Clouds have a dual personality. On one hand, they form beautiful piles of fluffy white cumulus, flat sheets of stratus and feathers of icy cirrus, all of which entertain with spectacular yellow, orange and red sunrises and sunsets.
On the other hand, they indicate the presence of otherwise invisible air currents within the atmosphere. Clouds usually indicate regions of rising air.
If the air cools below the dew point as it rises, then nano-droplets of water condense out of thin air and grow larger as more water molecules stick to the droplet.
The phase transition from gas to liquid in condensation and from liquid to gas in evaporation is as mysterious as it gets. It seems even magical if you think about it, but it is so common we usually do not.
Water just disappears into the air. Dishes dry in the dish rack, and streets dry after it rains. What a mystery that must have been long ago in the time of alchemy and magic, spirits and spells!
Water magically appears on the cold can that you took out of the fridge. How does that water just appear out of the air?
The magic of evaporation and condensation occurs on the molecular scale. Water molecules are slightly sticky and weakly stick together over a small range of temperatures at any given pressure.
The liquid state of water is rare. It seems ordinary to us because we live in a rare “Goldilocks” earthly environment where liquid water is common in a temperature range that is suitable for life. In addition, our environment happens to be one in which water can exist in all three phases — solid, liquid and gas — at the same time and place.
The planet’s mass, size, distance from the sun and atmospheric pressure are all determinants of that Goldilocks zone. A smaller planet located where Earth is, all other factors being equal, would be dry, having atmospheric pressure too low for liquid water.
Below freezing temperature the molecules form rigid crystals, like a room full of dancers holding hands. Above boiling temperature they are free, having too much thermal energy for the limited stickiness to keep them grouped.
Between 32 and 212 degrees Fahrenheit, the bonds between water molecules are weak enough that the molecules can slide over one another like oily dried beans in a jar. They are stickier at lower temperatures when their thermal motion is slower and form submicroscopic, semicrystalline clumps. The higher the temperatures, the smaller and shorter-lived are the clumps.
When water boils, it has reached equilibrium where heat carried away by evaporation equals heat input from the stovetop as molecules that are more energetic escape the sticky bonds. However, water does not have to boil in order to evaporate.
At any temperature, even below freezing, some molecules gain enough energy from collisions to escape and evaporate into the air. The warmer the water, the faster the motion, allowing more escapees, so warm water evaporates faster than cold water.
This everyday magic is one of the most incredible of all physical processes, yet we take it for granted because our place of residence is a water world, “Planet Ocean.”
Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.
