Casting word is:

Protectus Breezium

Great discussion of use cases here: https://forums.giantitp.com/showthread.php?562914-Warding-Wind

10 ft radius means it's a 20 ft sphere... right?

Yep!

Quote from Ylva_Baldr >>

10 ft radius means it's a 20 ft sphere... right?

 If by 20ft. sphere you mean the diameter then yes.

So... I love this spell, my sorcerer uses this a lot, but I had a question with sorcerer meta magic, what would happen if I twinned this spell. I know that this has a range of self, but is there anyway that this could be twinned so that you have 40 mph winds around you, nothing could move around you, no one can attack, and maybe, maybe, no one can breathe and they have to make constituion saving throws?

Twinned Spell states: To be eligible (to be twinned), a spell must be incapable of targeting more than one creature at the spell’s current level. The spell effects apply to all creatures in the area, so per Rules As Written, this spell is not eligible to be twinned.

yup!

 How do people rule this with CLoudkill?  I allowed it to "disperse" it, but I wasn't happy about it.  A 2nd level spell should not so easily dismiss a 5th level one.

Sounds like he is a smart player since the spell cloudkill says "It lasts for the duration or until strong wind disperses the fog, ending the spell." If it didn't say "ending the spell", I would say the cloud comes back when the wind stops. Since it did, it seems he came up with a clever counter. I personally enjoy when player are creative in using their spells.

I love creativity.  I don't like broken mechanics.  Lets take a couple simple examples.  Counterspell.  Literally designed to counter other magic, but balanced so that if it encounters a spell of higher level the counterspell caster must make a check to pull it off or the counter fails.  Light/Darkness.  You can dispel darkness with a light spell of equal or greater level.

See, both of these make sense and call for something to be done to account for effects of higher level.  It's kind of weird that a 5th level spell is so easily countered by a 2nd level one.  I think this is a troublesome mechanic.  Don't forget, these things work both ways.  If my player casts cloudkill and I just drop a 2nd level spell full of NOPE, how mad do you think that player would be?  There is a balance that is usually upheld in D&D and I personally think it's pretty important.  I'm not trying to poop on creativity, I'm trying to make sure things are balanced.  Imbalance is a game killer.

Anyway, this is a pretty isolated example, so I'm not too worried about.  I just find it VERY odd given the substantial level difference in the spells and how they work against one another.

I'd say this is an edge case. It will probably come up once in your life, twice if you keep using this encounter. Your goal as a DM is to make the players feel awesome for overcoming odds. Yes a challenge is great, but if you know your player has this spell, throw something else in the gas. Like an invisible stalker! Have fun holding that concentration spell now! If they fail, theyre in an even worse situation! The DM always, and I mean always  has a way to overcome the players, assuming said DM is imaginative. But even in my example, dont punish them for their spell working! Its lame and unsportsmanlike! Do something that;s just a different kind of challenge. 
So much in dnd 5e makes no sense, this isnt worth fretting over, my friend.

We're going to have to agree to disagree how to handle this.  I've been playing 40+ years and have no problems with anything you mentioned.  I'm saying "this isn't the right way to balance this and I'd like to see it changed" and you're making it out like I got stumped on how to deal with it.  Apples and oranges.  I mean, I appreciate the sentiment, but it just goes around the problem.  I'll die on this hill, but I won't play ball when I think something is out of line.  In an amazing coincidence, this has already come up a second time.  I think you might be on the money that in normal play, this is a unicorn.  I DM about 30 hours a week, so I'm in very different circumstances.

Looks like you just want to argue your point and be right and not take into account anything anyone else is offering. You're the DM, so change it yourself. Duh! Those 40+ years haven't taught you how to accept maybe some things are just imbalanced, just as they are in life. Listen to what these folks have brought you, because they aren't going to change the rule because of your inability to understand why they are allowing it to be.

If an Aarakocra used this spell could they still fly?

Because it says:

• The area is difficult terrain for creatures other than you.

I would assume that you would be able to fly without issue.

Gust of Wind, another 2nd level spell definitely disperses Cloudkill, so why shouldn't this spell?

It shouldn't.  In my opinion, they're both wrong.

Does this spell do anything to the fog around Barovia in Curse of Strahd?

The spell can only target self, so while the spell can only target one creature (while effecting more than one), it can't be twinned since it can't be applied to yourself more than once at a time. The twinned spell metamagic specifically says that it can't be used on a spell with a range of self.

Spells like fireball have a range, and an area of effect, but the only targets are creatures within the effected space. The spell doesn't target a point in space, but that point is chosen by the caster. The targets are the ones effected. Contrary to that, Warding Wind specifically targets only the caster, and then effects multiple creatures within range of the target (the caster).

perhaps the player could be allowed to upcast it or have to make some sort of skill check to disperse the cloud