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The Color of Clouds
The Color of Clouds
The color of a cloud depends primarily upon the color of the light it receives. The Earth’s natural source of light is the sun which provides ‘white’ light. White light combines all of the colors in the ‘visible spectrum’, which is the range of colors we can see.
Each color in the visible spectrum represents electromagnetic waves of differing lengths. The colors change as the wavelength increases from violet to indigo to blue, green, yellow, orange, red and deep red.
As a light wave’s length increases, its energy decreases. This means the light waves that make up violets, indigo and blue have higher energy levels than the yellow, orange and red.
One way to see the colors of sunlight is by the use of a prism. The velocity of light decreases slightly as it moves into the prism, causing it to bend slightly. This is called refraction. The degree of refraction varies with the energy level each wave.
The lowest energy light waves refract the least, while the highest energy waves exhibit the greatest refraction.The end result is a dispersion of light into a rainbow of colors.
Rainbows are partly the result of sunlight refraction through a rain drop, which acts like a prism.
So, if sunlight is ‘white’, why is the sky blue?
The atoms and molecules comprising gasses in the atmosphere are much smaller than the wavelengths of light emitted by the sun.
As light waves enters the atmosphere, they begin to scatter in all directions by collisions with atoms and molecules. This is called Rayleigh scattering, named after Lord Rayleigh.
The color of the sky is a result of scattering of ALL wavelengths. Yet, this scattering is not in equal portion but heavily weighted toward the shorter wavelengths.
As sunlight enters the atmosphere much of the violet light waves scatter first but very high in the atmosphere and therefore not readily seen. Indigo color light waves scatter next and can be seen from high altitudes such as jet airplanes flying at normal cruising altitudes.
Next, blue light waves scatter at a rate about four times stronger than red light waves. The volume of scattering by the shorter blue light waves (with additional scattering by violet and indigo) dominate scattering by the remaining color wavelengths. Therefore, we perceive the blue color of the sky.
If the sky is blue, why are clouds white?
Unlike Rayleigh scattering, where the light waves are much smaller than the gases in the atmosphere, the individual water droplets that make up a cloud are of similar size to the wavelength of sunlight. When the droplets and light waves are of similar size, then a different scattering, called ‘Mie’ scattering, occurs.
Mie scattering does not differentiate individual wave length colors and therefore scatters ALL wave length colors the same. The result is equally scattered ‘white’ light from the sun and therefore we see white clouds.
Yet, clouds do not always appear white because haze and dust in the atmosphere can cause them to appear yellow, orange or red. And as clouds thicken, sunlight passing through the cloud will diminish or be blocked, giving the cloud a grey color. If there is no direct sunlight striking the cloud, it may reflect the color of the sky and appear bluish.
Rayleigh and Mie
Some of the most picturesque clouds occur close to sunrise and sunset when they can appear in brilliant yellows, oranges and reds. The colors result from a combination of Rayleigh and Mie scattering.
As light passes through the atmosphere, most of the shorter blue wavelengths are scattered leaving the majority of longer waves to continue. Therefore the predominate color of sunlight changes to these longer wavelengths.
Also, as light enters the atmosphere, it refracts with the greatest bend in its path near the earth’s surface where the atmosphere is most dense. This causes the light’s path through the atmosphere to lengthen, further allowing for more Rayleigh scattering.
As light continues to move though the atmosphere, yellow wavelengths are scattered leaving orange wavelengths. Further scattering of orange wavelengths leaves red as the predominate color of sunlight.
Therefore, near sunrise and sunset, a cloud’s color is what sunlight color it receives after Rayleigh scattering. We see that sunlight’s color due to Mei scattering which scatters all remaining wavelength colors equally.
The Color of Perception
Sometimes, under direct sunlight, clouds will appear gray or dark gray against a blue sky or larger backdrop of white clouds. There are usually two reasons for this effect.
- The clouds may be semi-transparent which allows the background blue sky to be seen through the cloud. Thereby giving it a darker appearance.
- A more common reason is the contrast between the background (blue sky or additional clouds) and foreground cloud overwhelms our vision. In essence, our eyes are tricked with our perception of foreground clouds appearing dark relative to the overwhelming brightness of the background.
This latter reason is why sunspots look dark. Brightness of the sun is based upon temperature and a sunspot’s temperature is lower than the surrounding surface of the sun.
Relative to the surface of the sun, sunspots appear quite dark. However, if sunspots were isolated from the surrounding brightness, they would still be too bright to look at with the unprotected eye. The contrast in brightness between the two is what causes sunspots appear dark.
