MOOC on meteorology for sailors on French sailing school - What I learned on clouds, 18 November 2018

Going on retirement, I registered to the French National School for Sailing MOOC on meteorology, a three-week course for sailors wishing to improve their meteorological skills. The first week of the MOOC was devoted to meteorological principles and clouds. In this post, I will highlight what I got out of the MOOC regarding clouds.

By the end of the week, I came up with a representation of the ten "species" of clouds on which is built the international classification of clouds (pictures are extracted and adapted from Wikipedia). Basically, three rows related to height of the clouds, and two columns related to whether the clouds are mushrooming (cumuliform) or spreading in bands (stratiform), along with two clouds in-between, the cirrus (neither mushrooming not spreading in bands) and the stratocumulus (both mushrooming and spreading in bands). The two external columns are referring to clouds including the prefix or suffix "nimbus", which means "dark" in Latin, the darkness being related to the thickness of these two cloud species. Stratus clouds, are clouds in bands that do not show patterns, therefore resulting in homogeneous bands of clouds occulting the sun in different proportions: halo for Cirrostratus, sun shade for Altostratus, and homogeneous stratum for Stratus. As for Cumulus, the differentiation between the species refers to the relative size of the individual component of the cloud. Cirrocumulus are within on finger width of a hand stretched, while Altocumulus are within three fingers. On this representation, the two external columns are representing clouds extending over huge altitude range, the Nimbostratus, and the Cumulonimbus, both being the main couds generating rainfall.
A purely graphical representation adapted from wikipedia could look like below.

Adapted from wikipedia at https://en.wikipedia.org/wiki/List_of_cloud_types

Cloud classification is complex. Fortunately, there is an international organisation maintaining the classification, the World Meteorological Organisation (WMO). As a trained epidemiologist, the classification of clouds reminds me much of the classification of microorganisms, with genus and species characteristics. A good starting point is the Cloud Classification Atlas.

The classification comes up with the ten species (genera) shown on the first picture in this post. Each of the genus can be associated with a "variety". See below the table of correspondence between genus, species and variety:

Cloud species and the genera with which they most frequently occur

Cc	Cs	Ac	As	Ns	Sc	St	Cu	Cb
fibratus (fib)	●		●
uncinus (unc)	●
spissatus (spi)	●
castellanus (cas)	●	●		●			●
floccus (flo)	●	●		●			●
stratiformis (str)		●		●			●
nebulosus (neb)			●					●
lenticularis (len)		●		●			●
volutus (vol)				●			●
fractus (fra)								●	●
humilis (hum)									●
mediocris (med)									●
congestus (con)									●
calvus (cal)										●
capillatus (cap)										●

In addition to species, varieties can be defined as shown below.

Cloud varieties and the genera with which they most frequently occur

Genera Varieties	Ci	Cc	Cs	Ac	As	Ns	Sc	St	Cu	Cb
intortus (in)	●
vertebratus (ve)	●
undulatus (un)		●	●	●	●		●	●
radiatus (ra)	●			●	●		●		●
lacunosus (la)		●		●			●
duplicatus (du)	●		●	●	●		●
translucidus (tr)				●	●		●	●
perlucidus (pe)				●			●
opacus (op)				●	●		●	●

Quite complex multi level classification at the end, but worth looking at.

The WMO has developed a couple of algorithms for the identification of clouds.

• WMO cloud identification guide.
• The low level cloud classification aid.
• The middle level cloud classification aid.
• The high level cloud identification aid. 

By the ends of the week, I feel comfortable looking at the clouds out there and classifying them accordingly. Yet, I need to relate this classification to the meaning of the clouds in term of weather forecast.

On the Internet, there are couple of applications that are providing maps of cloud cover, such as Windy below. You can overlay the wind direction and isobars.

Fronts will be addressed in the second week of the MOOC. However, searching the Internet, I came up with some explanations on what to expect when fronts are passing by:

Copied from https://crystalsamuel.weebly.com/weather-and-climate.html 

Let's see what we get out of second week of the MOOC!

Related links:

• Windy: wind and cloud charts.
• The World meteorological organisation (WMO) cloud atlas.
• The Cloud appreciation society.