November 7, 2020

How clouds form
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How do clouds form

Clouds form at anywhere from 1,500 feet (~0.5 km) to 33,000 feet (10 km) above the earth.

In this post, I will outline the basics of cloud formation in 4 steps. 

I will conclude with a discussion why the 1,500 feet (0.5 km) mark above the Earth is an important metric for the lowest cloud formation.

What are the steps for cloud formation?

Swirling steam

Coffee with swirl of steam

Evaporation and condensation

Take a hot cup of coffee on the table. You can see the steam floating up in swirls (we never see steam sink down, do we?). The steam is made of evaporated water. It is as if the water molecules in the hot liquid are saying: "it is too chaotic in here; I have to leave".

The way a water molecule leaves a hot liquid is when its hydrogen atoms detach from other H2O molecules. The water molecule "evaporates" and floats upwards as a gas. This gas is called water vapor.

The reverse happens as water vapor molecules come together. When gaseous molecules converge, a liquid state is regenerated.  It is amazing how water vapor comes together to form a liquid, out of thin air. This reverse process is condensation. Liquid water droplets are created.

When a large mass of water vapor molecules condense, you can see it. The reason for that is that the condensation entraps air pockets of different sizes; these packed air molecules reflect white light back to us. If water vapor were condensing in a vacuum, it would be invisible! Or blue - if there were a large amount of water vapor condensing. This is why notilucent clouds look blue - the same reason oceans and lakes look blue (I am not sure I agree that the blue color is from ozone; the Mesosphere - where notilucent clouds are formed - has lower amounts of ozone than the Stratosphere). 

Thus, vapor from a steaming bowl of soup, cold breath, steam from a boiling kettle, and condensation on the bathroom mirrors appear as white ghostly mists. 

hydrogen bonds

"Hydrogen bonds”

Water loves hydrogen bonds

Cooler temperatures favor condensation. However, even without changing the temperature, water molecules prefer to be connected rather than separate. This is due to the formation of hydrogen bonds. Think of a hydrogen atom as a small child (the blue spheres) between two adults (the red spheres). It has one hand connected to one parent (an oxygen atom) with a strong grip (the solid line: a covalent bond) and the other hand with a weaker grip (a dotted line:  a hydrogen bond) to the second parent (a neighboring oxygen atom).

As a lone entity, a water vapor molecule is not attached to other molecules; the only bonds it has are the covalent bonds (the solid lines) attaching  each hydrogen atom to the "parent" oxygen atom. However, when this water vapor molecule comes closer to others, each hydrogen atom is encouraged to form bridges with the nearby oxygen atoms using hydrogen bonds (the dotted lines).

Adding a hydrogen bond decreases the overall energy of the molecule. Since physical states are always moving towards the lowest energy state at a given temperature and pressure, water vapor molecules will choose to connect with each other whenever possible. As enough of the gaseous molecules connect, a liquid state ensues.

water droplets in spider web

dewdrops in a web

Holding droplets in a cooled water vapor net

When water evaporates from the oceans, lakes, and rivers, it rises in the sky. Water vapor is less dense than air,  so the gaseous molecules float upwards. The molecules join others higher in the Troposphere. Temperatures start cooling to by 5.4º F every 1,000 feet (or ~ 3º C every 304 meters). Thus, those water vapor molecules are pretty chilled by the time they reach the lower levels of the Troposphere at which clouds live (1,500 feet or ~0.5 km).

In a liquid state at room temperature. each water molecule has an average of 2 hydrogen bonds (and 2 covalent bonds). However, as temperatures drop to around 0ºC(32º F), water molecules accumulate an average of 3+ hydrogen bonds/molecule. In the same way, gaseous water vapor molecules form extra hydrogen bonds when they are cooled. This is how snow forms; snow is a mass of very cold water vapor molecules that connect through frozen hydrogen bonds at higher levels in the Troposphere.

Hydrogen bonds are stronger the colder they are. Thus, at a height 2,000 - 6,000 feet water vapor molecules will bind together with strong hydrogen bonds.  In such situations, small groups of water vapor molecules will  come together to form fine webs of strongly connected molecules.

Dense fog: why is it so hard to see in fog

Dense fog: why is it so hard to see in fog

Is fog a cloud?

This how fog forms. A mass of - separate and invisible water - vapor molecules comes together and condenses into liquid when the surrounding air cools.  Fog formation is very similar to our misted breath in cold weather. 

