When I asked a friend to define a cloud, she retorted: “Aren’t you a meteorologist? Shouldn’t you know what a cloud is? They’re puffy white blobs in the sky and you’re obsessed with them!” She was right on one count; I am obsessed with them. Her definition of clouds as puffy white blobs in the sky is a bit narrow but contains one common aspect of almost everyone’s definition of clouds: you can see them, at least during the daytime and occasionally even at night.
The button above will take you to the definition of a cloud as envisioned in the American Meteorological Society’s Glossary of Meteorology. In general, however, there is no consensus on the definition of a cloud because it depends upon the application and sensor that is being used to observe the feature.
Let’s begin with the basics and let you customize the definition for your own application.
Technical requirements for a cloud that is visible to a human observer:
To be seen as a cloud by the human eye, a collection of suitable particles must redirect or absorb enough visibile light from surrounding sources to significantly alter or completely obscure the original incident direction and intensity of the visible light from these sources. When the absorption of visible light in the collection is negligible, which is the case for relatively pure liquid water droplets, a significant portion of the visible light from the surrounding sources must have been redirected by multiple encounters with particles in the collection before exiting. In atmospheric science we refer to these interactions in which more than one particle intercepts and redirects the incident radiation stream as multiple scattering. But how much incident radiation must be redirected before we deem it a “significant” amount? This is a subjective measure and will depend upon the application, but one measure that is useful is termed the “optical depth” or “optical thickness”. Press this button for short technical explanation of the optical thickness and why I choose a optical thickness of one as the threshold for defining a cloud. Optical Depth
- Cloud Base is defined optically as the bottom boundary of a region in which a large percentage of the light from a visible or near infrared source is backscattered toward the source. The source can be reflected light from the Earth’s surface or transmitted light from a reasonably columnated source, such as a spotlight, or a highly direction source such as a laser radar (LIDAR). In non-precipitating clouds, the optical cloud base may be determined using a cloud radar. Moving downward through an existing cloud, the optical cloud base height is defined during the daytime as the level at which the percentage of non-white light is significantly increased or the level at which the forward visibility increases to at least 50 m, while during the nighttime the optical cloud base is defined as the point at which a distant light source becomes visible or when backscattered light from a nadir beam becomes minimal or absent. Identification of the optical cloud base at sunrise and sunset when moving downward through a cloud may be difficult because insufficient radiation may be supplied to the top of the cloud or reflected off the surface toward the cloud to provide a visual contrast while inside the cloud. Cloud base is defined thermodynamically as the lowest level in a region or regions that indicate saturation in measurements made by an in situ sensor such as an aircraft or a weather balloon (radiosonde). From a cloud microphysical perspective, the optical cloud base may be defined as the lowest point in a cloud at which cloud particles ranging in size from 5-15 microns are measured in significant enough concentration to enable a large percentage of visible or near infrared light to be backscattered toward its source.
- Cloud Top: top boundaries of regions of visible obscuration when (a) viewed in the visible wavelengths from aircraft or elevated locations, (b) the radar echoes from the upper boundaries of hydrometeor layers observed by cloud radars in non- and weakly precipitating clouds, (c) the highest reported echo from a precipitation radar in moderate and heavy precipitation, (d) the highest level in an area of saturated atmosphere as measured by a weather balloon (radiosonde), or (e) the first cloud detection from a downward looking LIDAR on an aircraft or satellite.
- Cloud Thickness: the geometric height difference of the continuous obscuration of visible radiation along a zenith or nadir path length for an individual cloud layer or element between the cloud base and cloud top height.
- Cloud Coverage: percentage of the hemisphere above a location that exhibits cloud
- Cloud Fraction: percentage of a specified layer that contains cloud
- Cloud Volume Fraction: percentage of a specified volume that contains cloud
- Cloud Optical Thickness: Path integrated extinction of radiation due to single scattering for extremely thin liquid water, ice, or mixed phase clouds or due to a combination of single and multiple scattering for most liquid water clouds and mixed phase clouds and many ice clouds.
