For instance, vertical wind speed is very difficult to measure by any means, so not much is known about what the updraft velocities are. Along the way I did a lot of different simulations using a wide variety of initial conditions. Besides arriving at two plausible theories for explaining sub-visible cirrus cloud formation, his research indicated that cloud formation and stratospheric dehydration processes are symbiotically linked. In the tropical western Pacific, tropopause temperatures are at their lowest levels. Like typical cirrus, sub-visible cirrus can form when layers of slowly rising humid air cool and reach saturation.
Ice crystals can then nucleate, grow, and precipitate.
But compared to typical cirrus clouds composed of ice crystals of a few hundred microns' length, cirrus formations in the upper tropopause are almost certainly composed of very small ice crystals of 20 or 30 microns or less, Jensen said, because the growth rate of ice crystals at minus 70 to 80 degrees Celsius is much slower than at warmer temperatures. The smaller, lighter, ice crystals also precipitate at slower rates, so cirrus in the tropopause last longer than do typical midlatitude cirrus formations. The larger ice crystals of typical cirrus fall away quite rapidly, so that the clouds dissipate in a matter of hours, compared to days for sub-visible tropospheric cirrus. The modelers also simulated the formation of sub-visible cirrus due to outflow from deep convective systems circulating from the surface all the way up to the tropopause in strong upward drafts. The scenario resulted in the formation of huge anvil clouds covering thousands of kilometers.