If you’ve spent enough time looking at images of the worlds in our solar system that have atmospheres, you have probably noticed that every atmosphere has clouds or dust or haze (or all three!) as either local or global features. This semester I am teaching a course on Planetary Atmospheres and as I sat down to prepare for a lecture on clouds, haze, and dust I found myself running into a problem that is quite familiar to me as a person who studies haze. The problem is one of nomenclature (something I seem to write about often here) [she's not the only one -Ed.] but since communication is paramount in science, it is important to ensure that we are clear about word choice, usage, and meaning and this is one area of the study of planetary atmospheres where meaning is often hazy (pun very much intended).
Clouds, haze, and dust have something in common: they are all particles (liquid or solid) and they absorb and scatter light differently than gases, which is one of the reasons why they are so important to understand for planetary atmospheres. The word aerosol means particles suspended in a gas and includes clouds, haze, and dust. Aerosol should be used, although it is rarely happens, as the most general term. However, types of aerosol are also important and it is useful to have words for the different types including clouds, haze, and dust.
What is a cloud? You have probably never thought about the definition of a cloud because clouds are a normal part of our everyday existence. The American Meteorological Society has a handy glossary and they define a cloud as “a visible aggregation of minute water droplets and/or ice particles in the atmosphere above the earth’s surface”. According to that definition, clouds can only be made of water and they only exist on Earth, which isn’t particularly useful for our purposes. I checked the self-proclaimed “definitive record of the English language”, the Oxford English Dictionary, and found “a visible mass of condensed watery vapour floating in the air at some considerable height above the general surface of the ground”. According to the OED, clouds are only made of liquid droplets and I’m still trying to figure out what altitudes are included in “some considerable height above the general surface of the ground.” By the way, the OED notes that obsolete definitions for cloud include “a consolidated mass of earth or clay” which (a) I had no idea and (b) might turn out to be useful for exoplanets! I like Wikipedia’s definition the best: “a cloud is an aerosol comprising a visible mass of liquid droplets or frozen crystals made of water or various chemicals. The droplets or particles are suspended in the atmosphere above the surface of a planetary body.” It seems to be a combination of the above definitions, but it acknowledges that clouds can be made of substances other than water. All three definitions agree that clouds are visible and they are above the surface. The one part of the OED definition I would like to keep is “condensed” as I think this is one of the most important ideas that will help us separate clouds from haze and dust. I would define cloud as “a visible mass of liquid and/or solid particles suspended in an atmosphere that form from condensation of atmospheric gases.” For added confusion, cloud droplets prefer to form on preexisting particles (cloud condensation nuclei – CCN) and both haze and dust particles may serve as CCN.
What about dust? The AMS defines dust as “Solid materials suspended in the atmosphere in the form of small irregular particles, many of which are microscopic in size.” OED says “Earth or other solid matter in a minute and fine state of subdivision, so that the particles are small and light enough to be easily raised and carried in a cloud by the wind; any substance comminuted or pulverized; powder.” Wikipedia says “Dust consists of particles in the atmosphere that come from various sources such as soil, dust lifted by weather (an aeolian process), volcanic eruptions, and pollution.” All three agree dust is “particles” and the AMS and OED agree that dust is solid. My preferred definition of dust is “solid particles suspended in the atmosphere that did not originate in the atmosphere”.
So far, although they have lacked scientific precision and are often Earth-centric, the definitions haven’t been so bad. Everything falls apart when you get to haze. The AMS glossary defines haze as “Particles suspended in air, reducing visibility by scattering light; often a mixture of aerosols and photochemical smog”; a definition that somehow ignores the fact that photochemical smog is a type of aerosol and that particles suspended in air are aerosol by definition. Wikipedia uses this definition “Haze is traditionally an atmospheric phenomenon where dust, smoke and other dry particles obscure the clarity of the sky. The World Meteorological Organization manual of codes includes a classification of horizontal obscuration into categories of fog, ice fog, steam fog, mist, haze, smoke, volcanic ash, dust, sand, and snow. Sources for haze particles include farming (plowing in dry weather), traffic, industry, and wildfires.” And our definitive source on the English language defines haze as “An obscuration of the atmosphere near the surface of the earth, caused by an infinite number of minute particles of vapour, etc. in the air” which has so many things wrong with it I am not even sure where to start (“infinite number”?!?). I suspect that part of the problem here is that in common usage, haze is used to mean aerosol. If you look outside one day and there are obviously particles present but you don’t know what they are, you could just say “it is awfully hazy today”. If you live in Tucson, Arizona it is probably dust and if you live in LA it is probably smog and you might use those words instead if you are fairly confident you know what you are looking at. No one walks outside and says: “My, it is quite aerosol-y today.”
For the definition of haze, I’m just going to start from scratch. When we talk about haze for Titan or Pluto or Venus, we are referring to particles that are produced from chemistry in the atmosphere that is usually initiated by solar photons and/or an external source of energetic particles and results in the formation of solid products. In general, the products are solid at any combination of temperature and pressure that exist in that world’s atmosphere. (We may have to be flexible for this part of the definition for giant planets, where the range of pressures and temperatures in the atmosphere is very large.) Unlike a cloud particle, haze particles will not go through cycles of evaporation and condensation depending on the current atmospheric conditions. Cloud material can move back and forth between gas and solid/liquid phase. Haze material is on a one-way trip. In general, haze particles are smaller than clouds or dust because of their formation mechanism. So, unlike clouds, they aren’t always visible. I’ll define haze as “particles produced from chemistry in the atmosphere that results in the formation of involatile solids.”
