All About Clouds

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.


The relative size of water molecules to condensation nuclei.

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 5.4°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 5.4°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.


In an ideal atmosphere the saturation level of a parcel with a surface temperature of 85°F and a dewpoint of 65°F will cool to the saturation point at about 4,000 feet in elevation.

The Core Four

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 basic categories; cirrus, cumulus and stratus.

Cirro-form

The Latin word 'cirro' means curl of hair. Composed of ice crystals, cirro-form clouds are whitish and hair-like. There are the high, wipsy 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-form

Generally 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-form

From 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-form

Howard 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.


Combinations of the Core Four

Based on his observations, Howard suggested there were modifications (or combinations) of these core 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 today.

From the World Meteorological Organization's (WMO) International Cloud Atlas these clouds are Altocumulus, Altostratus, Cirrus, Cirrocumulus, Cirrostratus, Cumulonimbus, Cumulus, Nimbostratus, Stratocumulus and Stratus.

Dividing by Height

By convention, clouds are vertically divided into three étages (levels); low, middle, and high. Each étage is defined by the range of levels at which each type of cloud typically appears. Divided by their height the ten types of clouds are...

  • 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.

  • 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 between liquid-based clouds. They are 'high' relative to their low level liquid-based counterpart clouds Stratus and Cumulus. Altostratus can extend into the high level of clouds. Nimbostratus can extend into the high level as well but the base of the cloud typically decreases into the low level as precipitation continues.

  • 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.

The Height of Clouds

This change in tropopause height effects the altitude at which clouds occur. Except for low clouds, which are defined as occurring within the first 6,500 feet altitude in each region, the boundaries for mid and high level clouds overlap and vary.

The polar and subtropical jet streams are major dividers between the polar, 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.

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.

Level Tropical Region Temperate Region Polar Region
High Clouds 20,000-60,000 feet (6-18 km) 16,500-45,000 Feet (5-14 km) 10,000-25,000 feet (3-8 km)
Mid Clouds 6,500-25,000 feet (2-8 km) 6,500-23,000 feet (2-7 km) 6,500-13,000 feet (2-4 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.

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.

Visible light is only a small portion of the full electromagnetic spectrum.

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 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.


A prism will allow you to see the individual colors that comprise the source light. In this case, sunlight entering the prism is divided into the colors of a rainbow based upon the wavelength of each component. The lowest energy light waves refract the least, while the highest energy waves exhibit the greatest refraction.

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?

Even with Rayleigh scattering taking place in the atmosphere, over one-half of the sun's 'white' light continues through the atmosphere reaching the earth's surface.

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.


Looking toward sunrise, the blue sky, yellow Cirrus clouds and orange Altocumulus clouds result from both Rayleigh and Mie scattering. Rayleigh scattered produces blue sky and the color the clouds receives. Mei scattering is responsible for the color we see.

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.

A depiction of three hypothetical waves of light passing through the earth's atmosphere. A) Sunlight barely enters the atmosphere. Therefore only the violet and indigo colors are scattered. B) After violet and indigo colors are scattered first the sunlight penetrates further into the atmosphere where the greatest portion of blue scattering occurs. There is some bending of light by the atmosphere due to refraction adding some length to the light's path. Just as the light path begins to leave the atmosphere the color is predominantly yellow. C) Greatest refraction and longest light path with the most Rayleigh scattering.


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.

  1. The clouds may be semi-transparent which allows the background blue sky to be seen through the cloud. Thereby giving it a darker appearance.

  2. 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.