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As quoted from the AR5 IPCC report (chapter 7) here.

Most high cloud (mainly cirrus and deep cumulus outflows) occurs near the equator and over tropical continents, but can also be seen in the mid-latitude storm track regions and over mid-lati- tude continents in summer (Figure 7.6a, e)

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    $\begingroup$ Not enough for an answer, but for the worst airplane ride in your life, take a trip that goes from the northern hemisphere to the southern (or vice versa). I've seen stewards/stewardesses on such flights lock and then abandon their carts because even they had to sit down. Those ITCZ clouds reach very, very high. $\endgroup$ – David Hammen May 2 '14 at 0:48
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Clouds require moisture and therefore high clouds will exist in regions with upper-level moisture. Synoptic scale ascent is on the order of cm/s and this is not an efficient way to bring moisture from the surface to the upper levels.

Consider this water vapor imagery:
Water vapor Source: College of Dupage NEXLAB

In this image, the color represents the humidity of the mid and upper levels of the troposphere. Brighter colors represent moisture at higher levels. Darker colors represent moisture at mid levels. Black represents dry mid and upper levels.

The brightest whites (highest level moisture) at this time are along the ITCZ and associated a mid-latitude cyclone in the northeastern US. These are areas associated with convection, which unlike the synoptic scale flows is an efficient transport of moisture into the upper troposphere. This convective outflow will first be associated with the "anvil" top of the thunderstorm, but as it is advected at the upper levels can manifest as cirrus clouds. The ITCZ is driven by equatorial convergence and the upward branch of the Hadley cells. Mid-latitude cyclones have a so-called "wet conveyor belt" that transports near-surface moisture into the mid and upper levels which can then manifest as upper level clouds (or lots of rain/snow).

To answer "why are most cirrus clouds in the tropics?" I would associate that with the large transport of water vapor into the upper levels by the persistent convection in the ITCZ.

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  • $\begingroup$ (+1) for a really cool image. Do you know anything about the cause of the vortices off of the west coast of the US? $\endgroup$ – Chris Mueller May 2 '14 at 19:47
  • $\begingroup$ @ChrisMueller those are other cyclones (baroclinic storms) in the mid-latitude storm track. $\endgroup$ – casey♦ May 2 '14 at 20:41
  • $\begingroup$ Question: is the variation in tropopause height between equator and poles relevant? $\endgroup$ – kaberett May 4 '14 at 16:08

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