Their puffy appearance notwithstanding, clouds are actually large, cold, and wet accumulations of vapor, at least in the absence of Felis Umbrosus' spell The Billow of Eiderdown or other such magical interference. The air gets colder as you go up; it's not surprising that there are mountains in deserts that keep their snow all year, given how cold it is up at that altitude. If I hadn't previously researched a ward against the discomforts of the elements, Silence of the Marching Grog, my sailors would have come back with nasty colds and frostbite. As it was, I needed to work some spontaneous magicks to prevent ice from rendering the deck and the sheets and shrouds of the Aristeia dangerously slippery, and I intend to make this a small enchantment in the near future.
The freezing cold of the air lasts for many miles. The only things moving up there were the clouds, the wind, and the Aristeia; birds fly only so high. Whatever denizens of the upper air there may be, I spied none of them. (As I was only exploring, I did not check every cloud for the palace of a dragon or air-fairy, though since I lack Second Sight and Faerie Sight, the entire sky may have been filled with a family reunion of alven or an army of air elementals. If it was, doubtless they were too busy marvelling at a wooden vessel sailing by miles above the earth to trouble us.)
Perhaps ten miles up, the air begins to warm again as the Sphere of Air begins to be influenced by the Sphere of Fire. Within a few miles, the grogs were praising the power of my warding magicks in keeping them from both heat and cold. It seems that the wispy, high clouds have their distinctive appearance due to the warmth of the upper air making them more like steam than fog. The figure of 103 miles reported in Pliny's Natural History for the maximum height of clouds seemed, upon this day, greatly exaggerated.
At this point, I began to muse upon the nature of the interaction of the elemental spheres. If it were not that the Sphere of Earth is irregular in its interaction with the Sphere of Water, there would be no dry land and we would all be writing our musings about the Sphere of Air on fish-skin parchment. The Sphere of Water is smoother than the Sphere of Earth, but far more turbulent. If the trend continues, then one may expect great waves of air entering into the Sphere of Fire, and turbulent fire from above entering into areas below. While the wards on the Aristeia are strong, the power of the Sphere of Fire has yet to be measured, and I chose the path of caution: we returned to the ground.
In the future, I hope to journey into the Sphere of Fire itself, and determine whether it will be possible to touch the Lunar Sphere. This, however, will require a great deal more effort in the way of wards.