gates and appointed themselves guardians thereof, because they are afraid that one day a gate may fail to close and the whole sea will be drained to some other plane. The others can neither confirm nor disprove the possibility, but obviously can imagine the result, so Tritons are mostly left alone to deal with this as they see fit.
 * Planescape lore describes the opposite: an "elemental vortex" is a connection between an elemental plane and another accessibe plane that is maintained - and usually initiated - by the very flow of appropriate elemental substance in a correct way. Since it involves moving a lot of matter, elemental vortices tend to be very stable. E.g. turbulence near a high mountain peak on a Prime world sometimes can be enough to open a vortex to the plane of Air. Other three obviously tend toward much less accessible places. Same for paraelemental and quasielemental vortices - even though there are a few normal portals to those places, Lightning vortices open in the heart of a huge thunderstorm, which by definition tend to be short-living, Radiance vortex may be stable, but "dive into the blazing sun at just the right place" part is inconvenient; Magma vortices are common (most active volcanoes), but the place itself isn't very popular. Physics of planes being rather weird, the flow direction, if any, is a toss. E.g. there's Waterspout - two vortices arranged so that a great torrent of water is expelled from the first onto the plane of Air, flies about 500 yards and vanishes into the second. The system is very stable, but why the water moves like this indefinitely is just one of little mysteries of the Multiverse, especially since neither plane has objective gravity or stable pressure gradients to begin with.
 * Al-Qadim has high-level spell Maelstrom, which opens on surface of any large body of water a short-living whirlpool with 50' radius eye, with another 100' of dangerously strong current around it, pulling into the center. The inner ring is a vortex pumping to the plane of Water, so anything that gets sucked down there isn't about to simply float back up anytime soon.

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
 * 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.

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.
 * What If? xkcd actually does the math on the above, placing a portal at the bottom of the ocean and another on top of the poor Curiosity rover on Mars. Result:  The Netherlands; New Netherlands.
 * In The Unspeakable Vault of Doom portals evidently have a problem with this. What the fhtagn, indeed.

Western Animation

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