Are weather balloon launches NOTAM'ed?
Related, how frequent (periodically and spatially) are they for gathering data on wind-aloft, etc.?
Yes, they are NOTAM'ed. In the FAA case, the FAA wants to know first when you intend to launch, where from (distance and direction from a town on a map), how high you expect to go and what direction you expect it to travel in. 24 hours before launch, you have to call your local Flight Service Station (FSS) and file a Notice to Airmen (NOTAM) giving the launch site, date and time, expected max altitude, expected rate of climb, expected direction of travel and landing spot. Your local FSS will have an 800 number. These people can also give you the winds aloft if you ask. Most pilots have no use for winds aloft over 30,000 feet, so tell the FSS that you're launching a balloon and ask for wind speed and direction for as high as they have data for. This will typically be about 50-60 thousand feet. Try to get the winds aloft for a reporting station to the west of you because 24 hours later, those winds will have likely moved to your location.
The NOTAM will look something like this:
"High altitude balloon release 5.49 WNW of KAGC, E/SE bound reaching Flight Level 600 (60,000 feet) on June 6. Launching time between 1200 and 1300Z. E/SE direction of drift. Landing time no later than 1700Z"