Those condensed water vapor molecules create a mishmash of microscopic water droplets. These are held in tow between pockets of air as well as a cooled netting of interconnected water vapor molecules. This is why the droplets don't mix together; they are separated by air and a lacework of frozen water vapor. 

How similar is fog to clouds? I would make the argument that they are very different. In most circumstances, fog formation is due to local factors. With clouds, the sky's the limit. 

Cloud formation is subject to different altitudes, updrafts of air, and different fronts that collide with each other at higher levels of the Troposphere. These differences create the immense size, variable shapes, and power of clouds. 

Water droplets on spider's web

Water droplets on spider's web

Forming a frozen water vapor web at higher altitudes

Here's the thing. Water vapor molecules attach together when they are released in a very cold environment.  Rather than condensing into liquid water, they form ice crystals. The very cold water vapor freezes into a crystal netting before it has a chance to condense into liquid water. 

What about liquid droplets that are formed higher in the Troposphere? When microscopic liquid droplets are formed at higher altitudes, they have several competing forces that force them to change from liquid water to an iced water vapor network: 

  • It is true that at colder temperatures, hydrogen bonds stronger. This tends to keep liquid droplets firmly held together.
  • However, higher altitudes produce lower vapor pressures for water (water boils at 70º C at the top of Mt Everest, 30,000 feet above sea level). This predisposes liquid droplets to vaporize as they are lifted higher in the Troposphere.
  • Moreover, microscopic droplets have a minute surface area, which further predisposes the droplets to dissipate into vapor.

Thus, it is very likely that liquid droplets have a hard time remaining liquid when they are forced into higher altitudes. They revert back into vapor and then freeze into an icy water vapor webbing. Fortunately for us, the continuous evaporation of water from lakes and oceans replenishes the Troposphere. With more and more water vapor molecules joining others, clouds continually restock their liquid droplet supply through ongoing condensation. 

cirrus clouds

Cirrus clouds

Forming cirrus clouds 

Given that water vapor at higher altitudes tends to be frozen vapor rather than condensed liquid droplets, how will clouds look at high altitudes?

High altitude clouds are a type of cirrus cloud. These live at a height of 20,000 - 40,000 feet (6 - 12 km). Cirrus clouds are wispy, white, and light clouds. They look as if someone blew strands of ultra-white hair across the sky. 

Cirrus clouds have less water in them than lower clouds. Thus, they are the result of drier air that floated upwards or left-over cumulus clouds that have dispersed most of their liquid water droplets. Any leftover liquid droplets vaporize and freeze at high altitudes. Those  frozen water vapor molecules are dispersed by wind into the different patterns of lines, waves, and strands that make up cirrus clouds.  

It is as if a grouping of fine paint brushes were pulled across the sky by the wind, leaving streaks of wispy white paint trails.

Cumulus clouds

Updrafts: Pushing water vapor molecules together and pulling them apart

There is another factor that makes clouds a special phenomenum. Masses of air and water vapor are subject to updrafts. Can you imagine a large mass of moist air pushing up into a netting of cooled water vapor?  

The result would be like the picture above of a spider web with scattered water droplets: The updrafts push the water vapor molecules into forming together. However, as the updrafts push the mass into higher altitudes, a frozen water netting is produced. Both forces play a role at low and middle levels of the Troposphere; moisture condenses into drops of water that stay attached to a iced water vapor web.

Forming stratus clouds

 Do you remember the last time you walked under a leaden sky? It is as if someone drew a dark grey soft blanket across the sky. The sun's light barely makes it through; there is no fluffiness or whiteness to the clouds; only variations of dark and light grey.

Stratus clouds form on humid days or when a warm front hits cooler air in the lower Troposphere. The altitude is cold enough for water vapor to condense into liquid droplets. Moreover, since the air pressure is only slightly lower than sea-level, the droplets remain liquid. 

The result is a large mass of the liquid droplets that are suspended in a cold water vapor netting. As more and more moist air is pushed up into the cloud base, more liquid water droplets form. The increased density of droplets pushes the air out of the cloud. 

The resultant cloud is primarily droplets of liquid water, with little air. Light is absorbed when going through this relatively airless and wet cloud. There is little light leaving the cloud. The cloud looks grey, just as if you had placed a shroud between the sunlight and the earth. 

