Posted by: Aba Cohen | November 18, 2013

It is easy understanding a typhoon

It’s easy to understand why and how typhoons occur (or hurricanes or cyclones, names that vary by region of the planet where they happen): First it needs a portion of the sea, larger than an average size city, which might accumulate excess of thermal energy of the order of 10 17  to  10 19 Joules, or the equivalent to the energy released by THOUSANDS OF ATOMIC BOMBS of ~ 10 kilotons (the Hiroshima bomb released 10 14 Joules) and can flow to the atmosphere,  due to temperature imbalances, within a few days. Concentrations of energy with these characteristics may occur in tropical regions, concentrated in a volume of the sea at a temperature of ~ 27 ° C (Δt ~ 5 ° C or more, compared to an average temperature of 20 ° C), within lateral extensions of  some tens of kilometers in diameter and depths of 50 to 60 meters (that means Δt ~ 5 °C in tens of billions of m3 of warm sea).


The hot air rises up from this region for a few miles, towards the higher regions of the atmosphere, carrying a colossal mass of water, due to the intense evaporation of the sea at that temperature; such a flow of hot air out from the region transforms it in a ‘low pressure zone’, that means “a large ‘evacuated’ volume asking to be filled”, a factor that also favors the gales and the evaporation process. Upon approaching the stratosphere where the air is naturally cooler, the steam accumulates in the form of clouds and eventually condenses, liberating huge amounts of heat, and precipitates as storms -see the blue lines in the figure above, so as to compensate for the shortage due to low pressure over the sea. This pumping hot air and moisture up and a lot of water down in the form of storm generates a “convective flow” that lasts as long as the excess energy exists.

Up to this point we explained the storms but not the funnel (eye of the hurricane) nor the direction of the spinning wind that causes the huge damages by this phenomenon. This can be understood if we take into account the rotation of the Earth:

Coriolis Force

The figure above shows two points near the equator (A and A’) and two farthest points of this line (B , north of A and B’ , south of A’). Because the Earth’s rotation points A and A ‘, when viewed from a point outside the planet, move eastward at rotational speeds greater than the points B and B’ (points at the poles have no rotational speed – not to be confused with angular velocity). If an object is launched BY AIR from A to B (or from A’ to B’) it will hit the ground at a point slightly to the east of point B (or B’) because while in the air, in addition to the SN velocity, the body keeps its high “equatorial” speed. On the other hand, if the object is released THROUGH THE AIR from B to A (or B’ to A’) , it will reach the ground at a point further to the west of A (or A’) because, while in the air, in addition to the NS velocity, the body keeps its low original rotational velocity. In the same figure we see two points “O”, one in the Northern Hemisphere and the other one in the South, where winds come traveling in the directions NS and SN. It is easy to see that the wind in the Northern Hemisphere tends to rotate anti-clockwise (rotation contrary to the arms of a clock) while in the southern hemisphere the wind tends to rotate clockwise. This effect is caused by the “Coriolis force” that produces these trajectory deviations due to the Earth’s rotation.

Katrina and Catarina

The extent and devastation caused by a typhoon depend on the energy stored in the ocean. In the case of typhoon Haiyan, which killed thousands of people and devastated the Philippines in 2013, the diameter was ~ 600 Km ( not to be mistaken with the eye of the typhoon which is much smaller – see the pictures above) and winds reached 380 ~ km/h, a record speed. Upon arriving on the continent the typhoon loses strength because its source of energy (the hot water of the ocean) is kept back. The pictures above are satellite shots of the hurricanes Katrina , which struck the U.S. in 2005, turning in a counterclockwise direction and the Catarina, which struck southern Brazil in 2004, with clockwise rotation as explained above. The frequency and intensity of such an unwanted phenomena have increased significantly in recent times and the explanation is the global warming. Until this situation is not reversed, the prospects are the occurrence of more and more tragedies like the Raiyan, even in Brazil where such disasters rarely occurred in the former years.

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