How does a tropical storm become a hurricane?

Histogram Hurricane Florence 2018

Histogram of Hurricane Florence 2018

Changes in the estimated average wind speed classify the weather events over the sea as a tropical depression, a tropical storm, or a hurricane (tropical cyclone). The Saffir-Simpson scale is used to allow proper classification.
The attached curve reveals no abnormality when a tropical storm becomes a hurricane.
However, satellite images show a significant change when a hurricane occurs. An eye appears and is clearly visible from space. The system is highly ordered but much more dangerous because a hurricane is a mobile thermal engine above the ocean, with a huge motive power.
The goal of this blog is to try to clarify this apparent contradiction. What happens when a tropical storm turns into a hurricane? What are the relative influences of various parameters on the formation of a hurricane and on its regression to a tropical storm?
Lastly, we note that hurricane Florence may be downgraded to tropical storm. Other hurricanes behave the same way. This is an encouraging sign.

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Michel Pluviose
Michel Pluviose is an honorary professor of Le Conservatoire National des Arts et Métiers (CNAM). Formerly chair of turbomachines at CNAM. Previously, he served as laboratory director of the Technical Association for Turbomachines and Gas Turbines (ATTAG).