101: Clouds I

I am the daughter of Earth and Water,
  And the nursling of the Sky;
I pass through the pores of the oceans and shores;
  I change, but I cannot die.
For after the rain when with never a stain
  The pavilion of  Heaven is bare,
And the winds and sunbeams with their convex gleams
  Build up the blue dome of air,
I silently laugh at my own cenotaph,
  And out of the caverns of rain,
Like a child from the womb, like a ghost from the tomb,
  I arise and unbuild it again.

Percy Bysshe Shelley
The Cloud


The cloud is likely the most iconic element of weather that has captivated humans since ancient times. Clouds are frequently depicted in pieces of art, sung about in song, or captured in a poem, such as the one above. However, for as familiar as they are, the seemingly countless varieties can make general statements about their structure and origins elusive. True, the boring cloud overcast on a dreary day is a far cry from the gentle wisps floating high in the sky on an otherwise clear day. Fortunately, there is one statement that holds true for nearly every cloud: they are formed when moist air is cooled. This might seem overly simplistic, and in some ways it is, but it is a fundamental concept of the formation of clouds. There are many potential causes for the cooling of air, but what is important is that the air that is to become a cloud is cooled enough to make the air saturated with water vapor, leading to the formation of microscopic water droplets (or ice crystals in high clouds).

What is the connection between the temperature of the air and water droplets? The answer lies in a property of air that has a convenient analog in everyday life. Suppose you need to make a glass of salt water using ordinary tap water and table salt. For whatever reason, you decide to mix the salt in to the water while it is piping hot. After stirring the salted water until all traces of the salt crystals has dissolved, you decide that it is indeed too hot and put it in the refrigerator to cool down. Later, when you go to retrieve the saltwater, now cold, from the fridge you might find a small layer of salt crystals lying on the bottom of the glass. What happened? It turns out that much more salt could be dissolved in the hot water than in cold water, so as the saltwater cooled it reached a point where it could hold less dissolved salt than was currently present, so some of the salt had to precipitate (un-dissolve) out of the water. This same concept happens to air, where the air is like the glass of water and the water vapor is like the salt. Thus, as air cools, it is capable of holding less and less water vapor. Once it reaches a temperature where it can no longer hold all the water vapor, called saturation, small amounts of vapor begin condensing into water droplets (or depositing into ice crystals). It should be noted that in reality other factor such as particles called condensation nuclei play an important role, but for the purposes of this discussion the saturated air concept will suffice.

 
 

A cloud produced by the sudden drop in pressure inside the bottle

So, how does this cooling occur? Well, there are actually several ways, but they can be simplified into just two methods: rising air and heat loss. The loss of heat by a parcel of air is really only responsible for the formation of fog on a cold night. If the ground cools overnight by radiating heat into space, the air in contact with, or at least near the ground loose heat as it is conducted to the cooler surface. The loss of heat results in lower air temperature which can lead to the formation of water droplets (i.e. fog). The other mechanism, rising air, is responsible for the vast majority of clouds and is based on the concept that as air rises its pressure drops and thus any arbitrary parcel of air will expand. By expanding, the parcel will cool since all the molecules in it are further apart and so collisions, which is an expression of temperature, decrease. This happens on a small scale every time you open a bottle of carbonated soda. The carbonation in the soda (the bubbles) will have built up some pressure inside the sealed bottle. When this bottle is opened, the air inside suddenly expands to match the air pressure outside of the bottle. Occasionally, a small amount of “steam” will drift out of the bottle immediately after opening it; this is a cloud, a very small cloud. Since there are several reasons a parcel of air might rise, many clouds are divided in one of two categories based on the mechanism that caused the air to lift: stratiform and cumuliform.

 

 

One way of stratiform cloud formation is along a warm front.

Stratiform clouds are typically very wide clouds that have few features. These are the clouds that produce dreary overcast days. Some varieties of stratiform clouds produce precipitation, but it is rarely very heavy. The mechanisms behind the formation of these clouds are usually gentle accents due to large scale lifting, such as an approaching warm front. Other mechanisms include air that is forced to rise due to elevated terrain, called orographic lifting.

 

 

 

 



Very general example of cumuliform clouds

Cumuliform clouds are much more energetic and are often due to individual pockets of rising air, called cells. Much of the large clouds that often grow throughout the day and can develop into thunderstorms are due to convective processes, thus these clouds are sometimes referred to as convective clouds. Cloud growth in this manner begins when a pocket of air near the ground becomes warmer than the air around it. Since warm air is less dense than cooler air, the warm air parcel will rise due to buoyancy. The parcel will cool as it rises, but as long as it remains warmer than the surrounding air, it will continue to rise. At some point the parcel will have reached the temperature at which the air inside it becomes saturated, thus a cloud will begin to form, with this point becoming the cloud base. If conditions are right, the parcel will be able to rise through a significant portion of the troposphere, this is referred to as deep convection, and under ideal conditions, called unstable or conditionally unstable, the parcel will rise all the way to the tropopause. In particularly vigorous convection the parcel will have enough momentum that will rise a short distance into the stratosphere, the cloud features created by these parcels are called overshooting tops. Other sources of cumuliform clouds include cold fronts, where warm air is forced up sharply over the denser advancing cold air.

 

 

 

The ten genus of clouds

Categories such as stratiform and cumuliform are useful for describing the forces behind clouds, but in practice it is much more convenient to describe clouds by their physical properties, especially those that can easily be determined by an observer. The solution is a naming system based on Latin word parts and organized in a similar manner to living organisms in regards to genus and species. In this system there are ten officially recognized types of cloud, called genus, organized into three categories based on the height of the cloud base, called etage, with specific varieties of these clouds referred to as species. Below is a simple table of the ten types of clouds along with their etage and approximate height of their cloud bases in the mid-latitudes. In future posts I’ll go into detail about these clouds, along with a few unofficial cloud types.

 

Genus	Abbreviation	Etage	Mid-latitude cloud base
Cumulus	Cu	Low	Less than 2km
Cumulonimbus	Cb
Stratus	St
Stratocumulus	Sc
Nimbostratus	Ns
Altostratus	As	Middle	2-7km
Altocumulus	Ac
Cirrus	Ci	High	5-13km
Cirrostratus	Cs
Cirrocumulus	Cc

 

 

 



Symbols sometimes used on weather maps to indicate the dominant type of cloud currently being observed