Earth is the water planet, but perhaps it is equally appropriate to call Earth the water cloud planet. Although liquid water is a distinctive feature of planet Earth, water clouds are even more so.
Liquid water on the surface of a planet is rare, Earth being the only example that we know of. Just because there is liquid water on a planet does not mean that there would be water clouds.
WHAT’S IN A NAME
The two main cloud types are cumulus and stratus.
» Cumulus clouds, the puffy, cottony ones, are named for the Latin word for "heap."
» Stratus clouds, the flat, broad ones, are named for the Latin word for "strewn."
|
Earth’s atmospheric pressure is just right to allow water to exist in all three phases — gas, liquid and solid — at the temperature that our sun’s energy produces. The temperature and pressure are near the triple point of water, which is the temperature at which all three phases exist simultaneously. Away from the triple point are temperature and pressures where water can exist in any two of the three phases.
It is this characteristic of the Earth environment that allows for the hydrologic cycle whereby water evaporates, water vapor rises as it is heated by the sun, and it condenses into clouds that eventually drop the water as precipitation, which flows back into the sea either on or below ground level.
As seen from ground level, the bottommost clouds are flat. On an overcast day, we see the clouds from the bottom forming a flat layer stretching from horizon to horizon, but from above they form a dramatic landscape. The flatness is due to the condensation level, which is nearly the same over a broad area. This represents the altitude at which the air temperature has cooled to the dew point, or 100 percent humidity. The flat bottoms belie the beauty of the cloud tops. As seen from 7 miles up, clouds form a unique landscape of three-dimensional shapes that is never the same from minute to minute.
Clouds are sometime scattered as gentle thermals bubble up from the surface. At other times, they form a nearly continuous layer stretching out to the horizon as their billowy tops undulate like hilly meadows and forests, or flat layers stretch out like the Great Plains.
There are nearly always at least two distinct layers of clouds, sometimes three, a fact that is more obvious looking down than looking up from ground level. This also is lost on photos taken from high above the atmosphere.
The two main cloud types, cumulus and stratus, have many different variations. Cumulus clouds are the puffy, cottony or cauliflower-shaped ones that often occupy the lowest layer, sometimes very close to the ground and closer when the humidity is high. The term comes from the Latin word for "heap" or "to pile up," from which we also get accumulate and cumulative.
Stratus clouds are flat and broad. This term is also from Latin, meaning "strewn." We get our English word "stratify" from this root. Stratus clouds form in the lower layer of the atmosphere but are most common in the middle layer.
The next time you have the opportunity to fly, open the window shade for a free display of nature’s remarkable art.
Richard Brill is a professor of science at Honolulu Community College. His column runs of the first and third Friday of the month. Email questions and comments to brill@hawaii.edu.
