# Relationship between speed and power in wind/hurricanes

I was looking at hurricanes today when question crossed my mind.

Cat 1 Hurricanes are the ones going up to 95mph. Cat 5 goes above 157 mph.

That says a lot, but not all.

In motors or engines, RPM (analogy to MPH) is not the only important factor. Torque is equally as important. If one goes up, the other goes down unless you have more HP to sustain both at "desired" level.

How does this work with Hurricanes or wind for that matter?

I would assume that a slower wind gust with more torque could make more damage than a faster wind gust with less torque if that makes sense.

Is there such a relationship? How are both reconciled in this particular case?

• Where do you think there is torque with the hurricane? There's nothing rigid in the structure of the storm delivering force on some moment arm.... Commented Sep 17, 2018 at 0:36

the airflow equivalent of torque is pressure and of RPM is mass flow rate, at least in the sense that sustained pressure differences are what accelerate air masses to high velocities.

This sort of dynamic analogy is appropriate when dealing with air movement in ducts but it doesn't work so well when there are no ducts as such and where the scale lengths are of order ~hundreds of miles.

Instead, we look at the kinetic energy carried by parcels of high-speed air and the aerodynamic ("wind") loads imposed by those air parcels on things like trees, houses, cars and people.

Wind loads (expressed in terms of pounds per square foot) are proportional to the square of the wind velocity, which means doubling the wind velocity increases the loads by a factor of four. This effect is what makes hurricanes and tornadoes so destructive.

• Just to make sure I understood it. In mechanical engineering, there is a correlation between torque and rotations. In a Hurricane, there is not. The only way that 157mph winds can be generated is by achieving a certain fixed value of pressure. In other words, the "torque" in this case is constant. So, for winds of 100mph, you need X amount of pressure, for winds of 200mph you need 4x pressure (or generate 4x loads). Is that right? Commented Sep 17, 2018 at 1:09
• It's not that easy, but there is a lot that has been written about it. I recommend an entry-level text about atmospheric science and weather. regarding the analogies: these exist throughout the world of systems modeling. you begin with generalized variables of effort and flow. effort variables are torque, voltage, pressure and force. in order, the corresponding flow variables are RPM, current, mass flow rate and velocity. Commented Sep 17, 2018 at 2:00

There is no such relationship.

Your analogy of RPM vs torque assumes constant power from the engine. That doesn't apply to hurricanes: a category 5 hurricane has a MUCH BIGGER engine than a category 1.