Lighting from the base of a cumulonimbus cloud at sunset over Pyramid Lake, NV

Clouds come in many shapes and forms, and this week I thought I would provide some insight into what they are, how they form, their different shapes and sizes, and how they can be used to forecast the weather.

What exactly is a cloud anyway? Clouds are conglomerations of water droplets or frozen ice crystals that resist gravity and float in the atmosphere above us. When the air becomes saturated with water, the water is expelled from the air and forms visible clouds. This happens most often when warm air rises into cooler, less dense air, and the temperature at which this occurs is called the dew point.

Water in the air needs something to condense upon in order to form clouds. These particles are called condensation nuclei, and can include salt, ice crystals, or dust. Each particle is less than 0.2 micrometers in diameter, or approximately 1/100th of the size of the actual water droplets that coalesce on them to form the clouds that we see floating in the air. These particles are about 20,000 times smaller than a raindrop. As a comparison, human hairs are around 100 micrometers across, and spider’s cobweb are about 1 micrometer in diameter.

The current nomenclature and classification system of clouds was developed in 1802 by Luke Howard from Latin terminology, and adopted as the official classification system of the International Meteorological Commission in 1929. Howard divided his cloud naming system into three major branches – 1) cirriform, meaning thin, elongated, and wispy, 2) cumuliform, meaning bunched up, puffy, or 3) stratiform, meaning flat clouds that appear as sheets or continuous layers.

Cumulus clouds, from

Within those three categories, the descriptive terms for clouds are divided by height. High clouds occur in the atmosphere between 10,000 and 25,000 feet above sea level in the polar regions, between 16,500 feet and 40,000 feet in the middle latitudes, and between 20,000 and 60,000 feet above the tropics. These include cirrus clouds, which are the thin, bright white elongated wispy clouds, and variations of cirrus that begin to show forms closer to puffy than wispy – cirrocumulus, and cirrostratus. Middle clouds form between 6,500-13,000 feet above the poles, between 13,000 and 23,000 feet above the middle latitudes, and between 23,000 and 25,000 feet above the equator. Common middle clouds are altocumulus, and altostratus.

Low clouds form in the first 6,500 feet of the atmosphere, and take names starting with strato-. Examples include stratocumulus, and stratus. Within all of the above groups of clouds as segregated by elevation, there are other terms used to further describe the specific type of cloud – lenticularis, meaning lens-shaped, stratiformis, meaning flat, castellanus, meaning turreted, and floccus, meaning tufted. Fully descriptive cloud names are given in two words – for example Cirrocumulas lenticularis cloud would be a lens-shaped cirrocumulus cloud, lying in the upper part of the atmosphere at a height of greater than 16,500 feet above Tahoe. Beautiful examples of these clouds were seen above Carson City on Sunday night September 25th:

Lenticular clouds over Carson City

Clouds also provide a great albeit rudimentary way to forecast the weather. This skill used to be hugely important to backcountry travellers and mountaineers before the age of GPS, satellite phones, and people walking around with smartphones that work in the California wilderness. Sometimes you just can’t get a signal, or your batteries die, and some basic knowledge of how to read the weather from the clouds could save your life.

Approaching high cirrus clouds colloquially known as “mare’s tails” often foretell a change in the weather as a front passes, either from stormy to fine, or from fine to stormy. Cirrocumuls clouds that look like ripples or dapples like fish scales are called a “mackerel sky” and can be another indication of an incoming storm or hurricane. Cirrostratus clouds can mean rain in 12-24 hours. Altocumulus clouds, or sheets of puffy white clouds approaching overhead in the morning hours in summertime can indicate that thunderstorms will form that afternoon, especially if the clouds thicken out and up like broccoli and grow vertically into cumulonimbus clouds, aka thunderheads. It doesn’t take a rocket scientist to forecast rain when bands of low stratocumulus clouds appear and block out the sunlight.

Mare's tails, or altocirrus clouds, from

Humans have actually tried with varying degrees of success to develop ways to “seed” clouds, or spray artificial condensation nucleii into the atmosphere to try to get clouds to form in order to make rain. Typical artificial condensation nucleii agents are silver iodide, propane, or solid carbon dioxide (aka dry ice). Some ski areas have actually engaged in cloud seeding programs to try to get it to snow more in the wintertime, as the practice is more effective in winter months.

Cloud seeding process, from

Utah started seeding clouds over the Wasatch in 1973, and still engages in the practice today. Check out the areas of the state that seed clouds to enhance snowfall:

Areas of Utah wehre cloud seeding poerations are still conducted, from

This is yet another reason why Tahoe is better than Utah – in the same way that Lance Armstrong is better than Floyd Landis, Squaw doesn’t need performance enhancing substances to bring on the winter dumps. Squawlpine doesn’t need to do this though because we are blessed with one of the deepest snowpacks in the world. On both sides of KT-22, even though more often than not, the same storms seem to mysteriously snow about 8-10” more at Alpine Meadows. Only time will tell if this phenomenon will continue now that the two mountains are under one umbrella of ownership.

Ever wonder what a few months’ worth of Tahoe storms looks like in fast-motion? Check out this weather timelapse I put together last year:

2 replies on “On the Formation and Classification of Clouds”