- Cloud Liquid or Ice Water Content: mass of liquid or ice water in cloud droplets per unit volume of cloud ()
- Cloud Liquid or Ice Water Path: vertically integrated liquid or ice water content ()—represents the amount of water or ice per unit area that would be deposited on a horizontal surface (can be reported as a depth i.e. cm or mm)
- Particle Number Density: number of droplets or ice crystals per unit volume of air. Sometimes referred to as particle number concentration
- Liquid clouds: 50-1000 cm-3
- Particle Size: a representative dimension describing the particle
- Liquid clouds: radius 5-20 μm
- Mass to Diameter Ratio: Ratio of the mass of a particle to its diameter. Used to describe ice particles
- Mixed Phase: a volume of cloud that contains both liquid and ice particles
- Fog: a cloud with its base at the surface
- Entrainment Rate: Rate at which cloudy air is mixing with and being diluted by clear air.
- Drizzle: droplets that have sufficient terminal velocity to have net earthward motion
- 20-200 μm
- Precipitation Rate: rate of mass deposition through a specified horizontal layer, which can be the surface—mm/hr. Often referred to as the liquid water flux.
- Cloud Droplet Size Spectrum: Number of cloud particles per unit radius or diameter that are too small to have a terminal velocity that is greater than average updrafts.
It will not come as a surprise to you that clouds are made of water in the form of either liquid or ice or both. But it may come as a surprise that there is no scientific consensus as to the exact origin of water on Earth and thus no consensus as to the exact origin of clouds. Hence, we are faced with a basic question: was water always a part of the structure of our planet as it formed from the colossus of collisions between asteroids, which is termed “accretion”, or was water delivered to our planet from somewhere else? Theorists contemplate two possibilities, “dry accretion” and “wet accretion”, with the latter meaning that the materials that formed the planet contained water to begin with and the former meaning that they did not. Let us get one thing straight—early Earth was hot. So hot, in fact, that water could not have remained in the amalgamation of nebular material formed early Earth. So the young Earth had to cool a bit before water could have condensed on its surface. For decades it was believed that once the Earth had cooled enough, water was delivered to its surface by asteroids and comets. Comets originate from two distinct “belts” and asteroids originate from a collection of much less organized material that orbits the sun. Both are known to contain ice which, in the case of comets, gives rise to their long, spectacular tails as the ice evaporates. But recent data collected from comets proved that no more than 50% of Earth’s water was transported here by them and, most probably, much less than this amount . Similar studies suggest that the same is true for asteroids, so the hypothesis that the Earth’s water was delivered to the Earth entirely by comets and asteroids once it had cooled is unlikely. Suffice it to say that there are many unsolved questions, but one of the latest theories is that the water was carried by chondrites, which are small non-metallic meteorites that contain water. According to this theory, chondrites arrived en masse late in the Earth’s formative stages and placed a “late veneer” of water on the surface (Albarède, 2009). But equally plausible theories challenge this hypothesis suggesting that Earth had a “complex history of volatile accretion and loss ” (Wood et al., 2010), and there you have it, science has been yet unable to ascertain the origin of Earth’s water, and, as such, its clouds.
Let us assume that water made its way to Earth, offering the fact that it is definitely here as evidence. Before we form clouds, we need to create an atmosphere, which was created by the great volcanoes that covered the surface of early Earth. These volcanoes spewed enormous amounts of water vapor, carbon dioxide, nitrogen, and traces of hydrogen and carbon monoxide. A thin gas layer containing these gases was most probably the first manifestation of the atmosphere. You may have noticed that there is no oxygen in this list of gases and, indeed, Earth’s initial atmosphere is thought to have contained no oxygen—it was anoxic, a term normally reserved for oxygen levels in liquid water though equally applicable to the Earth’s earliest atmosphere. This violent and inhospitable world when Earth’s atmosphere first appeared marks the beginning of the period between 2.5 and 4 million years ago that is known to Geologists as the Archean Eon (pronounced Arc-e-an). Geologic evidence strongly suggests that life evolved during this period and began producing the oxygen that we breathe today.
