Physics Photo of the Week - August 25, 2023
Cumulus Clouds - Photos by Donald Collins
Cumulus clouds are the most familiar and most common cloud formations. Their formation is very simple: these clouds form on sunny days and result from air that has been heated by the Sun; the heated air rises to higher elevations because the heated air has expanded and has a lower density, so it rises like air bubbles in water. In rising to higher elevations the surrounding atmospheric pressure is reduced and the air expands. Air bubbles or carbon dioxide bubbles rising in a carbonated beverage do a similar expansion. A "bubble" of expanding warm air - up to several hundred meters in diameter - is "insulated" from the temperature of the surrounding air because of its sheer size. This is in contrast to the tiny bubbles rising in a beverage glass that have intimate thermal contact with the surrounding water-based beverage. When a parcel of air that is insulated from it's environment expands, its temperature lowers. That is because the parcel of air had to do physical work to expand and required the parcel to convert some of the internal thermal energy, which depends on temperature, into the work of expanding. This insulated expansion of a gas is called an adiabatic process. Adiabatic means thermally insulated. The drop in temperature of the rising air bubble eventually drops below the dew point and the water vapor in the air condenses into a cloud that consists of millions of tiny water droplets. The billowy fluffy appearance of these cumulus has resulted to the collection of many bubbles of rising air.
The left image was taken near Warren Wilson College looking over the Great Craggy Mountains. The large billowy cloud in the center is obviously the result of a collection of rising "bubbles of rising air". The right-hand image shows a towering cumulus cloud near the left center taken from Willoughby Lake in Vermont.
Under the right conditions - no wind shear that would "tear the clouds apart" - the cumulus clouds can grow to extreme heights. This is due to the fact that when the cloud forms into dew droplets, heat is released because condensing water vapor releases the heat of vaporization into the rising bubble. The released heat thus gives the bubble more buoyancy. In these conditions the cloud top keeps rising until it reaches the stratosphere - about 40,000 feet altitude above which the air becomes warmer with altitude. This thermal inversion prevents further rise of the air, and the bubbles spread out on top of the regular atmosphere forming an "anvil top". The air is so cold at the stratosphere that the mist freezes into ice crystals. The right hand image from Willoughby Lake above shows the anvil top of a more distant cumulo-nimbus cloud - a thunderstorm. The rising cloudy air reaches the stratosphere where it ceases to rise and is forced to spread laterally into a flat, anvil-shaped cloud.
Both of these images are animated. Click on the links below to view each animation which show the time-lapse videos played back at about 200 times faster than the actual speed.
The puffy clouds rise and recede with the prevailing wind, but the flat anvil in the distance is merely becoming more distant without further rising.
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Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us. Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.
All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion. These photos and discussions may be used for private individual use or educational use. Any commercial use without written permission of the photoprovider is forbidden.
