Different types of clouds explained

What's happening in the air to give us different types of clouds?

Different Types of Clouds

Some clouds are long, flat and horizontal, and we call these ones stratus. Some are high or medium in elevation, and accordingly they receive the labels cirrus and alto respectively. Some produce precipitation and get the name nimbus. Some are specialized - found only in certain atmospheric conditions or limited areas of the world.  Examples include the lenticular "lens-shaped" clouds shown above.  The most interesting ones are puffy, pillowy or piled high. Cumulus or cumulonimbus if they give rain; there's that "nimbus" again. 

Meteorologists often put two of these words together to get a more fully descriptive cloud name. For example, you can have alto-cumulus for mid-level puffy clouds. See the three examples above. Using combinations of this sort, we can then identify more than just four cloud types.

What gives the different types of clouds their shapes? The properties of the air surrounding the clouds, the "air parcel" containing the clouds.  There is quite a bit of science behind cloud formation.

Can you deduce all other relevant parameters for an air parcel from pressure, temperature and dew point? Yes. And from these, meteorologists easily decide which curve on a thermodynamic graph to follow if this air moves up or down.

Warning:  technical lingo ahead

Take a look at the drawing in this sample. The path traced by the air parcel goes up a curved line sloping upwards to the right called a moist adiabat if the humidity of that air has reached 100%, enough to produce cloud.  In the sample, it follows the line labelled "saturation adiabat".  If it is not saturated it follows the dry adiabat, the straight line going up to the left. When the air is moving downwards, follow the curved line for moist air only if cloud droplets are present telling us it’s a saturated parcel.

Let's have a look at an ideal situation now.  For the moment, do not worry about air mixing, known as entrainment.

Then, carry out temperature comparisons described in detail on this page. Doing so will tell you if it is stable, unstable or neutral.  How so?

Stability classifications and the different types of clouds that result from them

There's stable and there's "stable".  This means some atmospheric conditions are more stable than others.  Here are a few categories of stability:

Absolutely Stable - The external forces on a displaced packet of air (one that has been moved upwards or downwards) automatically try to return it to its original level. Regardless of moisture content.  We often see clear sky, fog, stratus or high cirrus clouds under these conditions.  More on this further below.

Absolutely Unstable  - The displaced parcel keeps going faster and further. No matter how moist or dry it is. This structural concept is theoretical and remains virtually nonexistent in reality, with short-lived exceptions occurring during conditions of external forcing (such as within storms).  It sure would give different types of clouds if it were to actually occur and persist. We would get strong cumuliform clouds. Big deep piles.

Conditionally Unstable
- We now examine stability that depends on whether the moving air is dry or saturated. We also call this potential instability. This condition gives us most of our clouds. 

Convective Instability of the Second Kind (CISK) – In this situation, latent heat (stored in vapour) within a storm core comes off as sensible heat (measurable with a thermometer) as soon as any condensation occurs, as usual.  This heat provides just enough power for storm development.  It gives rise (pun intended) to convection which would not otherwise occur.  It may have contributed to this storm.

Instability and cumulonimbus development

Neutral - This time the temperature of the air parcel remains the same as the surrounding air temp. Scientists call this a state of equilibrium; and though its stability also depends on water content, we don't see much cloud under these circumstances either.

What else are stability calculations used for?  Two scientists got together in the 1960’s and devised the Pasquill-Gifford system of classifications. They defined stability subdivisions and assigned them capital letters A through F. The system and its concepts are still used in air quality modeling today.

Air quality is greatly affected by atmospheric stability. Why? Because stable air traps pollution and different types of clouds near the ground or even in elevated layers.

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Here's an interesting situation...turbulent air eventually becomes well mixed, go figure! Then the environmental lapse rate in the mixed parcel becomes neutral with respect to dry air. In other words, if unsaturated air were to move up through it would experience no forces from the ambient air pushing it upwards or downwards and we would say it is neither stable nor unstable.

Since moist air is less stable than dry air, it will convect (move upwards) more easily. It is this upward motion that leads to cloud development. Moist air has the water supply needed for the production of different types of clouds, a necessary ingredient since clouds are made of water droplets.

What about absolute instability?

Do you think it will last? Not long. Not unless there is a good steady source of heat from below, special situations like cold air moving over a warm lake, a forest fire or strong sunshine quickly heating up the ground underneath the air parcel in question.

Super-adiabatic lapses will fill in spontaneously giving us auto-convection and even more different types of clouds.  Then we get turbulence and then a neutral lapse rate afterwards .

Stability and cloud formation or suppression go hand-in hand. An experienced meteorologist can tell what the temperature and moisture structure is like in the air above our heads just by examining the clouds.

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Different air motions result in several different cloud types. 

Air moving up and down give the clouds their shapes.

Are you concerned about Air Pollution in your area?

Maybe modelling air pollution will get you the answers you need for this problem.

That's what I do full-time.  Try it.

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