Clouds

What is a cloud?

The visible aggregate of minute particles of water and/or ice that form when water vapor condenses in the atmosphere.

What are the two ingredients needed for a cloud to form?

Water (water vapor) and nuclei (cloud condensation nuclei).

What are cloud condensation nuclei?

Minute solid and liquid particles in the atmosphere (such as smoke, ocean spray, or soil) with a radius of at least one micrometer, on which water vapor condenses to form cloud droplets. They are hygroscopic — meaning they attract water molecules.

How large are cloud condensation nuclei compared to cloud droplets?

Condensation nuclei are about 1/100th the size of a cloud droplet.

What does "hygroscopic" mean in the context of cloud formation?

It means the nuclei attract water molecules, allowing condensation to occur on their surface.

What is at the core of every cloud droplet?

A tiny speck of dirt, dust, or salt crystal (a condensation nucleus) — though the droplet itself is still essentially pure water.

What is the dew point?

The temperature at which air becomes saturated — the point where evaporation equals condensation (100% relative humidity). Air must cool to the dew point for a cloud to form.

What is the adiabatic process?

The cooling or warming of an air parcel due to expansion or compression (change in pressure), rather than from heat exchange with the surrounding atmosphere.

What is the dry adiabatic lapse rate?

The rate at which a rising unsaturated air parcel cools — approximately 5.5°F per 1,000 feet (9.8°C per kilometer) — until it reaches its saturation temperature.

Why do clouds constantly change shape and appear/disappear?

Because the atmosphere is always in motion. Dry air mixes into rising parcels, causing condensation and evaporation to occur simultaneously. Clouds grow when condensation exceeds evaporation and dissipate when evaporation exceeds condensation.

Who first categorized clouds and when?

Luke Howard, in his 1803 "Essay on the Modifications of Clouds," dividing them into cirrus, cumulus, and stratus, plus the special type nimbus.

What are the four core cloud types identified by Luke Howard?

Cirro-form, cumulo-form, strato-form, and nimbo-form.

What does "cirro-form" mean and what are its characteristics?

From the Latin for "curl of hair." Cirro-form clouds are high, wispy, whitish clouds composed of ice crystals. They are the first clouds to appear in advance of a low-pressure system.

What does "cumulo-form" mean and what are its characteristics?

Generally detached, fluffy, white clouds resembling cotton balls. They show vertical motion/thermal uplift. They have a flat base (at the condensation altitude) and dense, sharp-outlined tops.

What does "strato-form" mean and what are its characteristics?

From the Latin for "layer." These broad, widespread blanket-like clouds result from non-convective rising air. They tend to occur along and north of warm fronts and have diffuse edges.

What does "nimbo-form" mean?

From the Latin "nimbus" (rain). Howard combined cumulo + cirro + stratus to define the rain cloud category. Nimbo-form clouds produce the vast majority of precipitation and are generally the thickest clouds.

What are the three altitude levels for cloud classification?

High-level, mid-level, and low-level clouds.

What are the high-level clouds (name all three)?

Cirrus (Ci), Cirrocumulus (Cc), and Cirrostratus (Cs).

What are the mid-level clouds (name all three)?

Altocumulus (Ac), Altostratus (As), and Nimbostratus (Ns).

What are the low-level clouds (name all four)?

Cumulus (Cu), Cumulonimbus (Cb), Stratocumulus (Sc), and Stratus (St).

What is the approximate altitude range for high-level clouds in the Temperate Region?

16,500 to 45,000 feet (5-14 km).

What is the approximate altitude range for mid-level clouds in the Temperate Region?

6,500 to 23,000 feet (2-7 km).

What is the approximate altitude range for low-level clouds?

Surface to 6,500 feet (0-2 km) in all regions.

What is a Cirrus (Ci) cloud?

Detached, high-level clouds in the form of white, delicate filaments, patches, or narrow bands with a fibrous or silky sheen. Always composed of ice crystals. They hardly diminish sunlight and are often brightly colored yellow or red before sunrise and after sunset.

What is a Cirrocumulus (Cc) cloud?

