While the concept that balloons operate only under visual flight rules (VFR) is basic, trying to explain in a coherent fashion what is considered the minimum weather conditions to operate under VFR is consider by some to be next to impossible.
The appropriate minimums vary by flight visibility (as perceived from the aircraft and from the ground), cloud distance, airspace classification, altitude, and time of day. While Federal Aviation Regulation (FAR) § 91.155 attempts to show the effects of these factors in table form, the result leaves much to be desired.
Fortunately, in most instances, the applicable minimums are consistent, and once that general pattern is recognized, one can then focus on the exceptions to the weather minimums. The usual pattern of weather minimums that applies to most lighter-than-air flight operations requires a minimum visibility of 3 statue miles, a minimum distance from clouds of 500 feet under, 1,000 feet over, and 2,000 feet horizontally from the aircraft, and a ceiling is 1,000 feet above the surface. For the pilot who never operates over 10,000 feet above the mean sea level, also known as MSL) and who follows these separations, they will always be in compliance with these minimums.
But if a flight is to take a balloon over 10,000 feet above MSL or in mountain areas where the flight is below 1,250 feet above the ground (AGL), and over 10.000 feet MSL, special rules apply. While these circumstances are infrequent for ballooning, the language of FAR § 91.155 and 91.157 do not limit their applicability as to "free balloons" and thus, a flight operation of a balloon (whether hot air or gas) must meet all the specified minimum weather conditions.
Variations from typical operations
When a flight operation occurs at or above 10,000 feet MSL above the mean sea level, greater separations are required. Flight visibility must be at least five miles. Cloud clearances must be 1,000 feet above or below, and one statue mile horizontally from clouds.
In other situations, lesser separations are permitted. In these instances, the required distances are visibility of one statue mile , or clear of clouds, or both. These circumstances include CLASS G ("uncontrolled") airspace below 10,000 MSL. The one mile visibility separation only applies in daylight hours. (Nighttime requirements call for three statute mile visibility.) The usual cloud distances apply. However, if operations occur in Class G airspace at or below 1,200 feet AGL, then FAR § 91.155 permits lower cloud distances so long as one remains clear of clouds and have one statute mile visibility. and in mountain areas where the flight is both above 10,000 feet MSL and is less than 1,250 feet AGL during daylight hours.
At night, however, keep in mind the usual clearances for visibility and cloud distances apply with one exception. If a nighttime operation occurs within one-half mile of an airport runway and within the traffic pattern of an airport in Class G airspace below 1250 feet AGL, the minimum requirements are reduced to one statute mile of visibility and clear of clouds.
Class B airspace (formerly the Terminal Control Area) permits VFR operations clear of clouds so long as the three statute mile visibility requirement is maintained.
Except in Class G airspace, the usual minimum ceiling requirement is 1,000 feet, provided the report cloud ceiling is at or above 1,000 feet. Special VFR operations are authorized under FAR § 91.157, provided certain conditions are met, and provided the one statute mile and clear of clouds standards are maintained. For balloons, nighttime special VFR conditions do not exist, as instrument flight requirements apply. However, daytime operations in controlled airspace are permitted with air traffic control clearance under these rules.
