Travel within planetary atmospheres is a very tricky business, spelljammers are not really designed to stand up to the stress caused by wind and weather and in many ways landing on a planet is the most dangerous maneuver a spelljammer might routinely attempt. When a spelljammer is caught in a storm the vessel must make a Fortitude Save each turn. This save can be modified in several ways. If all sails are taken down and secured (reducing the ship’s MC to “F”) then the save is made at +4.
Additionally, if the helmsman has the Pilot Spelljammer Skill, he can add +1 to the roll for each two ponts he has in the skill. Regardless, if the roll fails the ship suffers a random critical hit. As some of these can be disastrous for a vessel within a gravitational field (such as spelljammer shock) atmospheric travel is often considered quite dangerous. Stronger storms increase the DC of the save, the DM informs the crew of this as appropriate. Even when the wind is calm spelljammers are limited to a move of 24 in the atmosphere, the rough equivalent of TR 1. If the ship is pushed beyond this limit the vessel must make a saving throw again for each turn spent beyond TR 1, further more, the roll is penalized by the amount over. A vessel flying at TR 4, for instance, would have a -3 on the save. Positive modifiers can be used here as well when appropriate. Again the ship suffers a random critical hit when the save is failed.
Additionally, spelljammers are inherently less maneuverable within the atmosphere, all MCs are reduced by one level. All spelljammers can hover, however, though they can’t turn while hovering unless they are MC “A” within the atmosphere. Note the saving throws for speed and weather are cumulative. If a ship is traveling through a hurricane at TR 6 that vessel would have to make two saves each turn, not one.
The Storm Belt
Finally, nearly all planetary bodies of size D and larger have a band of very high winds in the upper atmospshere, similar in most respects to the Earth’s jet stream. Passing through this band, whether on landing or take off requires a Fort Save as well. Again, if the save is failed a critical hit occurs. Ship’s suffer damage in this way often enough to keep spelljaming interference within most world’s affairs to a minimum. All the major campaign worlds, (Oerth, Krynn, Mystara, Toril) possess such a band of high winds. A few planets smaller then size D have it as well.
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