Looking at Clouds

My sabbatical is long over, alas, but like any good project it has reverberations and influences that last well beyond the submission date.  So I am going to keep this blog going, as a place to air thoughts as I learn more about climate science and its attendant implications for life on earth and for all our lives.

I’m going to start with clouds, because they are beautiful and interesting and vital to us in many ways.  They are also one of the important unknowns in climate science, as we’ll see below.  As I learn about them, traveling dizzyingly across scales from my home base in quark physics (10^-15 m) to cloud physics (10^-7 m and beyond), I am going to use this blog as a repository for information and references.  But I want to begin by acknowledging the place clouds have in human life and culture.  Like most people raised in India, I associate clouds at the end of the relentless summer with the promise of the monsoons.  Seeing the thunderheads piled high and dark in the sky before the first rain is an utterly thrilling experience that implies fields of plenty, food security, a cool respite from deadly heat, the washing away of summer dust, the world made anew.  No wonder clouds are the subject of innumerable songs and dramas!  In the ancient Sanskrit drama Meghadutam, by the poet Kalidasa, the central character is a cloud messenger that traverses the length and breadth of the country.  Thinking of clouds today I ponder the mixed messages they are giving scientists with regard to their role in Earth’s warming climate.

But let us start with some basics.  I learned a lovely new word in the course of my exploration: hydrometeor.  This refers to any water or ice particle formed in the atmosphere due to condensation or sublimation, or blown from the ground into the air.  Hydrometeors include clouds, fog, rain, and snow.

Specifically a cloud is a visible cluster of tiny water and/or ice particles in the atmosphere.  Here is a picture from NASA  that shows the range of droplet sizes:

I had heard vaguely about the different kinds of clouds, but upon further digging I found a veritable zoo of types.  They are organized into 4 genera, 14 species and 9 varieties, as elucidated in this handy Cloud Atlas from the World Meteorological Association:

Cloud Classification

The present international system of Latin-based cloud classification dates back to 1803, when amateur meteorologist Luc Howard wrote a book The Modifications of Clouds. WMO currently recognizes ten cloud genera (basic classifications), which describe where in the sky they form and their approximate appearance:

• High clouds (CH): Cirrus, Cirrocumulus, Cirrostratus;
• Middle clouds (CM): Altocumulus, Altostratus, Nimbostratus
• Low clouds (CL): Stratocumulus, Stratus, Cumulus,
• Cumulonimbus

These genera are subdivided into 14 species (secondary classifications), which describe shape and internal structure, and 9 varieties (tertiary classifications), which describe the transparency and arrangement of clouds. Not all genera have all species, and not all species have all varieties, but in all there are about 100 combinations. In additional to the first three levels of classification, certain supplementary features and accessory clouds are also defined.

The international Task Team has given consideration to possible new cloud classifications, and proposes recognition of a new species Volutus (from the Latin volutus which means rolled) It also proposes some new “special clouds” like Homogenitus (from the Latin homo meaning man and genitus meaning generated or made. Its common names include Contrails (from aircraft)

En route to finding this information I learned that there is a measure of what fraction of sky is covered by clouds, evidently invented by octopi, because it is called an okta.  It measures the sky in fractions of 8, so that 8 oktas corresponds to an overcast sky.

How do clouds form?  According to this site from the University of Alaska, Fairbanks,  cloud physics has undergone most of its development after the 1940s, although its roots go back to the 1700s.  This site is an invaluable resource for those just beginning to think about cloud physics.  Here is an excerpt on the basics of cloud formation:

Clouds form from water vapor molecules in the atmosphere that condense into liquid droplets. These droplets are extremely tiny, with a radius on the order of about 10 micrometers. Water is constantly evaporating and condensing into and out of the air when the relative humidity (the amount of moisture in the atmosphere relative to its capacity to hold water vapor) is less than 100%. However, to get a big enough concentration of water droplets in the air that eventually forms a cloud that is visible to the naked eye requires a relative humidity of near 100%. This generally occurs when the air is relatively cool, because the amount of water vapor that the atmosphere can “hold” is proportional to the air temperature. The warmer the air is, the more water the atmosphere can hold in vapor form, because there is more energy available. When the relative humidity is 100%, the air is said to be “saturated” with as much moisture as it can hold, and therefore water vapor can only condense back into liquid. This greatly increases the number of water droplets floating in the air, and this is what creates clouds. However, just having the humidity at 100% is not enough. Tiny airborne particles called “condensation nuclei” are necessary for water vapor to condense onto. Although the air may look clean on an ordinary day, as many as 150,000 particles can exist in a volume of air approximately the size of your index finger. These particles are extremely small and light, with many having a mass less than one-trillionth of a gram. Without them, relative humidities of several hundred percent would be required for water vapor to condense.

The article goes on to describe how precipitation occurs, and how you get snow from clouds that are mostly liquid water droplets.  Fascinating!

Let’s look for a moment at the reference to ‘tiny airborne particles called “condensation nuclei”,’ without which it would be very difficult for clouds to form.  Solid particles suspended in air are called aerosols – there is a whole lexicon of these as well.  Aerosols range from dust, pollen, sea salt and volcanic ash to black carbon and other pollutants.

Humans have changed cloud formation to such an extent that it is difficult to separate ‘natural’ and human-pollutant-influenced cloud formation, unless one goes to the middle of the ocean or to the Amazon.  This NOVA article about cloud formation in the Amazon  is about the research of Scot Martin, a Harvard environmental chemist who works in the Amazon because the particulate count there is around 300 particles per cc, instead of 10,000 to 100,000 per cc in cities.  Apparently the sweet range for cloud formation is in the 300 to 1000 particles per cc; beyond that, particle count does not greatly influence cloud formation.  In the Amazon, particles are mostly dust and pollen.  But urbanization, as in the city of Manaus, which sits in the middle of the rainforest, is changing that.  Here is a link to the research.

One of the really cool things I learned from the article is about African dust blowing across the Atlantic, fertilizing the Amazon rainforest.  There is a very neat video embedded in the previously mentioned NOVA article, and a research report from Nature. Very mind-boggling to see phenomena at such global scales, that confound our human artificial categories of nations and regions!  How very provincial we are as a species!

Talking of global scales, there’s a very interesting and controversial new hypothesis from two Russian scientists, Victor Gorshkov and Anastassia Makarieva: they proposed in 2007  that enormous rainforests like the Amazon act like a ‘biotic pump.’  According to the hypothesis the condensation of water vapor over such forests creates a low pressure region that drives ocean-to-land circulation.  This effect is, it is claimed, far larger in scale than previously considered.  Their paper was greeted with much skepticism, because this condensation effect has been assumed to be negligible compared to other drivers of Earth’s climate.  Whether their idea is correct or not, we don’t know yet, but a 2013 paper by the two scientists and their collaborators published in Atmospheric Chemistry and Physics elaborates on the earlier work, concluding that “condensation and evaporation merit attention as major, if previously overlooked, factors in driving atmospheric dynamics..”

If this is indeed the case then rainforests are even more vital than we previously imagined.  The fact that the Amazon suffered two hundred-year droughts in a decade is scary.  Perhaps the rapid rate of deforestation is affecting the great atmospheric river that flows above the Amazonian sky.  From the reference:

Atmospheric rivers are a key feature of nature’s atmospheric water supply pipeline. While scientists and forecasters have long recognized that the water vapor that fuels rain, snow, stream flow and storms is transported by a variety of atmospheric processes, it has only recently become apparent how much of this is focused in very narrow regions of the atmosphere—the so-called rivers—that move with the storms.

(If the idea of an atmospheric river isn’t wonderful enough, there’s the discovery in 2011 of an underground ‘river’ running 13,000 feet under the Amazon.  Truly, the Amazon is a remarkable place.  If we can stop killing it who knows what other wonders it may reveal?  According to the World Wildlife Fund , globally some 46-58 thousand square miles of forest are lost each year—equivalent to 36 football fields every minute).