Conspicuously absent from the list of volcanic gases emitted during the beginning of the Archean is ozone, which plays a crucial role in the present atmosphere. Ozone is a molecule composed of three oxygen atoms, unlike than the two oxygen atoms found in a molecule of oxygen molecule and is as essential to life as oxygen itself because it performs the vital service of absorbing harmful, cell-destroying ultraviolet (UV) light from the sun. This UV absorption occurs mostly in the modern-day stratosphere, which begins at an average depth of about 15 km above the surface, higher in the tropics and lower at the poles. Ozone in the stratosphere is formed from oxygen when energetic photons from the sun divide some of the individual oxygen molecules into their two constituent atoms. These two newly liberated “free agent” oxygen atoms bond with two separate intact oxygen molecules that are nearby to produce two ozone molecules. The successive reactions that result in ozone were first described by Sydney Chapman in 1930. This life-giving stratospheric ozone should not be confused with its harmful counterpart, which forms near the surface because of photochemical reactions involving anthropogenic gases, which are produced by the combustion of fossil fuels.
You may wonder why this excursion into the history of oxygen and ozone when the subject at hand is clouds? It is because the stratosphere, a byproduct of the presence of ozone, effectively defines the maximum level of vertical development for clouds in the contemporary atmosphere. Our stratosphere is the layer immediately above the top of the present-day troposphere, or “weather layer”, where most Earth’s clouds reside. By absorbing incoming sunlight, ozone heats the air in the stratosphere and raises its temperature relative to the air in the weather layer below, which is home to our clouds. This heating in the stratosphere creates a thermal boundary that effectively blocks clouds from developing higher into the atmosphere.
This discussion, of course, raises the question of the origin of oxygen itself and the answer is that, like water, there is no definitive explanation. We know that around 2.7 billion years ago there was an event known as the Great Oxygenation. In a short period, by geological standards, oxygen flooded the atmosphere like a great Phoenix. Geologists have found evidence in support of this event all around the planet and the source of this oxygen was probably life itself. Photosynthesis.
We are faced with our first conundrum and our first connection between clouds and life on Earth. Oxygen and ozone were likely the products of the beginning of life on our planet and, as we have noted, ozone inhibits the vertical development of clouds through its warming of the stratosphere, so it is reasonable to assume that Earth’s earliest clouds were altered by the initiation of life, their vertical growth being slowly stunted as ozone collected in and heated the stratosphere. Through the Archean, gradual changes in the Earth’s clouds are virtually guaranteed by this simple fact. Alas, as in the case of water, we are unsure of the exact origin of life and its oxygen and ozone byproducts and, consequently, unsure of the origin of the stratosphere and its capping effect on cloud vertical development. One thing is for sure: the appearance of life on the planet likely facilitated considerable changes to its early clouds.
So let’s take a step back and imagine Earth’s earliest clouds before the first life emerged and oxygen was produced. Geological formations suggest that Earth’s average surface temperature during the Archean was similar to today’s even though the energy output of the sun was approximately 30% less back then. Less solar input and same surface temperature, but how? This paradox is evidently resolved by assuming that the Earth’s initial atmosphere must have contained enormous concentrations of water vapor and carbon dioxide relative to today’s atmosphere and, perhaps, clouds that trapped more outgoing energy than modern clouds. Water vapor and carbon dioxide trap energy leaving the Earth’s surface and function conceptually like a greenhouse, though a real greenhouse warms the air mostly by prohibiting the air inside the greenhouse from mixing with cooler air outside. Water vapor and carbon dioxide and other less important greenhouse gases act as an insulator by trapping outgoing energy and redirecting some of it back toward the Earth’s surface and this trapped energy warms the planet. These greenhouse gases enable our planet to support life and allow for all three phases of water to exist: vapor, liquid, and ice.
Assuming that higher concentrations of greenhouse gases were sufficient to completely offset the effects of a faint young sun, the reflection of incoming solar radiation by the planet and its atmosphere (termed its albedo) must have been significantly lower than its present value to enable the surface to absorb more energy and preserve liquid water (Kasting, 1993; Catling and Claire, 2005). Differences between the cloud structure during the Archean and modern clouds is the likely cause of any albedo reduction that occurred. Many models of the Archean atmosphere neglect changes in cloudiness altogether while others link changes in cloud structure to increased surface temperatures (Rossow et al.,1982; Kasting and Ackerman, 1986). Changes in cloud morphology and microphysical structure have been proposed as mechanisms leading to cloud-induced changes in albedo. These include increasing the sizes of cloud droplets due to changes in the abundance of cloud condensation nuclei (Rosing et al., 2010), unspecified changes in cloud cover, and a blanket of cirrus over the tropics. Simple models and have provided evidence in support of these hypotheses.
