Nimbostratus clouds


Squeezing out the rain: three steps to understanding rain clouds

By Juman Hijab

Reading time: minutes

Original date: July 28, 2023  

Updated: September 1, 2023

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Nimbostratus clouds

Nicholas A. Tonelli. Spotlight. Storm clouds, northeast Lehigh Valley., Sept 25, 2006.

Why does it rain?

Our typical rain clouds: the nimbostratus clouds

The image to the left shows the dark clouds that make up the nimbostratus clouds

The nimbostratus clouds produce a leaden gray sky. They result in continuous rain over several hours. 

Why does rain fall from the cloud? Why does it fall in rain drop fashion rather than larger amounts if the "sky is opening up"? In this article, I will use the analogy of a wet sponge to explain rain clouds.

Here are the steps to producing thick, dense, and dark gray rain clouds:

  1. Rain clouds start with an altostratus cloud
  2. Rain clouds increase their water content by several fold
  3. Rain Clouds with lots of water droplets are heavier and absorb light
altostratus clouds

Maxwell Hamilton. Unusual Clouds; Altostratus undulatus., March 30, 2015.

First: Use a cloud that is extensive and filling the sky

An altostratus cloud is one with large amounts of ice crystals spread across the higher troposphere. 

This cloud is produced by a lifting moist air mass. The water vapor in the air condenses into ice crystals and supercooled water at the level of the mid-troposphere.

The altostratus clouds are pretty thin. This allows the sun to shine weakly through. They also tend to be somewhat more gray than the highest clouds, which are pure white (A simple explanation for why clouds are really white).

What is really interesting is that as you add more water vapor to a cloud, it gets heavier and more gray. Thus, a white cirrostratus descends in the sky to become a gray altostratus. And a gray altostratus descends in the sky to become a dark gray/black nimbostratus.

nimbostratus - altostratus

Nicholas A. Tonelli. Convergence, Nimbostratus streaks, Easton,, March 24, 2007.

Second: Add a warm front that has a lot of warm water vapor

Let's take the instance of dark or gray clouds, such as nimbostratus or cumulonimbus clouds. As soon as the sky darkens above us, we instinctively look for cover. The darkening signifies a heavy downpour.

Rain clouds are thicker and deeper clouds. The nimbostratus cloud develops from the higher altostratus cloud as it encounters a warm front. The excess warm water vapor creates increasing condensation within the cloud. In turn, this encourages the formation of larger water droplets.

The image above shows an altostratus cloud converging into a nimbostratus cloud.

Have you noticed that as soon as the rain stops that the clouds no longer have that darkened structure?

nimbostratus with rain

Melissa Gutierrez. Nimbostratus clouds. Same clouds as the previous picture, only after getting into them and the accompanying light rain., July 29, 2001.

Third: The heavy water-laden cloud is squeezed out like a wet sponge

As the cloud gets heavier, it lowers itself down in the Troposphere. The air mass of the lower Troposphere pushes upwards against the lower surfaces of the cloud. This added pressure increases the condensation of water vapor into droplets within the cloud. The droplets in our nimbostratus cloud grow even larger.

This is probably the point at which the sky starts darkening.  At this point, the water droplets are large enough to absorb the light creating a dark and foreboding presence. 

The heavy cloud descends in the sky and meets the tighter air mass below. It cannot go any further down in altitude. The cloud is squeezed like a sponge between its "heaviness" as it continues to accumulate water droplets and the upward pressure of the mass of air below it. 

As the heavy cloud hits the air mass "floor", the increased pressure squeezes the larger rain drops out. And that is when the rain starts falling. 


clouds, rain, water vapor

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