There is so much that we don’t know.  Recent breaking news tells of the discovery that ocean plankton, the chlorophyll containing forms of which are responsible for over 50% of the oxygen we breathe, also aid cloud formation.  According to this NASA article, research reveals that

… in summer when the Sun beats down on the top layer of ocean where plankton live, harmful rays in the form of ultraviolet (UV) radiation bother the little plants. UV light also gives sunburn to humans.

When plankton are bothered, or stressed by UV light, their chemistry takes over.

The plankton try to protect themselves by producing a chemical compound called DMSP, which some scientists believe helps strengthen the plankton’s cell walls. This chemical gets broken down in the water by bacteria, and changes into another substance called DMS.

DMS then filters from the ocean into the air, where it breaks down again to form tiny dust-like particles. These tiny particles are just the right size for water to condense on, which is the beginning of how clouds are formed. So, indirectly, plankton help create more clouds, and more clouds mean that less direct light reaches the ocean surface. This relieves the stress put on plankton by the Sun’s harmful UV rays.

The article ends with:

The next step for the researchers will be to see how much the added clouds from plankton actually impact climate. By figuring out how plankton react to light, scientists now have the information they need to use computer models to recreate the impacts of plankton on cloud cover. Since the white clouds can reflect sunlight back out to space, the researchers believe the plankton-made clouds may have some affect on global temperatures.  

The compound formed by some plankton and bacteria, DMSP or dimethyl sulphoniopropionate, is also responsible for the characteristic smell of the sea.

This research is new, reported in mid-July, just a couple of weeks before this writing.  It is clear that to fully understand clouds, physics and chemistry are not sufficient – we must understand ecosystems and the role that living beings play in their environment.  Science drives us to specialize, to define an area of study and go deep – but the climate crisis compels us to simultaneously look at the big picture, beyond boundaries of discipline, nations, continents.

One of the unanswered key questions in climate science has to do with clouds.  We know that they can have a warming effect AND a cooling effect, so the question is, which wins out as the climate warms? This — and related questions — turn out to be important enough that climatologist Sandrine Bony and her collaborators have issued an appeal in the pages of the august journal Nature Geoscience, as reported in Nature News: Climatologists to Physicists: Your Planet Needs You. 

Last week in Nature Geoscience, Bony’s team outlined four of the field’s deepest questions, including how clouds and climate interact and how the position of tropical rain belts and mid-latitude storm tracks might change in a warming world (S. Bony et al. Nature Geosci. http://doi.org/3gb; 2015). The questions are best tackled, says Bony, by creating more realistic climate simulations — an approach that she hopes will appeal to physicists.

Water vapor is a greenhouse gas; it absorbs and scatters infrared radiation, thus clouds have a warming effect.  But white clouds also reflect sunlight back into space, thus having a cooling effect.  Whether one effect dominates over the other depends on the kind of cloud, and its altitude.  We don’t know how cloud formation and properties such as albedo (reflectivity) might change in a warmer climate – earlier some climatologists had suggested that warmer temperatures would imply more water vapor in the atmosphere and consequently more cloud formation and greater reflectivity.  But things turn out to be more complicated, as this NASA GISS article suggests.

 It had been thought that brighter clouds would partly “save” us from significant global warming, by reflecting more energy into space. Instead, these results suggest that clouds are not necessarily the white knight that will rescue us from climate change.

One of the things that we want to know about clouds is how they might affect CO₂ equilibrium climate sensitivity (ECS) – that is, the equilibrium temperature increase corresponding to a doubling in atmospheric carbon dioxide concentration.  Current estimates of climate sensitivity – in the ultra-cautious words of the IPCC Fifth Assessment Report (AR5) “there is high confidence that ECS is extremely unlikely less than 1°C and medium confidence that the ECS is likely between 1.5°C and 4.5°C and very unlikely greater than 6°C.”

When I was a kid growing up in New Delhi, I used to sometimes listen to a radio program, the only one of its kind at the time that would play popular music from the West, including oldies.  One of these songs was by Joni Mitchell.  I still remember her singing:

I’ve looked at clouds from both sides now/ From up and down and still somehow/It’s clouds’ illusions I recall/I really don’t know clouds at all.

This may well become the anthem of today’s climate scientists, unless physicists — and various other species of scientists — join them in their quest to understand the complicated nature of clouds.