Ensconced in a water-vapor clogged early Archean atmosphere and with no constraint on cloud vertical development because there was no stratosphere to stop clouds from growing taller, early Archean clouds must have been impressive, if not downright scary. Imagine a world with tropical thunderstorms stretching high into the young atmosphere to depths that are hard to imagine, perhaps even double the depths of today’s deepest thunderstorms. And the great preponderance of water vapor would have made conditions much more humid. This vast atmospheric reservoir of water vapor would have been conducive to a hyper version of the current hydrologic cycle. This early Archean atmosphere would have played host to these enormous early Archean thunderstorms and weather systems of all types that would have stretched much higher in the atmosphere, precipitated more, and lasted longer.
It is likely that the vertical distribution of water vapor in the early Archean atmosphere would have differed too. Water vapor in today’s atmosphere is concentrated near the surface, on average, and there is steadily less as we move upward to the base of the stratosphere, above which the concentration is effectively negligible. The extraordinarily deep thunderstorms coupled with frequent volcanic eruptions that thrust large amounts of water vapor high into the early Archean atmosphere suggest that appreciable concentrations of water vapor may have been present much higher in the Archean atmosphere than in today’s atmosphere. Owing to the much colder background temperature that would have existed at these levels in the early Archean atmosphere, we may safely assume that this great volume of cooled water vapor would have given birth to a thick layer of ice crystals, perhaps the forbearers of the fibrous-looking ice clouds that we call cirrus. Such ice clouds, which we will term Archeocirrus, if they were similar in characteristics to today’s cirrus, would have allowed a considerable amount of the sun’s then feeble amount of energy to percolate to the Earth’s surface and been extremely effective at blocking radiation attempting to leave the surface and return to the heavens. The possibility that Archeocirrus could have assisted greenhouse gases in trapping outgoing energy has been entertained in numerical simulations, though the cirrus were located beneath a stratosphere and, as such, are more relevant to the late Archean after life had been initiated and the stratosphere created by it. Allowing Archeocirrus to have existed in greater concentration and much higher in the early Archean atmosphere would trap even more outgoing energy and, potentially, warm the atmosphere and surface beneath even more. Thus, Archean clouds like Archeocirrus and Archeocumulonimbus (a thunderstorm) may have helped maintain the early Archean surface and atmosphere at a temperature like that of today’s climate even though the young planet was receiving 30% less energy from the sun. The thick greenhouse blanket allowed cozy conditions to prevail and may have contributed to the viability of life itself on Earth.
Albarède, F., 2009: Volatile accretion history of the terrestrial planets and dynamic implications, Nature, 461, 1227-1233.
Catling C. and M. Claire, How Earth’s atmosphere evolved to anoxicstate: A status report, Earth Planet. Sci. Lett., 237, 1-20, 2005.
Kasting, J.F. and T.P. Ackerman, 1986; Climatic consequences of very high carbon dioxide levels in the earth’s early atmosphere, Science, 234 (4782), 1383-1385. [DOI:10.1126/science.11539665]
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Rosing, M.T., D.K. Bird, N.H. Sleep, and C.J. Bjerrum, 2010; No climate paradox under the faint early sun, Nature, 744-749, doi:10:1038/nature08955
Rossow, W.B., A. Henderson-Sellers, and S.K. Weinreich, 1982; Cloud Feedback: A Stabilizing Effect for the Early Earth?, Science, 217 (4566), 1245-1247. doi:10.1126/science.217.4566.1245.
Sandu, I. and Stevens, B. (2011). On the factors modulating the stratocumulus to cumulus transitions. Journal of the Atmospheric Sciences, 68, 1865-1881. doi:10.1175/2011JAS3614.1.
