As in the single cell circulation model of the atmosphere, single cells are thought to carry the energy from the equator to the poles, but this doesn't describe the whole phenomenon. Instead the general circulation is preferably applied having three different cells. Among these cells Hadley cells rises near the equator due to differential heating and move up to the tropopause, and then spread horizontally near the subtropics (mid lattitude 30 degree) where they again sink which is attributed to the cooling of the air mass which makes them heavy. Why does this cooling or heavy air mass take place exactly near the subtropic? Why not at other latitudes?
I have read, that air in polewards flowing upper troposphere moves more and more zonally due to Coriolis force, so at tropics it flows mainly zonally, not meridionally, so has only one possibility - to sink.
So the structure and flowing regime (Hadley or Rossby) is dictated by the Earth rotation.
Dear Dhirendra,
i am sending you a short text from a book chapter i am currently working on:
Within the equatorial zone the heated air masses rise up to the upper atmosphere and are transported to higher latitudes by the anti-trades, which partially descent at about 25-35 degrees latitude (Lauer 1993). However, the denomination “anti-trades” proves awkward since it indicates a similarity to the surface trade winds although the anti-trades are neither equal in strength nor in steadiness. Instead, meridional components are weak and zonal componentsare strong. This could be traced back to the influence of the Coriolis force which is growing larger with increasing distance to the Equator. Because of earth’s rotation moving air is deflected, veering to the right of the moving direction in the Northern Hemisphere and to the left in the Southern hemisphere. As a result, the upper air flows become more and more westerly at about 25-35 degrees latitude and are named subtropical jets (STJ). Over the southern hemisphere the subtropical jet consists of strong winds at approx. 12000 m altitude, which prevail throughout the year over 25-35 degree southern latitude. Over the northern hemisphere a similar jet stream is limited to the winter season. During the summer it is replaced by a strong easterly air flow over Asia and Africa, the Tropical Easterly Jet (Nieuwolt 1977, McGregor and Nieuwolt 1998). Between 25-35 degree latitude the cooling of air masses, the increased earth’sangular momentum and the accumulation of air masses due to the reduced size of the earth's surface lead to a broadsubsidence of the air towards the ground. These areas are characterized by a relative high surface pressure and are called the subtropical highs.
Kind regards!
The average latitude for subsidence of the Hadley circulation is determined by the balance of energy input and energy loss. Input is solar heating -- mostly as latent heat via water evaporation from the tropical ocean that drives the strong upwelling at the inter tropical convergence zone (ITCZ). Loss is a combination of radiative loss to space and surface drag, drag being the controlling factor. The same physics control for example the bands of Jupiter, so given the right conditions you can wind up with many of these cells. This was described (including the fact that the ITCZ moves very quickly from one tropic to the other, much faster than the sub-solar point) in a paper by Isaac Held and A Y Hou rather a long time ago now:
Held, Isaac M., and A Y Hou, 1980: Nonlinear axially symmetric circulations in a nealy inviscid atmosphere. Journal of the Atmospheric Sciences, 37(3), 515-533.
Dear Dhirender
i think none of the answers posted above deal with what actually you are asking. the answer to this question perhaps does not lie exactly in subject of physics or for that matter science. it has just to do with the purest and innocent of all subjects which is Maths i.e. the geodesy of our planet or its shape (and a little of conservation of angular momentum) . the air subsides at 30 degree because of such geometry of earth. (and of course the long time scale stabilized mean meridional velocity) which causes mass convergence of air at that degree. see this link--
https://courseware.e-education.psu.edu/courses/meteo241/Section04p03.html
your answer is in tan(phi), the latitude. if shape of earth (not size) would have been different that phi would have been different. may be you can check for other planets with atmosphere. what are their tan(phi) for this equation.
good luck with your CLM run
The air sink at 30 degree latitude as it is very cool enough at that time. Cooler air will have a higher density which will make the air to sink to the Earth surface create a high pressure area. Just try huhu.
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