Ten Basic Clouds
Ten Basic Clouds
Based on his observations, Luke Howard suggested there were modifications (or combinations) of the core four clouds between categories. He noticed that clouds often have features of two or more categories; cirrus + stratus, cumulus + stratus, etc. His research served as the starting point for the ten basic types of clouds we observe.
From the World Meteorological Organization’s (WMO) International Cloud Atlas, the official worldwide standard for clouds, the following are definitions of the ten basic cloud types. Divided by their height the ten types of clouds are…
High Level Clouds
Cirrus (Ci), Cirrocumulus (Cc), and Cirrostratus (Cs) are high level clouds. They are typically thin and white in appearance, but can appear in a magnificent array of colors when the sun is low on the horizon.
Cirrus (Ci)
Detached clouds in the form of white, delicate filaments, mostly white patches or narrow bands. They may have a fibrous (hair-like) and/or silky sheen appearance.Cirrus clouds are always composed of ice crystals, and their transparent character depends upon the degree of separation of the crystals. As a rule, when these clouds cross the sun’s disk they hardly diminish its brightness. When they are exceptionally thick they may veil its light and obliterate its contour.
Before sunrise and after sunset, cirrus is often colored bright yellow or red. These clouds are lit up long before other clouds and fade out much later; some time after sunset they become gray.
At all hours of the day Cirrus near the horizon is often of a yellowish color; this is due to distance and to the great thickness of air traversed by the rays of light.
Cirrocumulus (Cc)
Thin, white patch, sheet, or layered of clouds without shading. They are composed of very small elements in the form of more or less regularly arranged grains or ripples.Most of these elements have an apparent width of less than one degree (approximately width of the little finger – at arm’s length).
In general, Cirrocumulus represents a degraded state of cirrus and cirrostratus, both of which may change into it and is an uncommon cloud. There will be a connection with cirrus or cirrostratus and will show some characteristics of ice crystal clouds.
Cirrostratus (Cs)
Transparent, whitish veil clouds with a fibrous (hair-like) or smooth appearance. A sheet of cirrostratus which is very extensive, nearly always ends by covering the whole sky.During the day, when the sun is sufficiently high above the horizon, the sheet is never thick enough to prevent shadows of objects on the ground.
A milky veil of fog (or thin Stratus) is distinguished from a veil of Cirrostratus of a similar appearance by the halo phenomena which the sun or the moon nearly always produces in a layer of cirrostratus.
Mid Level Clouds
Altocumulus (Ac), Altostratus (As), and Nimbostratus (Ns) are mid-level clouds They are composed primarily of water droplets, however, they can also be composed of ice crystals when temperatures are low enough.
In Latin, alto means ‘high’ yet Altostratus and Altocumulus clouds are classified as mid-level clouds. ‘Alto’ is used to distinguish these “high-level” clouds and their low-level liquid-based counterpart clouds; Stratus and Cumulus.
Altocumulus (Ac)
White and/or gray patch, sheet or layered clouds, generally composed of laminae (plates), rounded masses or rolls. They may be partly fibrous or diffuse and may or may not be merged.Most of these regularly arranged small elements have an apparent width of one to five degrees (larger than the little finger and smaller than three fingers – at arm’s length).
When the edge or a thin semitransparent patch of altocumulus passes in front of the sun or moon, a corona appears. This colored ring has red on the outside and blue inside and occurs within a few degrees of the sun or moon.
The most common mid cloud, more than one layer of Altocumulus often appears at different levels at the same time. Many times Altocumulus will appear with other cloud types.
Altostratus (As)
Gray or bluish cloud sheets or layers of striated or fibrous clouds that totally or partially covers the sky. They are thin enough to regularly reveal the sun as if seen through ground glass.Altostratus clouds do not produce a halo phenomenon nor are the shadows of objects on the ground visible.
Sometime virga is seen hanging from Altostratus, and at times may even reach the ground causing very light precipitation.
Nimbostratus (Ns)
Resulting from thickening Altostratus, This is a dark gray cloud layer diffused by falling rain or snow. It is thick enough throughout to blot out the sun. Also, low, ragged clouds frequently occur beneath this cloud which sometimes merges with its base.The cloud base lowers as precipitation continues. Because of the lowering base it is often erroneously called a low-level cloud. Both Altostratus and Nimbostratus can extend into the high level of clouds
Low Level Clouds
Cumulus (Cu), Stratocumulus (Sc), Stratus (St), and Cumulonimbus (Cb) are low clouds composed of water droplets. Cumulonimbus, with its strong vertical updraft, extends well into the the high level of clouds.
