"I may as well clear this up once and for all. The rain and snow were my doing. After I had told you to rely on the Mercurians and described the landmark, I discovered that the Mercurians were nitwits and the landmark a false alarm. That meant that, once you landed, you'd never find me except by accident. That put it up to me to find you. "As you may have heard, normally there's no such thing as rain or snow on Mercury. But there is water. And there is a continual process of transfer going on. The water flows through subterranean channels to the Hot Side, evaporates, and is carried over in the air to the Cold Side. There it deposits on the ground eventually as ice, melts, and goes through the whole process again." "Why doesn't it rain?" "Because there's no dust in Mercury's air. The absence of a rapidly alternating day and night means that the air doesn't circulate on the same scale as on Earth. Practically no wind, combined with little erosion, means little dust. The water-laden air cools off and becomes super-saturated at the Twilight Zone. But there are no clouds, and there's no precipitation because the water needs either dust or ions to condense on. In a Wilson cloud chamber, an experimenter furnishes it with ions. Here on Mercury I furnished it with dust. "I gave the Mercurians rifles and explosive bullets, and taught them to shoot into the air. It was quite a job, but they learned. The explosion spreads a cloud of dust, the water condenses, and you have rain or snow, depending on the temperature. I impressed it upon their brains, such as they are, that the presence of human beings calls for a Fourth of July celebration--shooting into the air. And there you are. I had the occurrence of rain and snow reported to me, moved toward wherever the snow was thickest, and found the ship." "Another thing--" "I've talked enough. That dame won't wait forever. Which will it be, Carvalho, the Lady or the Tiger?" They listened in curiosity as Carvalho, tight-lipped, tapped out a short message in code. They didn't ask him what it was. As the _Astrolight_ drove upward away from Mercury, Lamoureux had one last glimpse of the Mercurians shooting into the air. The snow was coming down in enormous flakes two inches across, and Carvalho, staring after the ship, was shivering and cursing. After they reported the facts to the Interplanetary Commission, a ship would be sent to pick him up--but it might take some time. "Let me tell you about this dame," said Kalinoff. Lamoureux listened patiently, got out his contract, and waited, with pen ready, for the interplanetary screwball's signature. End of Project Gutenberg's The Weather on Mercury, by William Morrison