Dewdrops in a web

dewdrops in a web

Forming Cumulus clouds

Cumulus clouds are those fluffy heaped up clouds. Those can be 1 Km across and 1 km high. Cumulus clouds, even though they start at 2,000 feet (3/4 Km), can extend quite a bit upwards. This means that there can be temperature differences of  almost 10º C (18º F) between the bottom of the cloud and its top,  The atmospheric pressure - which is ~ 90% that at sea level at 2,000 feet - drops to 80% that at sea level at the top of the cloud. 

The factor that gains prominence in forming cumulus clouds is an updraft of air. When there are strong updrafts, the mass of liquid water droplets, iced water vapor netting, and air is whooshed higher into the Troposphere. As that happens, the higher altitudes with their lower pressures favor expansion of the mass of water and air within the cloud. With expansion, there is further cooling. 

As noted above, liquid water droplets vaporize into an icy netting at higher altitudes. It is as if one had a giant standing on Earth and blowing moist air upwards through a stratus cloud. The stratus cloud puffs up and forms fluffy white mounds. Everything - cool air, cold liquid droplets, ice crystals - is held captive in thin frozen layers of a frigid water vapor web.

The net effect is an enlarging mass of liquid water droplets and air encased in an iced water vapor web.  In fact, the act of moving a large amount of water vapor molecules into the Troposphere will create a cloud instantaneously as a direct result of the colder temperatures and lower pressures. 

Fluffy cumulus clouds

Fluffy cumulus clouds

Types of Clouds

The types of clouds that form is based on amount of moisture in the air versus the outside temperatures and pressures: 

  • As moisture decreases (lower altitudes have higher moisture), temperature decreases (higher altitudes have lower temperatures), and as pressures decrease (higher altitudes have lower pressures),: 
    • Clouds move from stratus/nimbus --> Cumulus ---> cirrus clouds
Swirling steam

Evaporation and condensation

water droplets in spider web

 Holding droplets in a cooled water vapor net

Spider web holding water droplets

Forming a frozen water vapor net at higher altitudes

Cumulus clouds

Updrafts: Pushing water vapor molecules together and pulling them apart

The lowest clouds: 

Why 1,500 feet (1/2 km) is an important starting point for cloud formation

The major factors that lead to cloud formation are water vapor, temperature, pressure, and air updrafts (as well as dust particles and other condensation nuclei).

If we only had water vapor at warm temperatures, we would feel the mugginess in the air; we would not see the water vapor. However, with lower temperatures and condensation nuclei, water vapor condenses into liquid droplets as well as a interconnected cold vapor netting.

At 1,500 feet (1/2 km) the combination of low temperatures and lower pressures allow a tight frozen water vapor netting that entraps much of the water vapor that is rising from the Earth. As more and more water vapor molecules are stuffed into that netting, much of it condenses into microscopic liquid water droplets.

It is as if one were adding more and more people into a crowded hall. This is what forms the stratus clouds at the lower altitudes of the Troposphere.

Thus, it is a combination of factors that come together at 1,500 feet (300 meters) that allow cloud formation.

Picture credits: 

  1. Joao Alves. Clouds. Flickr photo-sharing, Taken on May 13, 2009.
  2. waferboard. coffee steam 2. Coffee break. Flickr photo-sharing, taken on June 21, 2012.
  3. Thomas Brueckner “Hydrogen bonds” Flickr – Photo Sharing! Taken on Oct 7, 2005
  4. Ben Pollard. Alcatraz: Spiderweb on some greenery at the bottom of the island. Flickr photo-sharing, taken on December 29, 2007.
  5. Ankur Panchbudhe. Dense fog on the way to Temi Tea Gardens from Namchi. Flickr photo-sharing Taken on April 29, 2018.
  6. Mark Braggins. Water droplets resting on spider's web. Flickr photo-sharing, taken on Aug 8, 2012. 
  7. Jan Remund. Cirrus clouds, Värmdö, Sweden. Flickr photo-sharing. Taken on July 19, 2013.
  8. Images by John 'K'. "Flat bottomed clouds". Flickr photo-sharing, taken on March 4, 2009.
  9. Peter shanks Dewdrops in a web. Flickr photo-sharing, taken on April 16, 2008.
  10. Nicholas A. Tonelli. Fluffy Midday cumulus clouds near Carroll, Clinton County. Flickr photo-sharing, taken on July 31, 2010.

About the author 

Juman Hijab

Juman has been in clinical practice as a physician for more than three decades. Her lifelong interest has been in the chemistry of life.

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