Cumulus (Cu)
Detached, generally dense clouds and with sharp outlines that develop vertically in the form of rising mounds, domes or towers with bulging upper parts often resembling a cauliflower.The sunlit parts of these clouds are mostly brilliant white while their bases are relatively dark and horizontal.
Over land cumulus develops on days of clear skies, and is due diurnal convection; it appears in the morning, grows, and then more or less dissolves again toward evening.
Cumulonimbus (Cb)
The thunderstorm cloud, this is a heavy and dense cloud in the form of a mountain or huge tower. The upper portion is usually smoothed, fibrous or striated and nearly always flattened in the shape of an anvil or vast plume.Under the base of this cloud which is often very dark, there are often low ragged clouds that may or may not merge with the base. They produce precipitation, which sometimes is in the form of virga.
Cumulonimbus clouds also produce hail and tornadoes.
Stratocumulus (Sc)
Gray or whitish patch, sheet, or layered clouds which almost always have dark tessellations (honeycomb appearance), rounded masses or rolls. Except for virga they are non-fibrous and may or may not be merged.They also have regularly arranged small elements with an apparent width of more than five degrees (three fingers – at arm’s length).
Stratus (St)
A generally gray cloud layer with a uniform base which may, if thick enough, produce drizzle, ice prisms, or snow grains. When the sun is visible through this cloud, its outline is clearly discernible.Often when a layer of Stratus breaks up and dissipates blue sky is seen.
Sometimes appearing as ragged sheets Stratus clouds do not produce a halo phenomenon except, occasionally at very low temperatures.
The Four Core Types of Clouds
The Four Core Types of Clouds
While clouds appear in infinite shapes and sizes they fall into some basic forms. From his Essay of the Modifications of Clouds(1803) Luke Howard divided clouds into three categories; cirrus, cumulus and stratus.
Cirro-formThe Latin word ‘cirro’ means curl of hair. Composed of ice crystals, cirro-form clouds are whitish and hair-like. There are the high, wispy clouds to first appear in advance of a low pressure area such as a mid-latitude storm system or a tropical system such as a hurricane.
Cumulo-formGenerally detached clouds, they look like white fluffy cotton balls. They show vertical motion or thermal uplift of air taking place in the atmosphere. They are usually dense in appearance with sharp outlines. The base of cumulus clouds are generally flat and occurs at the altitude where the moisture in rising air condenses.
Strato-formFrom the Latin word for ‘layer’ these clouds are usually broad and fairly wide spread appearing like a blanket. They result from non-convective rising air and tend to occur along and to the north of warm fronts. The edges of strato-form clouds are diffuse.
Nimbo-formHoward also designated a special rainy cloud category which combined the three forms Cumulo + Cirro+ Stratus. He called this cloud, ‘Nimbus‘, the Latin word for rain. The vast majority of precipitation occurs from nimbo-form clouds and therefore these clouds have the greatest vertical height.
The Height of Clouds
The traditional division between the Polar and Temperate Regions is the Arctic Circle (66.5°N) in the Northern Hemisphere and the Antarctic Circle (66.5°S) in the Southern Hemisphere. The division between the Temperate and Tropical Regions are the Tropics of Cancer (23.5°N) in the Northern Hemisphere and the Tropics of Capricorn (23.5°S) in the Southern Hemisphere.
The actual division between these regions varies from day to day and season to season. Between the Polar and Temperate Regions lies the jet stream in both hemispheres, while the Sub-Tropical Jet Stream divides the Temperate and Tropical Regions.
One effect of these cores of strong wind is the maximum altitude of the tropopause decreases in each region as one moves from the equator to the poles. Generally, as the tropopause’s height decreases, the elevations at which clouds occur also decreases.
The exception is for low clouds which are officially said to have cloud bases within the first 6,500 feet (2,000 meters) of the surface in each region. But even that is not always the case.
The base of cumulus and cumulonimbus clouds can sometimes be higher than 6,500 feet (2,000 meters). During summertime, the base of these convective clouds will be well in to the mid-level cloud range in the non-mountainous areas of the southwest United States.
Cumulus cloud bases have been observed up to 9,000 feet (2,750 meters) over North Central Texas and thunderstorms, with cloud bases from 11,000 to 12,000 feet (3,350 to 3,650 meters), have occurred near San Angelo, Texas.
