Beatiful Fluid-Mechanics

Here are some questions you may want to think about. I don't have good answers (or only partial answers).

• This picture shows the sandy bottom near the beach (Calabria, Italia). It is taken through a one meter layer of calm sea-water. Wind created waves in the order of 10 cm. The sun creates the beatiful lines on the bottem. Why lines?
  

• Water is heavier than air. Why do clouds not fall down?
• Cavitation: In fast flowing water, behind an obstacle the pressure can be negative - despite gravity, setting the pressure of the surrounding air as zero. Once this occurs, air bubbles form within the water. In this foto you see my fore-arm and water flowing very fast around it. The white region is formed by air bubbles that are formed at the hairs of the arm (and transported with the fluid). At which pressure form bubbles? What gas is inside the bubbles and where does it come from?
  
  
• If you stear your tea, the tea-leaves will eventually go to the center of rotation and to the bottom of the cup. Why?

  A partial answer to this question is: There appears a secondary flow that looks like convection. But where does this flow come from?

• What mechanism created these beautiful structures?

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• This picture here shows an aircraft hitting the sound barrier (?), german: es durchbricht die Schallmauer. How do you read the picture? What is happening?

• If you have a roll (Broetchen) of the other day, it's hard and you don't want to eat it anymore. Here's a trick: Put it in the micro-wave oven and heat it at low rate for about 10 seconds. It will be soft and nice to eat. The trick works, I use it. But what's happening?

• This picture shows a thin film of water running down a vertical plane. It forms nice structures. Can you predict (depending on the amount of water and the materials) the typical size and/or the typical shape of the structures?
  
• The picture shows a beautiful cloud over a city on top of a mountain near the ionian sea. Why forms the cloud here?
  Meanwhile I found a partial answer: wind creates a standing wave behind the mountain. This explains the nice stable UFO-shape. Enjoy also this second cloud.