Common usage is beyond the scope of what I am concerned with right now. My interest here lies in the fact that in planetary science we tend to use three words—clouds, haze, and dust—to describe particles present in planetary atmospheres and the usage is not consistent both within and between communities. Why does this happen? Shouldn’t scientists be precise with language? My sloppily collected anecdata suggest that the problem results, as problems often do in planetary science, from the fact that we slowly turn points of light into worlds. It is often the case that during this process we first can tell only that there are particles, but we know little about their size, shape, composition, origin, fate, etc. and without that information, it is difficult to precisely categorize them. This is most obvious right now in the field of exoplanets, which seem to frequently have particles in their atmospheres (as do all of the atmospheres in the solar system), but we do not have a lot of information about what kind of particles. Are they clouds? Are they haze? We don’t really know, and people have a tendency to use those words interchangeably even though they mean different things and imply different formation mechanisms, sizes, and compositions. As we turn points of light into worlds, scientists are not just communicating with each other, they are also communicating with the world. Describing a newly characterized exoplanet or the spectacular images from the Pluto flyby as "showing the presence of aerosols" is not nearly as compelling as "cloudy exoplanets" or "hazy Pluto". I understand that scientific precision sometimes loses to the human desire to communicate the wonders of the universe. But that doesn’t change the fact that when scientists are talking to each other, they should at least understand the words being used. I have selfishly written this post in the hope that professional planetary scientists read it and adopt the definitions I have suggested. Barring that, my hope is that people will at least be better about defining words as they are using them in the papers that they are writing so that their meaning is clear, even if we disagree about usage.
Why does this matter? It matters because every world in the solar system with an atmosphere has more than one kind of aerosol. Mars has dust and clouds (multiple kinds), Titan has haze and clouds and dust, etc. Let’s take Titan as an example. Titan’s characteristic orange color is the result of photochemically produced involatile (solid in all Titan conditions) aerosols in its atmosphere. This is haze. Titan also has clouds. We know that Titan has methane (CH4) clouds. It seems also to have ethane (C2H6) clouds. It currently has a giant HCN ice cloud at its south pole. And there is evidence for a number of other kinds of clouds (HC3N, C6H6, C4N2, etc). Here it is important to point out that, with the exception of methane, the material in all of those clouds was produced by photochemistry, but unlike the material in the haze particles, the cloud material can and does exist in the gas phase in Titan’s atmosphere (it is volatile), which is why those are clouds, not haze.
Eventually, some fraction of the aerosols in Titan’s atmosphere (both clouds and haze) end up on the surface through sedimentation (settling because of gravity) or rain/snow. Titan has extensive dune fields, and we know that the dune “sand” particles are made out of organic material that is almost certainly produced in the atmosphere. This means that winds on Titan transport particles across the surface and that particles are being lofted from the surface into the atmosphere—which fits our earlier definition of dust. I would argue in this case, even if the particles were originally produced in the atmosphere, once they reach the surface if they are lofted into the atmosphere they are no longer cloud or haze particles, they are dust. Hopefully this example shows why language is important and why the definitions are not straightforward. Some combination of composition, formation mechanism, size, origin, fate, etc. is required to define the particle.
Note that there are other types of aerosol that I have completely ignored. I ignored them mostly because we already have perfectly good words for them—ash, smoke, sea spray—all have relatively clear definitions. We may have sea spray, which forms from bubbles bursting at the atmosphere-ocean interface, on Titan- it will obviously be different than sea spray on Earth (and different from clouds, haze, and sand on Titan described above) but we have a perfectly good word for it. While writing this post I ran into some words I have never heard of like “suspensoid”, which as far as I can tell is the same thing as aerosol but the particle can only be solid (except that the context that it was used included fog and mist, only further demonstrating the nomenclature problems in this field). I also encountered the word smaze, which is apparently a combination of smoke and haze, even though almost every definition of haze I found included smoke. If you have a lot of free time, the 681-page Guide to Meteorological Instruments and Methods of Observation by the World Meteorological Organization is an oddly compelling read.
My favorite part of this whole nomenclature mess is that it demonstrates the need to study other worlds. Definitions that we think are abundantly clear for Earth fall apart for Titan, and if we somehow get the nomenclature sorted out for the solar system worlds, exoplanets come along with the possibility of clouds made of material that exists as rock everywhere it's found in our solar system. In trying to define dust, my instinct was to require it to originate from the surface, but what if there exists a planet where micrometeorite infall is a significant source of material and so micrometeorites are a source of opacity in the atmosphere. Maybe that was even true earlier in the history of our own solar system. As our knowledge of the kinds of planets that exist rapidly expands, we are forced to confront the underlying physical and chemical processes that define how planets form and evolve. This is the beauty of planetary science. This is why we turn points of lights into worlds. Because the more worlds we know, the better we understand how worlds work, including our own.