Wood, B.J., Halliday, A.N., and Rehkämper, 2010: Volatile accretion history of Earth, Nature, 467, E6–E7 (2010). doi.org/10.1038/nature09484.
Clouds around our planet vary considerably in composition, internal motions, form, and lifecycle. We refer to the size and number of liquid water droplets and ice crystals that compose clouds as the cloud microphysical structure. Updrafts and downdrafts inside clouds, exchanges of mass between clouds and their surroundings, and changes in the internal or external wind structure induced by clouds, are referred to as cloud dynamics. In the broadest context, clouds can appear as individuals or as a continuous layer, which is the fundamental definition of their form. These two categories give rise to the “cumuliform” and “stratiform” designations used in the familiar “cloud type” designations, cumulonimbus (a thunderstorm) or stratus (a layer cloud), for example. In all cumuliform clouds, air inside the cloud is moving “with the cloud material”. That is, if you watch a time-lapse video as a cumuliform cloud develops over a period of ten or twenty minutes, the vertical and horizontal extent of the cloud will change and you will perceive that these changes are associated with the changes in the wind structure inside and outside the cloud. In most stratiform clouds, the air is also moving “with the cloud material”, but the vertical motions are muted and horizonal motions often just seem like the clouds in the layer are being blown along by the wind. There is a special class of stratiform clouds in which the wind is moving “through the cloud”. This type of cloud is associated with atmospheric conditions in which the layer where the cloud forms resists vertical motion and cloud formation, but the layer encounters a mechanical force such as an obstruction that forces the layer to move vertically enough to cause saturation and cloud development. Mountains and the tops of deep cumuliform clouds are examples of physical obstructions that can produce layer clouds in which the wind is blowing “through the cloud”. Because an obstruction is required to produce these layer clouds, they will appear nearly stationary relative to the obstacle and not far downstream from the obstruction they will magically disappear. Strong vertical wind shear across a layer that resists vertical motion and cloud development can induce organized “mechanical mixing” that will also produce this type of cloud.
A complete distinction between cumuliform and stratiform clouds may be difficult to ascertain in many circumstances. A great many of the Earth’s clouds are complex hybrids that may contain connected cumuliform and stratiform components. Thin layers of cloud composed of tightly packed shallow cumuliform elements are known as stratocumulus, the name indicating that this form is a mixture of the two basic forms. Alternatively, shallow cumulus rising into a layer of stratiform cloud above is also a form of stratocumulus; sometimes this form is referred to as cumulus-coupled stratocumulus.
The composition, internal motions, form, and lifecycle of clouds around the planet vary by region. Two main factors dictate the distribution of clouds: latitude and the underlying surface. Latitudes may be subdivided into distinct zones that have generally similar cloud structure: the tropics (0-30 N and S) , the mid-latitudes (30 -60 N and S) , and the high latitudes (>60 N and S). The latitudinal distribution of clouds, on average, ranges from cumuliform clouds in the tropics to stratiform clouds in the high latitudes, so mid-latitude clouds are composed of a mixture of these two cloud genera. Within these latitude zones, cloud structure also varies according to the underlying surface, which is extremely diverse, but characterized generally as being land, ocean, or cryosphere. Borders between these underlying surfaces, coastlines for example, constitute hybrid classifications. Let’s begin with the influence of latitude upon the planetary distribution of clouds.
Tropical clouds are primarily driven by the heat released when water vapor condenses during cloud formation. Cloud development is facilitated by the warm surfaces that exist in the tropical latitudes because of the intense solar heating in these latitudes, and by the availability of moisture, which is supplied by evaporation from the tropical oceans. Cumuliform clouds dominate the tropics and some of the deepest clouds on the planet are found there. Three locations in the tropics are particularly favorable for deep cloud development: the West African Sahel, the Tropical Western Pacific, and Southern India. In two of these three areas, the West African Sahel and Southern India, deep cloud development is aided by monsoon circulations driven by land ocean temperature differences during summertime. Deep clouds above the Tropical Western Pacific Ocean are fueled by a reservoir of extremely warm ocean water in that region, which is known as the “tropical warm pool”. Tropical clouds are generally embedded in a steady, but lazy, east-to-west flow known as the trade winds. The tropics are also home to converging surface winds known as the intertropical convergence zone, or ITCZ, which straddles the boundary between wind circulations in the northern and southern hemispheres. Deep cumuliform clouds in the tropics transport liquid water high in the atmosphere where it freezes and is transported over long distances by the wind. These high level ice clouds, known as tropical cirrus, are a predominant feature of the tropics and have a much greater areal coverage in the tropics than the deep cumuliform turrets that generate them. They are thin enough to allow considerable sunlight to pass through them and reach the surface, but thick enough to trap a considerable amount of the energy leaving the surface before it reaches space and redirect it back to the surface. Hence, these tropical cirrus warm the surface and atmosphere below them.
