How to explain it rains only/always when there is no daylight?

Question

• How can I explain that it only rains at night (i.e. from dusk till dawn) in a certain region?
• Can or must I make this meteorological phenomenon apply planet-wide?
• Can or must I restrict rainfall to a certain season?
• Could it rain each and every night?
• What would happen during a total solar eclipse?
  (On an alien planet this could happen more often than on Earth.)
• Is there a place like that on actual Earth, in which climate?
  (I know it tends to rain each evening before sunset in tropic regions which can support rain-forests.)

The preferred, but not required, climate is moderate to warm with little influence of the seasons on highest (~ 30 °C), average and lowest (> 0 °C) daily temperatures. The sun must not shine too hot or bright, i.e. Earth-like vegetation (grass, bushes, trees, crops) must be possible. Geologic features like mountain ridges, ocean sides etc. can be chosen freely, as can humidity.

Answer 1

I would suggest the concept of "rain cloud seeding" - using chemicals that promote the formation of clouds (and rain) could, on another planet with the right chemicals, create regions or an entire planet that is subject to nightly rain. With a little imagination and research, you could possibly tune this to the extent you would want it to work.

https://en.wikipedia.org/wiki/Cloud_seeding

For example, during the day, the heat evaporates water from the surface (and the surface covered by more or less water than Earth). Simultaneously, chemicals that seed clouds might remain very high because of the heat. As night falls, the water vapor hits the high altitude seeding chemicals as they condense, causing rain.

Another approach might be that the seeding chemicals are ineffective when energized by the sun, and then be reactive as the sun sets.

You would need to be careful to research seeding chemicals and how they would impact the ecosystem in general. It might be a stretch, but perhaps there are one or more chemicals that could co-exist in a significant enough quantity while also allowing Earth-like life to form.

I would suggest that, given the narrow range of temperatures you require, it is unlikely you could find the right climate to support rainfall only at night at any or all places on such an alien planet. The one exception might be a slowly rotating planet that retains daylight heat on the surface, allowing the atmosphere to cool long enough that by morning, the air is too dry to ever be capable of producing rain. But a heat retaining planet that does not get too hot during the day (or too cold at night), to me seems much more far-fetched than an atmosphere with cloud seeding chemicals that biological life can tolerate or that stays out of the biosphere.

Answer 2

The daily rains that we know happen in this way:

1. Sun warms the surface
2. The surface warms the air above it while moisture evaporates into that air
3. The warm, moist air rises and hits the cold air above, forming clouds
4. As sunlight intensity wanes, the air cools more and it rains.

To have it predictably rain every night, you would want the air at cloud altitude to stay warmer and slowly cool during the night. This will allow more cloud buildup and a longer if lighter period of rain.

If your planet has a layer of gas/particles at the top of the troposphere that not only can reflect radiated heat from the ground back down to the clouds, but also retain some of that heat, it might create the effect you want. This is not too dissimilar from the effects of some kinds of pollution we know on Earth, but I can't tell you which exactly.

You might have this locally with specific geography. If your land is between a sea and mountains and has a steady wind blowing in moist air from the sea, it will always rain in front of the mountains, as the air again is pushed up into colder air and sheds its moisture in the form of rain. Again, the sun causes evaporation over the sea during the day, so rain will fall as many hours later as it takes the clouds to reach the mountains.

Answer 3

You might consider having your world and life-forms rely on another liquid than water. Different elements theoretically might respond differently to light. That will change a lot of stuff, probably (maybe even all the rules), but it's something to think about. This article may be insightful. It talks about how life could theoretically be based off of different elements and compounds.

Answer 4

During night, water could condense from the air as the sun's heat would not be forcing it to remain a vapor. Yes, this does not happen on Earth, but if the concentration of water-normally-suspended-in-the-air-because-of-diffusion was higher, it could. It could not be planetwide, however, as only half of a planet is in shadow at any given time.