Thin, white, high-level clouds arranged in a patchy sheet with small grains or ripples, each element less than ~1° wide (little-finger width at arm's length). Predominantly ice crystals. Short-lived; often form from degraded cirrus or cirrostratus.

What is a Cirrostratus (Cs) cloud?

A transparent, whitish, veil-like high cloud with a fibrous or smooth appearance that can cover the entire sky. Distinguished from fog by the halo phenomenon it produces around the Sun or Moon.

What weather feature is characteristic of Cirrostratus clouds?

A halo around the Sun or Moon, nearly always produced when light shines through a layer of Cirrostratus.

What do increasing Cirrostratus clouds typically indicate?

A change from fair weather to possibly rainy or snowy conditions is approaching (often associated with an approaching warm front or low-pressure system).

What is an Altocumulus (Ac) cloud?

White and/or gray, patchy, mid-level clouds made of water droplets (sometimes ice). Elements appear 1-5° wide (between 1 and 3 fingers at arm's length). The most common mid-level cloud. When thin edges pass in front of the Sun/Moon, a corona (colored ring, red outside, blue inside) appears.

What is an Altostratus (As) cloud?

Gray or bluish mid-level cloud sheets or layers that partially or fully cover the sky. Thin enough to reveal the Sun as through ground glass. Does NOT produce a halo. May produce virga (streaks of rain that evaporate before reaching the ground).

What is a Nimbostratus (Ns) cloud?

A dark gray mid-level cloud layer that results from thickening altostratus. Thick enough to blot out the Sun. Associated with continuous rain or snow. Its base lowers as precipitation continues.

What is a Cumulus (Cu) cloud?

A detached, dense, low-level cloud with sharp outlines developing vertically as rising mounds, domes, or towers with cauliflower-like tops. Sunlit parts are brilliant white; bases are flat and dark. Forms over land during the day via diurnal convection.

What is a Cumulonimbus (Cb) cloud?

The thunderstorm cloud — a heavy, dense low-level cloud that extends into high altitudes. Its upper portion is smoothed and often flattened into an anvil or plume shape. It produces heavy precipitation, hail, virga, and tornadoes.

What is a Stratocumulus (Sc) cloud?

Gray or whitish, patchy, low-level clouds with a honeycomb/tessellated or rolled appearance. Elements are more than 5° wide (more than 3 fingers at arm's length). Non-fibrous. The most commonly observed cloud type worldwide.

What is a Stratus (St) cloud?

A generally gray, uniform, low-level cloud layer with a flat base. May produce drizzle, ice prisms, or snow grains. The Sun's outline is visible through it. Does NOT produce a halo (except at very low temperatures). When it breaks up, blue sky is revealed.

What cloud type produces an anvil-shaped top and why?

Cumulonimbus (Cb). When the updraft reaches the tropopause (top of the troposphere), it can no longer rise and spreads horizontally, creating the characteristic flat anvil or plume shape.

What does "virga" refer to?

Streaks of precipitation (rain or snow) falling from a cloud that evaporate before reaching the ground, visible as trailing wisps beneath the cloud base.

What is the difference between Altostratus and Cirrostratus regarding halos?

Cirrostratus produces a halo around the Sun or Moon. Altostratus does NOT produce a halo — instead the Sun appears as if seen through ground glass.

What is the NWS Cloud Chart?

A standardized chart developed in the 1930s that depicts 27 cloud classifications (individual clouds and combinations) used to describe the "state of the sky" during weather observations, allowing meteorologists to interpret atmospheric conditions.

Why are cloud symbols no longer plotted on U.S. surface weather maps today?

Because weather observations became computerized in the 1990s, and automated observing systems can only detect cloud height, not cloud type. Human observers elsewhere in the world still use cloud symbols.

How does the NWS Cloud Chart classify Cumulus clouds differently based on development?

Cumulus of little vertical extent = Low 1. Cumulus with moderate vertical growth = Low 2. Cumulonimbus without a visible anvil = Low 3. Cumulonimbus with an anvil = Low 9.

