Why are storm clouds dark? When storm clouds roll in, it is like the sky is wearing a cloak of darkness that sends shivers down your spine.
Those dark clouds are a sight to behold, but they also make your heart skip a beat.
These gloomy clumps of compacted H2O and frozen bits appear to go against the grain of clouds’ typical fluffiness and whiteness.
And that makes many people wonder, “Why are storm clouds a dark color?”
Storm clouds are dark because of the complex interaction between sunlight, water droplets, and the science involved in cloud formation.
Why are Storm Clouds Dark?
There are a number of factors at play and you have to consider all of them to answer exactly why storm clouds appear so dark.
Here is more about some of those reasons:
The Outcome of Water Droplets and Ice Crystals
Clouds form when water vapor and ice crystals saturate the atmosphere.
There are several types, but nimbus clouds are associated with rain, producing precipitation from a drizzle to hail and snow or torrential downpours.
They grow darker and taller as more and more vapor and ice clump together sometimes developing into towering cumulonimbus clouds.
These are the anvil shaped thunderheads that are typical of storms.
They are huge masses of unstable air that produce high winds and severe downdrafts not just rain.
Fact: The nimbus clouds may only be 1000ft above sea level but can stretch into the atmosphere to reach 50,000ft.
Presence of Pollutants and Dust Particles
Because of their deep convection, cumulonimbus clouds are very powerful.
But, studies show that smoky air makes it much harder for them to form and although pollution energizes them, in large extreme amounts it can bring their development to a halt.
Even so, clouds generally need some form of particle, or aerosol, for the vapor to clump around and become a rain droplet.
There are many kinds, like volcanic ash, sea salt and pollen which are naturally occurring.
Also, there are man-made pollutants, such as sulfates and nitrates adding particles into the atmosphere which in moderate conditions allow developing clouds to grow taller.
Interaction between Light and Cloud Particles
Clouds that appear white have water droplets that are tightly packed. They reflect back much of the sunlight that hits them.
The droplets clumping together creates space between them but even less light can penetrate the cloud so it appears darker.
The thicker and denser the cloud becomes the more sunlight is scattered and the darker it grows, at first along the bottom of the cloud where the cloud above blocks more of the sunlight.
Fact: Over heavily polluted areas the sheer number of particles present also reduces the amount of sunlight that reaches the cloud making it look darker.
Absorption of Shorter Wavelengths of Light
The color of a cloud depends on how much light it receives although we only see a small portion of it.
The droplets and ice crystals that make a cloud act like prisms and are ideal for splitting and scattering light.
Usually the shorter waves, blue and violet, are the first to scatter as they enter the atmosphere.
But because the waves from the droplets and sunlight are similar in size, a different kind of scattering happens when sunlight hits thicker clouds.
Fact: The Mie effect treats all the wavelengths as if they are the same length and we perceive all the light scattered as white.
Scattering of Longer Wavelengths of Light
The Rayleigh and Mie effects are two types of scattering, the third is non-selective scattering.
The effects occur under different conditions but all three happen simultaneously.
Atmospheric atoms and molecules are much smaller than wavelengths of light and collisions scatter the larger wavelengths forming phenomena like sun dogs and glories in the Rayleigh effect.
The Mie effect causes storm clouds to look white above but have very dark bottoms as their heavy aerosols linger nearer the ground.
According to Mie theory, it is the magnetism of these particles, dust, smoke and pollen, and the like, that distorts the rays’ progress through the droplet.
Relationship between Cloud Thickness and Darkness
As more water vapor is added to a cloud it develops in thickness and density.
It steadily gets harder for any of the light wavelengths to penetrate the cloud and reach the individual droplets.
As a result, even more of the wavelengths are scattered off in random directions.
The effect becomes greater the heavier the droplets get as they proportionally become more efficient at absorbing light rather than scattering it.
Instead of white, with more moisture, the clouds appear grey with darker tones at the bottom layers.
The thicker and denser the cloud, it looks darker and greyer tending towards black.
The Build-up of Moisture and Condensation
On its own, the building moisture is not enough to trigger precipitation.
To fall, the droplets need to get heavy enough to overcome gravity but they need the particles to form.
The dust particles function as condensation nuclei onto which vapor condensates or freezes depending on the temperature, which also falls as less light penetrates.
Until gravity takes over, the suspended droplets begin absorbing whatever light reaches them adding to the impact of moisture on the darkening cloud.
Fact: Droplets won’t form unless the air is dirty enough and precipitation won’t happen until the droplets are too heavy for the air to support.
The Interplay of Light and Shadow within Clouds
As clouds are forming there are strong air currents within them that are rising and falling in constant motion.
The ascending updrafts and descending downdrafts create chaos and turmoil so that the cloud’s density and temperature vary throughout.
Air pockets form, some with less moisture than other areas of the cloud, making them appear lighter.
Pockets that are heavy with moisture appear darker and being heavier are closer to the ground.
As some parts of the cloud billow upwards where sunlight can penetrate, other layers are caught in their shadow further adding to the darkness of the cloud.
Effect of Contrast on Perceived Darkness
Clouds appear white where there are fewer droplets and more sunlight.
As the droplets increase in concentration, less sunlight can reach them to scatter the light and we see the cloud as dark.
Much of the effect we see from the ground is the result of the interplay of shadow and sunlight on the different cloud densities.
The movement of the clouds and the changes in the angle of sunlight have a role to play here.
Some clouds being almost transparent, reveal the sky through them which darkens them or a cloud mass in the foreground appears darker in contrast to the blue of the sky.
Sun’s Angle and Position in the Sky
The angle of the sun in relation to the clouds has an enormous impact on how they look from Earth.
Low, thick clouds reflect shortwave solar radiation back into space but the higher clouds reflect significantly less and as a result we see them lighter grays or even white.
However, when the sun is low on the horizon the light is scattered in all directions because the direction of the rays means they have to pass through much more of the width of the entire cloud.
Fact: The color of a cloud depends on the cloud's thickness and how much reflected light makes it back to the observer.
Effect of Atmospheric Conditions on Color Perception
Our perception of color in the atmosphere is dependent on how much of the sun’s light is refracted or scattered.
And this changes with the cloud’s density, its particles, its altitude and the position of the sun.
But, primarily the color of a cloud depends on how much light it receives.
Besides developing rain clouds there are other atmospheric conditions that produce the same effects such as excess dust or vehicle pollution.
Just as in a cloud, these contaminants form a layer through which the rays of the sun pass, where they are refracted and scattered or even blocked.
Takeaway
Why are storm clouds dark? Several elements interact to give storm clouds their black appearance.
Because of their enormous thickness and height, clouds absorb and scatter sunlight due to the high concentration of water droplets and ice particles within them. Since less light can penetrate them, the clouds appear black.
The presence of atmospheric moisture and dust particles can contribute to this darkening impact.
The science behind the gloom of storm clouds is fascinating, and understanding it helps us better appreciate nature’s awesome power and beauty.
