History of the Cumulus Cloud Rule

Cumulus clouds with tops above the freezing altitude can develop into cumulonimbus (thunderstorm) clouds very rapidly. Observers have reported periods as short as four minutes from the first appearance of a storm's electrical field outside of such a cloud to the first natural-lightning discharge. Cumulus clouds are thus a potential threat for natural lightning as well as triggered lightning.

The cumulus cloud rule specifies the unacceptable meteorological conditions for a launch, in terms of that cloud type.

The cumulus cloud rule specifies the unacceptable meteorological conditions for a launch, in terms of observed cumulus clouds. The rule has developed throughout the life of the space program. While the early days (before Apollo 12) saw the complete absence of such a rule because triggered lightning was not originally perceived to be a threat, today's rule provides for launch availability while still assuring safety. As the following chronology shows, improvement of the rule has come at the expense of increased complexity. (Each item names a vehicle or program, with the pertinent year in parentheses, preceding the rule printed as it stood during that year.)

Post-Apollo 12 (1970). The space vehicle will not be launched if the nominal flight path will carry the vehicle through cumulus clouds with tops at 10,000 feet or higher.

Apollo-Soyuz (1975). The space vehicle will not be launched if the nominal flight path will carry the vehicle through cumulus clouds with the freeze level in the clouds. This rule may be relaxed at the discretion of the launch director when electric field measurements in the launchpad area are stable and measure less than 1 kilovolt per meter. It may be further relaxed provided that airborne and ground electric field measurements meet [certain] criteria.

Shuttle (1979). The space vehicle will not be launched if the nominal flight path will carry the vehicle through or within five statute miles of any [cumulus] clouds where radar shows virga or precipitation and the tops extend...above the -10 degrees Celsius temperature altitude and four or more field mills within the launch site have changed more than 500 volts per meter.

Shuttle (1986). No launch if vehicle path is within 5 nautical miles of any convective cloud whose top extends to the -20 degrees Celsius isotherm with virga/precipitation.

Atlas (1987). Flight path of the vehicle should not be through cumulus clouds with the freezing level in the clouds.

Post-Atlas-Centaur 67 (1988). The vehicle will not be launched if the planned flight path is

• through cumulus clouds with tops that extend to an altitude above the 5 degree Celsius temperature level; or
• through or within 5 nautical miles of cumulus clouds with tops that extend to an altitude higher than the -10 degrees Celsius temperature level; or
• through or within 10 nautical miles of cumulus clouds with tops that extend to an altitude higher than the -20 degrees Celsius temperature level; or
• through or within 10 nautical miles of the nearest edge of either a cumulonimbus cloud or a thunderstorm including the associated anvil.

Post-First ABFM Campaign (1996). Do not launch if the flight path will carry the vehicle:

• through a cumulus cloud with its top between the +5 and -5 degrees Celsius levels, unless
• the cloud is not producing precipitation; and
• the horizontal distance from the furthest edge of the cloud top to at least one working field mill is less than the altitude of the 0.0 degrees Celsius level or 3 nautical miles, whichever is smaller; and
• all field mill readings within 5 nautical miles of the flight path are between -100 and +1000 volts per meter for the preceding 15 minutes
• through cumulus clouds with tops higher than the -5.0 degrees Celsius level.
• through or within 5 nautical miles (horizontal or vertical) of the nearest edge of cumulus clouds with tops higher than the -10.0 degrees Celsius level.

Current (as of 1998). Do not launch if the flight path will carry the vehicle

• within 10 nautical miles of any cumulus cloud with its cloud top higher than the -20 degrees Celsius level;
• within 5 nautical miles of any cumulus cloud with its cloud top higher than the -10 degrees Celsius level;
• through any cumulus cloud with its cloud top higher than the -5 degrees Celsius level;
• through any cumulus cloud with its cloud top between the +5 and -5 degrees Celsius levels unless
• the cloud is not producing precipitation; and
• the horizontal distance from the center of the cloud top to at least one working field mill is less than 2 nautical miles, and all electric field measurements at the surface within 5 nautical miles of the flight path and at the mill(s) specified above have been between -100 and +500 volts per meter for 15 minutes.

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