I don't have much for hard numbers on this, but have seen the vent pipe configuration you described used extensively in the petroleum industry, though I'm not sure it was intended to prevent storm water intrusion, as it is used on gas flare-stacks...most likely it is intended to prevent flame-out during high-wind events, and/or, to improve oxidation of the fuel for a cleaner burn when a more extensive/complicated forced-air flare system is not used. I see the theory behind your intended use for this configuration, and it does make sense to me, as long as there is at least a light breeze during precipitation. What I've seen appears to be a 3:1 cap-pipe to vent-pipe diameter, and roughly a 4:1 on vertical, or, for every inch in diameter of cap pipe, it extends 4 inches vertically. Lastly, the cap overlap on the vent looks to be around 1/4 to 1/3 the length of the cap-pipe...for 24" tall cap, the vent is inserted 6-8". That's just based on my observations after 31 years working in the industry.
An alternative to the above is a simple upright low-profile cone elevated over the vent stack which allows water to drop alongside the exterior of the pipe when there is not any wind during precipitation, though, with light to moderate wind, it is less effective...maybe consider using the two in combination if you desire protection from water intrusion under a wide variety of wind conditions.
Good luck on your build and don't forget to post her maiden voyage!!!
....BUT, many of us smoke in all weather conditions, so we do our best to mother nature-proof our rigs...
Not a bad idea...good idea, actually. That would work quite well with a centered swivel and a directional fin, if you didn't want to have to monitor wind direction changes. Even a full 90* elbow would work. Either one would need a fair amount of vertical run to get above anything close enough to cause turbulent air which may result in erratic partial rotation or improper direction, but yes, it's do-able. A slightly over-sized elbow over the pipe with a small amount of over-lap...good to go.
If it's a nice sliding fit it can be rotated as the wind changes direction too.
I like it, too. In theory, it should give relatively small grate temp variations, being such a short run from the fire box and out the vent. Sweet rig!!! Just guessing here, gauging from the tire size, but it looks to be about a 3 large whole brisket main grate capacity, with some wiggle room. Add a sliding or static grate above that and toss in 8 or so pork butts. You've got a lot of smoker there, stonemill.
Let us know how it cooks!!!