What determines the color of a cloud?

Primarily the color of the light it receives. The Sun provides white light (all visible wavelengths combined), which is then scattered or transmitted differently depending on cloud thickness and composition.

What is Rayleigh scattering?

The scattering of sunlight by gas molecules (much smaller than light wavelengths) in the atmosphere. Shorter wavelengths (blue, violet) scatter much more than longer wavelengths (red, orange), making the sky appear blue.

Why does the sky appear blue?

Due to Rayleigh scattering — blue light scatters about four times more strongly than red light. Combined with violet and indigo scattering higher up, blue dominates the color we perceive from the ground.

What is Mie scattering?

Scattering that occurs when light waves encounter particles of similar size to the wavelength of light (such as water droplets in clouds). Mie scattering affects all wavelengths equally, so all colors scatter the same — making clouds appear white.

Why are clouds white?

Because cloud droplets are similar in size to light wavelengths, causing Mie scattering. Mie scattering does not favor any particular wavelength, so all colors scatter equally and we perceive the result as white light.

Why do clouds appear gray or dark?

When clouds thicken, less sunlight passes through them. Also, contrast with a brighter background (blue sky or other white clouds) can make a foreground cloud appear darker than it actually is — a perceptual effect.

Why are clouds yellow, orange, or red at sunrise and sunset?

At low sun angles, light travels a longer path through the dense lower atmosphere. Rayleigh scattering removes shorter (blue) wavelengths first, leaving yellow, then orange, then red as the dominant remaining wavelengths. Mie scattering within the cloud then scatters these remaining colors equally.

What is refraction of light?

The bending of light as it passes from one medium to another (e.g., air to a prism or raindrop) due to a change in its velocity. Higher-energy (shorter wavelength) light bends more. This separates white light into its component rainbow colors.

How does a rainbow form?

Sunlight refracts and reflects inside raindrops, which act like prisms. The different wavelengths bend by different amounts, separating white light into the spectrum of colors we see as a rainbow.

What does the visible electromagnetic spectrum look like in order of increasing wavelength?

Violet → Indigo → Blue → Green → Yellow → Orange → Red → Deep Red. Shorter wavelengths have higher energy.

Why do cirrus clouds appear yellow or red near the horizon?

Because of the long distance light travels through the atmosphere at a low angle, Rayleigh scattering removes blue wavelengths, leaving longer yellow and red wavelengths to color the cirrus clouds.

What is the relationship between jet streams and cloud altitude limits?

Jet streams divide atmospheric regions (Polar, Temperate, Tropical). As the tropopause height decreases toward the poles, the altitude of clouds generally also decreases. The jet stream follows the sun seasonally, shifting cloud altitude limits accordingly.

Why can cumulus cloud bases sometimes be found at mid-level altitudes?

In very dry lower atmospheric conditions (e.g., the southwestern U.S. in summer), air parcels must rise much higher before cooling to the dew point. Cloud bases have been observed at 9,000+ feet over North-Central Texas.

What cloud is identified by a halo vs. a corona, and what causes each?

Halo: caused by Cirrostratus (Cs) — ice crystals refract light into a large ring around the Sun/Moon. Corona: caused by Altocumulus (Ac) — thin cloud edges diffract light into a small, colored ring (red outside, blue inside) very close to the Sun/Moon.

What cloud type is overcast stratus associated with in terms of atmospheric stability?

An overcast layer of Stratus (St) indicates a stable atmosphere, implying little change in current conditions for the next few hours.

What is the significance of the Tropic of Cancer and Tropic of Capricorn in cloud altitude classification?

These latitudes (23.5°N and 23.5°S) mark the division between the Temperate and Tropical Regions, which have different cloud altitude ranges. The Tropical Region has the highest cloud altitudes, reaching up to 60,000 feet for high-level clouds.

How does cloud classification differ for observers at different elevations?

Clouds are classified based on the observer's elevation. A cloud seen as altocumulus from sea level might be reported as stratocumulus by an observer on a mountaintop — two observers looking at the same cloud can report different classifications.