Mid-latitude clouds are strongly influenced by the upper level westerlies, which give rise to large storms (cyclones) and large regions of fair weather (anticyclones), which circulate air over vast distances. These large swirls capture air masses formed in the tropics and in the high latitudes into the mid-latitudes The structure and life cycle of clouds in the mid-latitudes varies depending upon the source of air mass that is present in a given location and how the air mass has been modified by the underlying surface. Tropical air masses drawn northward into the mid-latitudes give rise to cumuliform clouds and cold air masses plunging southward typically generate stratiform clouds. The centers of cyclones and extended frontal zones that separate air masses in mid-latitudes often contain deep multi-faceted cloud systems that include cumuliform and stratiform clouds. Mid-latitude ocean regions along the eastern margins of the ocean basins often host vast low-level cloud sheets characterized by a perplexing array of cloud configurations. Outbreaks of cold air moving from the continents over the oceans give rise to lines of low clouds called cloud streets and to vast sheets of low-level clouds that transform from stratiform to cumuliform over time. Marine low clouds are the most ubiquitous clouds on the planets surface and have a profound impact on the Earth’s radiation budget, perhaps the greatest of any cloud system.
High-latitude clouds range from multi-level cloud systems forming in the this region and moving south to decaying storm systems moving north from the mid-latitudes. These storm systems produce copious mid-level and moderately deep precipitating cloud layers, but relatively few high level ice clouds. There are also extensive sheets of low-level stratiform clouds, particularly over the ocean regions in the high latitudes. Low-level clouds in the high-latitudes are generally mixed-phase, which means that they contain both water and ice regardless of their proximity to the surface. Temperatures in the polar regions are sometimes cold enough at the surface to cause ice crystals to form without an obviously visible cloud present; this “diamond dust” produces a speckle effect when present. Ice crystals in the form of hexagonal plates and columns often produce a haze-like layer in these polar regions giving rise to some of the best displays of ice crystal optics on the planet.
Land, oceans, and the cryosphere place there own imprint on the clouds above. The influence of the surface below may exhibit a complicated relationship with the clouds above depending on the nature of the surface and the conditions that are present. There are some general differences. Clouds over land tend to have stronger updrafts and smaller cloud droplets than those over the oceans, which have generally weaker updrafts but larger cloud droplets. Clouds over the cryosphere also have weaker updrafts and larger cloud droplets. Prominent topographical features such as mountains (termed orography) strongly modulate cloud structure, but clouds tend to remain relatively stationary with respect to these features and do not always move along with the wind. Forested regions have a complicated relationship with clouds given that trees prefer partly cloudy skies, so-called diffuse solar radiation, and partly control exchanges of water vapor between the surface and atmosphere, which may impact cloud structure or coverage. In contrast, many low-level clouds over the oceans are driven from the cloud-top down. These clouds derive their energy from the loss of thermal radiation at the top of the cloud, which fuels the uplift of water vapor from the ocean surface to cloud base.
There are two primary methods for observing cloud structure. Sensors carried aboard weather balloons, aircraft, and drones make “in-situ” observations of clouds. These observations range from simply defining the depth in the atmosphere that contains clouds to complicated assays of the size and number of particles that comprise a cloud. In addition to in-situ measurements, remote sensors positioned at the Earth’s surface, placed aboard research aircraft, or positioned on satellites observe the details of global cloud coverage and in some instances provide relatively detailed surveys of the vertical structure and composition of clouds.