Water droplets condense on clouds and eventually fall as rain. It is a prevalent sort of precipitation across the globe. Clouds arise as a result of the instability in the air, causing a disturbance in the atmosphere. When the magnetic field that keeps the raindrops together is disrupted, the rain begins to fall because the raindrops’ electrical charge is no longer balanced between the positive and negative poles.
Clouds are obviously necessary for rain
The serein, best described as “rain without clouds,” is a somewhat unusual occurrence that happens mostly in the summer or in tropical locations with marine air. Given that the distribution of condensation nuclei (“cloud seeds”) is occasionally skewed toward large hygroscopic nuclei in comparison to a population of less efficient nuclei, it stands to reason that only the first nuclei to use the available vapour phase would do so.
And that under these circumstances, provided that competition from other nuclei has not yet developed, only the “big nuclei” will absorb the atmospheric water.
Thus, development may occur rapidly, and be fairly large, but few drops can fall on the ground in the absence of clouds, at least sometimes. This phenomenon is shown at high elevations in tropical areas by the appearance of a rainbow in an otherwise clear sky, despite the fact that the rain typically does not fall to the ground.
This “virtually cloudless sky” could be illusive. Because when we see what seem to be raindrops falling on a sunny, (mainly) blue sky, we know that rare clouds are actually there.
Serein
But continuing with the theme of the serein, there are actually times when it seems to rain even if there are no actual clouds in the sky. This is called a serein (not to be confused with dew).
The serein is produced by the rapid condensation of water vapor suspended in the air, caused by nighttime cooling and a lack of clouds. Serein works more like an unseen fog.
Rain that falls from a clear sky is called “serein.” Usually occurring after sunset, this kind of precipitation is described as a light, fine drizzle. The name originates from the French word “serene,” which means “calm,” or “clear.” The word “evening” in Old French (serain) is a possible source too.
Cloud droplets evaporating at the same moment raindrops develop might be to blame for serein, or “cloudless rains.” Another possibility is that powerful winds in the atmosphere are blowing rain from a cloud far away and depositing it where it falls (which may not be felt at ground level due to a high vertical wind shear).
