In a fantasy world I've been working on Dragons are venerated by many for their ability to seemingly summon rain where ever they roam. It's common for the peasantry of the south to leave offerings of both animals and incense to not only attract dragons to a region, but also as thanks for the blessing of rain.
What kind of gas or chemical would a Dragon have to exhale to cause rain? The effect doesn't have up be spontaneous, but anything quick (a few hours) would be preferable.
Note: don't worry about how a dragon produces the chemical too much. That will be the topic of another question.