PHY198S

Physics at the Cutting Edge (2020-21) -- PHY198S

More about hurricanes

Here is a resource on hurricanes by Dr. Kerry Emanuel of MIT recommended by Week 2 colloquium speaker Dr. Jennifer Kay. It aims to explain why increased global warming probably intensifies the strength of hurricanes. Along the way it discusses some neat facts, like the role of water vapour in the atmosphere in moderating wind speeds, making air travel feasible.

The NWS JetStream site has a more elementary introduction to tropical cyclones (a general name for hurricanes and typhoons). You may prefer to start with this.

The Kerry Emanuel article gets a bit more technical, and mentions some new terms that you may not be familiar with, so here is a very brief summary of their meaning which I hope is helpful. The Knight textbook covers all the following ideas, and more, in Chapters 18-21.

Brief glossary

Entropy represents how much of a system's thermal energy is unavailable for conversion into mechanical work. More intuitively, it can be thought of as the degree of disorder of the system. A messy desk has high entropy; a tidy one has low entropy.
Adiabatic is an adjective describing a thermodynamic process in which there is no heat transferred in or out.
Isothermal is an adjective describing a thermodynamic process occurring at constant temperature.
A heat engine converts heat into useful work, like pushing a piston. Any heat engine requires a working substance, usually a fluid, for example an ideal gas. A refrigerator is a heat engine run in reverse: instead of putting heat in to get work out, it puts work in to get heat out. In both cases, the efficiency is less than perfect: there is always some wastage, even theoretically.
A Carnot engine is the most efficient heat engine imaginable: it wastes the least heat in converting thermal energy into work. It is an idealization useful for thought experiments; a real-life car engine is much less efficient than this. The Carnot engine operates in a repeating cycle with four parts, each of which is a reversible process: (1) isothermal compression while in thermal contact with a cold reservoir at temperature $T_C$; (2) adiabatic compression while thermally isolated; (3) isothermal expansion while in thermal contact with a hot reservoir at temperature $T_H$; (4) adiabatic expansion while thermally isolated. Its efficiency is $(1-T_C/T_H) \times 100$%, so the only way to obtain 100% efficiency is for $T_C$ to be absolute zero or for $T_H$ to be infinity. Neither of those options is realistic.
A reversible process is one in which you can wind the clock backwards to get both the system and its environment back into their original states. This requires that the process be quasi-static, i.e., done slowly enough that you can essentially maintain thermal equilibrium throughout. By contrast, sudden processes are irreversible.
For a mixing process, which is irreversible, the entropy produced is known as the entropy of mixing. Another irreversible process that produces entropy is dissipation through friction.

A digression on the Laws of Thermodynamics

On Entropy and the Laws of Thermodynamics