September 5, 2019
[Image from weather.com, https://weather.com/storms/hurricane/news/2019-09-02-hurricane-dorian-labor-day-bahamas-florida-georgia-carolinas]
This week, Hurricane Dorian crossed the Bahamas. On Monday, while I was writing this, it was still there. It was a Category 5 hurricane on the Saffir-Simpson scale, which meant that it had winds somewhere within the storm blowing at over 157 miles per hour for at least a minute...i.e., sustained winds, not short-lived wind gusts. The wind speeds are comparable to an Enhanced Fujita (EF)-4 tornado, except that this storm is going on for days and is spread out over a much, much larger area than any tornado.
The winds are driven by an intense pressure differential between the eye of the storm and the surrounding atmosphere. At sea level, atmospheric pressure averages to somewhere just above 1 bar.
(1 bar = 100,000 Pascals, Pascals being the natural SI / metric unit of pressure. A fluid with a pressure of 1 Pascal, acting on one side of an object 1 kilogram in mass and 1 square meter in size, would accelerate it at 1 meter per second, per second into a pure vacuum. One kilogram is accelerated 1 m/sec/sec by 1 Newton of force, and 1 Pascal is 1 Newton spread over 1 square meter. You physicists, think through that explanation and contact me with corrections, but I believe that's correct.)
The Weather Channel graphic above notes that Dorian's central pressure was 919 mb, or 0.919 bar. That's a difference of almost 10% from normal pressure, and that's a huge pressure gradient, which is what accelerates the air to these howling velocities.
This massive difference in pressure also sucks the water of the ocean upward into the core of the storm, which is at least part of the phenomenon of storm surge. The water is pushed into the gap by the ambient air pressure outside the storm.
What causes this region of strongly lowered air pressure at the core of the storm in the first place? This is where my meteorological knowledge ends, although rising of warm air near the warm surface of the ocean is a crucial part.
I don't know what will be left standing in the northern Bahamas, and at this point we don't know how much of the U.S. east coast this monster will hammer, and how hard. It is nearly stuck in place at the moment.