How do clouds form
Clouds form at anywhere from 1,500 feet (~0.5 km) to 33,000 feet (10 km) above the earth. How do clouds form? In this article, I will outline a helpful way to understand cloud formation. I have listed 5 steps:
- Evaporation and condensation
- Cooling water vapor molecules as they rise in the atmosphere
- Creating a frozen water vapor netting
- Creating the whiteness of clouds
- Adding water droplets to the clouds
Step 1: 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.
How do clouds form? The first step in cloud formation is evaporation of water molecules from large bodies of water (lakes, oceans).
Step 2: Cooling water vapor molecules as they rise in the atmosphere
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).
How do clouds form? The second step in cloud formation is having water molecules rise in the atmosphere to the cooler temperatures.
Step 3: Creating a frozen water vapor netting
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 (blue sphere) to the "parent" oxygen (red sphere) 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.
Condensation of water molecules together (reverse of evaporation)
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 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).
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. Thus, when water vapor molecules are released up into the very cold environment of the higher troposphere, rather than condensing into liquid water, they form ice crystals (this is how snow forms). The groups of water vapor molecules will come together to form webs of strongly connected molecules. The very cold water vapor high in the troposphere freezes into an ice crystal netting before it has a chance to condense into liquid water.
How do clouds form? The third step in forming clouds is forming a frozen water vapor netting. The image below shows dew in a spider's web. That is the picture I want you to have of a frozen ice crystal net holding minute water droplets.
Keep in mind that the spider's web netting is very fine and hard to see. The only reason we can "see" it in the image below is that it is dotted with minute water droplets.
Step 4: Creating the whiteness of clouds
When a large mass of water vapor molecules condenses (attach to each other through hydrogen bonds) to form a web of connected vapor molecules, 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 maybe why notilucent clouds 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 in the bathroom appear as white ghostly mists.
How do clouds form? The fourth step of cloud formation is trapping air pockets in the frozen water vapor crystal netting. This makes the clouds look white.
Step 5: Adding water droplets to clouds
What about the liquid water droplets that are formed in clouds in the cold 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 separate water vapor molecules
- Finally, there are the updrafts within the cloud that force the droplets higher into the troposphere to repeat the cycle of vaporizing the liquid water and freezing the water vapor molecules into an icy netting
Staying liquid in the sky
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. When warm humid air rises (or a warm humid front comes through) the water vapor molecules in the air push into the bottom layer of the cloud. It is as if one is jamming more and molecules into a small room. The lowermost level of the cloud becomes more dense with water vapor as more molecules crowd in.
The pressure of the water vapor molecules on each other causes condensation of the water vapor into droplets of liquid water. This would be like running one’s finger gently over a steamed window/glass: as our finger brings the fogged water molecules together, they coalesce into liquid water droplets. It is a pressure effect of compressing a lot of hydrogen-bonded vapor molecules together.
The lowermost cloud layers are warmer given a continued addition of warmer water vapor molecules from the earth. By definition, warmer water vapor has less hydrogen bonding, which allows the creation of liquid water droplets, Liquid water has less hydrogen bonding than ice and snow and it has intermittent hydrogen bonding, allowing it to stay liquid. Keep in mind that the water droplets in the clouds are minute and spread across a large surface area of frozen ice crystals netting.
How do clouds form? The fifth step of cloud formation is continuously adding warm water vapor molecules into the cloud (rising up from the earth). These force condensation of the water vapor molecules thinto minute water droplets which are then held together in the frozen water vapor netting.
In the image below, I again demonstrate how the water droplets are held together in a spider's web to mimic what happens in a cloud. But imagine that web multiplied millions of times to visualize the size of a large cloud with a huge mass of water droplets.
When there is a lot of warm water vapor molecules pushing up into the cloud and forming water droplets, the frozen hydrogen bonds weaken. The droplets of water that develop within the cloud are no longer held suspended through the help of a frozen netting of hydrogen bonds; these droplets fall down as rain.
Note that it is not only the size of the droplets that allow them to fall; it is the loss of the holding power of hydrogen bonds. One can have small or largish droplets within the clouds. As long as there is attachment to the hydrogen-bonded frozen web of water crystals and enough buoyancy from the surrounding air, the droplets will remain suspended in the clouds. Of course, if the drops become large enough (through a mass of warm and humid water vapor rising into the cloud), even frozen hydrogen bonds will not be strong enough to hold them in place.
Can you see the rain falling when warm water vapor molecules (or warm humid air) rises into the cloud and the frozen web bonds disintegrate?