14 CFR § 101.33

Unless otherwise authorized by ATC, below 2,000 feet above the surface within the lateral boundaries of the surface areas of Class B, Class C, Class D, or Class E airspace designated for an airport;

At any altitude where there are clouds or obscuring phenomena of more than five-tenths coverage;

At any altitude below 60,000 feet standard pressure altitude where the horizontal visibility is less than five miles;

During the first 1,000 feet of ascent, over a congested area of a city, town, or settlement or an open-air assembly of persons not associated with the operation; or

In such a manner that impact of the balloon, or part thereof including its payload, with the surface creates a hazard to persons or property not associated with the operation.