Hurricanes on the rise
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With the problem of climate change due to global warming intensifying, are hurricanes becoming more volatile? And what actually causes them in the first place?
The short answer is simply a combination of heat and wind! The longer answer is much more complicated, depending on many factors. Let's explore some of these factors and see if it's possible to resolve some of the complexity by breaking it down into a few basic physics principles.
First, hurricanes are examples of tropical cyclones, other examples being typhoons (the nomenclature depending on where in the world they occur). Tropical cyclones are vast rotating weather features, typically hundreds of kilometres across and ten to fifteen kilometres deep. To be classified as a tropical cyclone, the storm must feature winds in excess of 74 mph for a so-called category 1 storm, rising to wind speeds in excess of 157 mph for a category 5 hurricane. The winds are least strong in the calm central region known as the eye of the storm. The eye wall sees the strongest winds, with the wind speeds easing off with increasing distance from the eye wall out to the furthest extent of the cyclone, where they are nevertheless still of storm force with the potential for damage.
The crucial elements required to get a tropical cyclone up and running are converging winds with low vertical wind shear (change of strength and direction), warm tropical water and the Coriolis Effect. There is no need to invoke such esoteric ideas as the 'butterfly effect' beloved of chaos theorists. Hurricanes form due to a combination of warm rising air and converging winds (deflected by the spin of the Earth) causing bodies of air to be deflected in their paths. The Coriolis effect is an example of a fictitious force (much like the ubiquitous centrifugal force we feel on a roundabout). If you were lucky (or perhaps unlucky) enough to be at a fixed point in space watching a mythical giant hurling a rock towards the equator from the north pole, it would appear to travel in a great circle. However, if you observe the same action from the surface of the Earth, the same rock will appear to travel in a curved path with respect to the surface; the planetary rotation carries you away from the great circle of the rock's trajectory. This phenomenon affects winds travelling around the Earth towards and away from the equator.
Because the surface of the Earth at the equator is moving faster than at the poles (like the edge and centre respectively of our aforementioned roundabout), winds travelling north (away from the equator) will be deflected towards the east, as the speed of the Earth's surface beneath the wind decreases relative to the wind speed. Conversely, winds travelling south from the north pole wind appear to be deflected towards the west as the spin of the earth's surface relative to the wind speeds up. The simple fact of the Earth being a sphere (actually an oblate spheroid) spinning around its axis combined with the inertial mass of the moving bodies of air, is responsible for tropical storms spinning counter-clockwise in the northern hemisphere and clockwise in the south.
The next ingredient to be added is solar energy which has greater intensity in the tropics than at the poles (as the sunlight is more concentrated per unit of area), consequently creating zones of greater heating than at higher latitudes. This disparity of surface temperature results in latitudinally rotating convection cells, with regions of low pressure where warm air is rising, and high pressure where cooled air is descending. These cells give rise to the trade winds - themselves seasonal due to the tilt of the Earth's rotational axis. Converging winds combined with rising warm, moist, tropical oceanic air, aided and abetted by the Coriolis effect, conspire in the formation of tropical storms. These often fizzle out, however. When and where there is enough heat energy in the ocean to feed the storm (sea temperature of typically 26 degrees Celsius or higher being required), tropical cyclones may develop with plumes of tropical air rising through the troposphere relative to the cooler air due to the warmer air's lower density. This process is strengthened by the moisture in the rising column condensing at high altitudes, the subsequent release of latent heat energy at the top of the storm columns pumping them up further, whilst increasingly lowering air pressure at their base, which in turn increases the speed at which the converging winds are sucked into the system, accelerating as they spin around the rising core of the storm. The higher the ocean temperature, the stronger the convection currents, hence the possibility of a correlation between global warming and hurricane intensity.
The formation of a hurricane’s eye, still the source of a degree of controversy, probably develops as a central region of cooled, clear sinking air surrounded by an ascending air mass which has gained angular momentum by virtue of decreasing in radius (much like a proverbial spinning skater pulling their arms in). There is insufficient centripetal force to allow the spiralling air mass to 'fall' into the very central core of the storm. The subsequent track that the body of a storm follows is determined by a complex combination of prevailing winds, areas of high and low pressure and precession due to the system’s angular momentum.
Incidentally, please note that the direction of the Earth's rotation has no effect on the way your bath water spirals down the plug hole, regardless of which hemisphere you're in. The scale of a bath or sink is simply too small to be affected in this way.
Hopefully, all of the above, which has been simplified (for obvious reasons) has convinced you that notwithstanding the increasing amounts of data science and mathematics involved, meteorology is first and foremost a job for physicists!
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