Square clouds, or clouds that appear to have grid-like patterns, form due to Rayleigh-Bénard convection, a process involving heating from below and/or cooling from above, often through evaporation. This creates organized, repeating patterns called Bénard cells.
Understanding Rayleigh-Bénard Convection
Rayleigh-Bénard convection is a type of natural convection driven by temperature differences in a fluid (in this case, air containing water vapor). Here's a breakdown:
- Heating from Below: When the air near the ground is warmer than the air above, the warm air rises.
- Cooling from Above (Evaporation): Evaporation near the top of the cloud layer cools the air, making it denser.
- Convection Cells: This temperature difference creates convection currents. Warm air rises in some areas, cools, and sinks in other areas, forming organized, repeating patterns called Bénard cells. These cells are often hexagonal, but when viewed from below in cloud formations, they can appear roughly square or rectangular.
- Cloud Formation and Boundaries: Clouds form where the warm, moist air rises and cools, causing water vapor to condense. The sinking air inhibits cloud formation. This creates clear boundaries between the rising and sinking air, resulting in the grid-like or "square" appearance of the clouds.
Analogy
Imagine boiling water in a pot. The heat from the bottom causes water to rise, while cooler water sinks. This creates a circular motion that can be visualized with food coloring. Rayleigh-Bénard convection is a similar process on a much larger scale in the atmosphere.
Summary
In short, the "square" appearance of some cloud formations is an optical illusion resulting from the organized convection patterns (Bénard cells) driven by temperature differences within the atmosphere. These differences are caused by heating from below (ground) and cooling from above (evaporation).
