What a "normal wind" ("ruach metzuya") would do affects various laws of sukka construction. For example, in this guide from the Star-K to using sechach mats, they require that sechach-eligible measures be taken to make it so that the mat would not fly off in a normal wind before one ties sechach down to preserve it in the case of an abnormal wind.

How, practically, should one assess what would or would not happen under "normal wind" conditions?

I suppose one way would be to experiment with different setups over the years and see what happens, but that seems excessively risky, time-consuming, and unreliable due to small sample size. Also, the results would be completely unavailable the first few years. Another method could be to get someone who already has a great deal of sukka experience to assess the situation, but given that every site is different, and sukkas come in all different designs, it would seem to be difficult to get a reliable assessment for one sukka and site based on anecdotal experience in others. I expect that a full-blown engineering study would do the trick, but it seems unlikely that that's required, as "the Torah wasn't given to the ministering angels" (Berachot 25b), a class of which engineers are probably a subset.

  • Not most engineers I know... – Ypnypn Sep 22 '14 at 21:44

From your assessment about the impracticality of observing and experimenting it seems your understanding of ruach meyzuyah is a wind that under normal circumstances can and might eventually blow, I would suggest that this is not the case. Rather, Ruach metzuyah means a wind that does blow. Practically constantly.

First of all is the Mechaber in hilchos succah siman 630 siff 10 about using hanging trees like willows as succah walls the mechaber says they must be tied well and be strong 'so the ruach metzuyah doesn't blow them CONSTANTLY'.

There are two other places that discuss ruach metzuyah and I think we should all be willing to assume its all the same wind.

In Taanis 3b we find Rabbeinu Chananel and Rashi explaining the ruach metzuyah in the Gemara means it is 'eino ne'etzarim' never withheld, much like dew, and unlike rain.

Also in Choshen Mishpat siman 418 which discusses the mazik called fire we find a term in siff 9 'ruach sheino metzuya tamid'. The Sma quotes the Magid Mishnah to explain 'ruach sheino metzuya means (a wind) that the world does not have constantly, but rather only at times, even though it comes often. And we don't even need to mention a hurricane that only blows from time to time'.

The normal wind when you step outside is called ruach metzuyah. Put your schach up and if it doesn't get blow down almost immediately, you are good to go. The wind gust is the ruach sheino metzuya about which you don't need to worry.

  • @Isaac Moses thanks for the edit. I had just realized I left that part out and went back to put it in, but you beat me to it:) – user6591 Sep 23 '14 at 1:30
  • Thanks for correcting my misconception, with support from the relevant sources! – Isaac Moses Sep 23 '14 at 1:30
  • Well I'm still looking for the smoking gun, anyone in hilchos succah that says this outright. Because we all are tought that as pshat and I just can't go along with it in light of the points I mentioned. But I really am still hoping to find it somewhere explicitly. – user6591 Sep 23 '14 at 1:38
  • The Gemara in Sukkah 23a seems to imply that even if it only blows away after an overnight, it is still not Kosher. And dew is not constant, just daily. Although the Star-K article quoted in the other answer says the way you do here. Ping @IsaacMoses. – Yishai Sep 23 '14 at 2:31
  • @Yishai I saw that gemara. It wasn't conclusive. The story happened to be that it blew away overnight. Which btw got me wondering where was Rabi Akiva sleeping? But after that Abaye said if it can't stay in ruach metzuya of dry land than its worthless acc to everybody. Rashi mentions its not even a diras aray. That comment can be taken in two ways. Either its physically unable to stay even aray, or its halachicaly not good enough to be called aray, which is what we would need, even though it physically may stand aray. – user6591 Sep 23 '14 at 2:39

This Star K article states: (highlighted words are mine)

If a normal prevailing wind (ruach metzuya) can blow the schach off the sukkah, one may not tie the schach down to hold it in place. The reason is that an item which is not kosher for schach (e.g. string) may not be used to hold down kosher schach. Instead, one must place a wooden beam (e.g. 2x4) on top of the schach which is resting on the walls thereby sandwiching the schach between the beam and the top of the walls. This beam should be placed perpendicular to the bamboo sticks, so that the beam would hold down the sticks without the presence of the fishing wire. Once this is done, one may then tie a string around the schach to anchor and protect it from falling off in a gusting wind (ruach she'ayna metzuya).

I don't think you are required to take an anemometer to calculate this. I think the term "prevailing" applies to your location. If you placed your Succah atop Mt. Washington, NH (Thanks @DoubleAA!!) (assuming it's not too cold for you, then), where it is almost always windy with frequent winds above 40 MPH, that's the prevailing wind, there. If you live near the ocean where strong sea breezes are common, at say, 20 mph, that's your prevailing wind. In most inland NE U.S. metro areas, during October the "prevailing wind" on most days is calm to under 15 mph. In most cases, such a wind would not blow down a large bamboo schach mat, but if it does - then you know that's a problem. Similarly, that same bamboo mat that works fine in the metro area could not be used in your Mt. Washington succah, since it has a much stronger prevailing wind.

  • I don't think you've answered the question, which takes for granted that "normal" is defined locally. That makes the problem harder, in a way, since a published, one-size-fits all metric is impossible, and local knowledge is required. – Isaac Moses Sep 22 '14 at 22:56
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    How do you know that "[i]n most inland NE U.S. metro areas, during October the 'prevailing wind' on most days is calm to under 15 mph"? – Double AA Sep 22 '14 at 23:51
  • @Isaac Moses I think DanF made an extremely important point by highlighting the word prevailing. Most people it seems think ruach metzuyah means a wind that will probably happen as opposed to eino metzuyah which means probably will not. This is not true. Mutzuya means it does happen, practically constantly. The normal wind when you step outside is called ruach metzuyah. Put your schach up and if it doesn't get blow down almost immediately, you are good to go. The wind gust is the ruach sheino metzuya about which you don't need to worry. – user6591 Sep 23 '14 at 0:46
  • @user6591, if you can bring a source for this definition (that we're talking about the wind that's out there constantly as you're setting up), that plus your advice equals an answer. – Isaac Moses Sep 23 '14 at 0:50

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