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

NoFlowPortal 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, [[PhysicsGoof for some]] [[ArtMajorPhysics reason]] doesn't let it through even when the portal is working. TheHero 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 [[ViewersAreGeniuses 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 - SandIsWater, 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 LethalLavaLand, for instance, would be pretty toasty on the other side if ConvectionSchmonvection was done away with. If combined with TimeTravel, this trope aversion is usually closely allied with SanDimasTime.

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 [[RuleOfPerception 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 FridgeLogic 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. MST3kMantra is usually enough to dismiss it.

May be justified by AWizardDidIt. Not to be confused with PortalPool, which is where a body of water ''is'' the portal.


[[AC:Comic Books]]
* In ''ComicBook/{{Watchmen}}'', a superheroine has her mind wiped and is trapped in a marriage with an abusive and worthless husband. The friend who rescues her to restore her memory delivers an ultimatum to the "husband" who is in on the deception: walk through that door and disappear, and you'll never hear from us again. He walks, only to find, very briefly, he is at the bottom of the Pacific Ocean being crushed by thirty thousand feet of water. The transition between states is not detailed, nor is the mechanism that prevents six square miles of water under serious pressure suddenly gushing into a bedroom in the USA.

[[AC:Film -- Live-Action]]
* {{Justified|Trope}} and {{ZigZagg|ingTrope}}ed in ''Film/PacificRim''. It's repeated throughout the movie that humans tried to close the Breach with nuclear weapons but they never worked. This is because the Breach [[spoiler:only opens when it recognizes Kaiju DNA]]. Of course, this doesn't stop other things ([[spoiler:radio signals, the Kaiju HiveMind]], and water of the Pacific Ocean) to flow through the portal at different times in the movie.

* Averted in Creator/DavidWeber and Linda Evans ''Literature/HellsGate'' 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.
* In ''Literature/{{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.
* In Terry Goodkind's ''Literature/SwordOfTruth'' series, there is a sentient magical portal called The Sliph. It requires the passenger to have both Additive and Subtractive magic. Also justified in that the Silph was constructed with a person as the base and is actually alive.
* In Creator/PhilipJoseFarmer's ''Literature/WorldOfTiers'' series, portals are used as a narrative device for characters to transit between Tiers. The hero crosses through a ring portal only to find himself thirty thousand feet up and falling fast. As he began at ground level, the books are silent as to how the atmospheric pressure and temperature differentials are dealt with, so that the difference between the two does not cause a hurricane-force wind between the two levels connected by the portal.

* In the first series of ''Series/{{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 [[FeatheredFiend Hesperornis]] getting through.
* ''Series/StargateSG1'' and ''Series/StargateAtlantis'', 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, [[InvokedTrope the gate can differentiate between a specific object entering it and ambient pressure]], only allowing the former through.
* One episode of ''Series/{{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.

* This is generally played straight for portals in ''TabletopGame/DungeonsAndDragons''[[note]]AWizardDidIt, often literally[[/note]], though ForgottenRealms 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.
* Played straight with the [[PortalNetwork Pandora gates]] in ''TabletopGame/EclipsePhase''. As they are [[SufficientlyAdvancedTechnology sufficiently advanced]], they use some sort of force field to keep the atmospheres on opposite sides of the portal from interacting.

* In ''VideoGame/Spyro2RiptosRage'', the one side of a portal from Aquaria Towers to Summer Forest is underwater and yet the GhibliHills level isn't flooded at all.
* In ''VideoGame/SpyroYearOfTheDragon'', the portals to a chinese fireworks factory, an abandoned ghost ship and a SlippySlideyIceWorld 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 ''Videogame/CrashBandicoot3Warped'', Crash can take a Time Twister portal from the WarpZone straight to the underwater levels and no water escapes from there to the present.
* ''VideoGame/{{Portal 2}}'':
** Averted the ending: [[spoiler: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 GrimyWater 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.

* [[AvertedTrope Averted]] in [[http://www.awkwardzombie.com/index.php?page=0&comic=091310 this]] ''Webcomic/AwkwardZombie'' comic.
* Discussed in ''Webcomic/{{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. This is a reference to the fact that in the Halflife universe, the combine are actually doing this, in order to drain the Earth's water for their other planets.

* In ''WesternAnimation/TransformersPrime'' the Autobots space bridge can open into space without sucking the air from the base into a vacuum.