This happens when, despite the dry lower level of the atmosphere, the atmosphere in the mid-levels is fairly moist and unstable. The dryness of the lower level is such that parcels of air need to rise up to two miles (3 km), and sometimes more, before the they cool to the point of condensation.
| Level | Polar Region | Temperate Region | Tropical Region |
|---|---|---|---|
| High Clouds | 10,000-25,000 feet (3-8 km) | 16,500-45,000 Feet (5-14 km) | 20,000-60,000 feet (6-18 km) |
| Mid Clouds | 6,500-13,000 feet (2-4 km) | 6,500-23,000 feet (2-7 km) | 6,500-25,000 feet (2-8 km) |
| Low Clouds | Surface-6,500 feet (0-2 km) | Surface-6,500 feet (0-2 km) | Surface-6,500 feet (0-2 km) |
Since the jet stream follows the sun, it shifts toward the equator as winter progresses. Therefore, the polar region expands and the temperate region moves toward the equator. In summer, the Tropical Region expands shifting the temperate region toward the poles while the polar region shrinks.
How Clouds Form
How Clouds Form
There are two ingredients needed for clouds to become visible; water, of course, and nuclei.
Nuclei
In one form or another water is always present in the atmosphere. However, water molecules in the atmosphere are too small to bond together for the formation of cloud droplets. They need a “flatter” surface, an object with a radius of at least one micrometer (one millionth of a meter) on which they can form a bond. Those objects are called nuclei.Nuclei are minute solid and liquid particles found in abundance. They consist of such things as smoke particles from fires or volcanoes, ocean spray or tiny specks of wind-blown soil. These nuclei are hygroscopic meaning they attract water molecules.
Called “cloud condensation nuclei”, these water molecule attracting particles are about 1/100th the size of a cloud droplet upon which water condenses.
Therefore, every cloud droplet has a speck of dirt, dust or salt crystal at its core. But, even with a condensation nuclei, the cloud droplet is essentially made up of pure water.
Temperature’s role
But having water attracting nuclei is not enough for a cloud to form as the air temperature needs to be below the saturation point. Called the dew point temperature, the point of saturation is where evaporation equals condensation.Therefore, a cloud results when a block of air (called a parcel) containing water vapor has cooled below the point of saturation. Air can reach the point of saturation in a number of ways. The most common way is through lifting of air from the surface up into the atmosphere.
As a bubble of air, called a parcel, rises it moves into lower pressure since pressure decreases with height. The result is the parcel expands in size as it rises. This requires heat energy to be removed from the parcel. Called an adiabatic process, as air rises and expands it cools.
The rate at which the parcel cools with increasing elevation is called the “lapse rate”. The lapse rate (the rate the temperature lapses or decreases) of unsaturated air (air with relative humidity <100%) is 5.5°F per 1000 feet (9.8°C per kilometer). Called the dry lapse rate, for each 1000 feet increase in elevation, the air temperature will decrease 5.5°F.
Once the parcel reaches saturation temperature (100% relative humidity) water vapor will condense onto the cloud condensation nuclei resulting in the formation of a cloud droplet.
But the atmosphere is in constant motion. As air rises drier air is added (entrained) into the rising parcel so both condensation and evaporation are continually occurring. So cloud droplets are constantly forming and dissipating.
Therefore, clouds form and grow when there is more condensation on nuclei than evaporation from nuclei. Conversely, they dissipate if there is more evaporation than condensation. Thus clouds appear and disappear as well as constantly change shape.
Introduction to Clouds
Introduction to Clouds
We see clouds almost daily. They can grow very tall or appear flat as a pancake. They are typically white in color but also appear in different shades of grey or in brilliant yellow, orange or red. They can weigh tens of millions of tons yet float in the atmosphere.
Clouds can be harbingers of good weather or bad. Their absence can be a good thing after a flooding rain or a bad thing during a drought.
They provide relief from the heat of direct sunlight but also act as a blanket to warm the earth.
Clouds help water the earth by providing precipitation but can hinder driving by reducing visibility.
They come in infinite shapes and sizes yet we often recognize more familiar objects or animals.
Clouds can be carried along by winds of up to 150 mph (240 km/h) or can remain stationary while the wind passes through them.
They can form behind high flying aircraft or can dissipate as a plane flies through them. They are not confined to earth but are found on other planets as well.
What are clouds? They are “the visible aggregate of minute particles of water and/or ice”. They form when water vapor condenses. Become “Cloudwise” by learning about clouds and how they form.
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