CLOUD ATLAS

by Michal Lumsden and Meggie Winchell

Introduction:

We’ve all looked up at the sky and said, “Hey, that cloud looks like a dragon, and that one is a bunny rabbit!” But, have you ever wondered what the scientific name is for that bunny- like cloud? Well, this atlas will give you the basics of cloud identification as well as some pointers about what clouds are made of and how clouds can show what is happening in our atmosphere. Also described are some optical phenomenon—what exactly makes a rainbow anyway?

Background:

Types of clouds can be categorized by height. There are high clouds at ~25,000’, middle clouds at ~10,000’, and low clouds at ~5,000’. High clouds are composed of ice crystals while middle and low clouds are mostly made of tiny water droplets. The names of clouds are based on their height as well as their appearance. The following vocabulary list will be helpful in cloud identification:

Stratus—“Stratus” means “layer” and refers to the group of clouds that form in big sheets covering the entire sky.

Cumulus—In Latin “cumulus” means “heap.” These are the clouds that we often say looks like bunnies or giant castles.

Alto—The word “alto” means “middle.” This refers to clouds that are in the middle layer of our atmosphere.

Cirrus—The word “cirrus” means “curl” in Latin. These clouds are high up and look like wisps of hair.

Nimbus—“Nimbus” comes from the Latin word for “rain.” Whenever there is precipitation there are nimbus clouds.

Given this, let’s identify some clouds….

1. Low Level Clouds:

CUMULUS:

Fair weather cumulus clouds form when a parcel of air rises and water vapor condenses into water droplets.

1.1 Fair Weather Cumulus

STRATUS:

This is a classic case of stratus clouds. The sky has a sheet-like grey covering and the sun is not visible.

1.2 Stratus Clouds

STRATOCUMULUS:

Stratocumulus clouds form a clumpy layering across the sky. They are not as well- defined or as well-separated as fair weather cumulus clouds are.

1.3 Stratocumulus Clouds

There are times at dusk when sunlight shines through the breaks in the stratocumulus covering. The effect is often call “Jesus Rays” however the formal name is crepuscular rays.

1.4 Crepuscular Rays shining through stratocumulus

2. Middle Level Clouds:

ALTOSTRATUS:

Altostratus is a uniform light grey layer covering the sky. Usually the sun is dimly visible through these clouds.

2.1 Altostratus. Notice that the sun can be faintly seen through the clouds.

ALTOCUMULUS:
This cloud type is similar to fair weather cumulus. However, the puffs are higher, more numerous, more uniform, and smaller. If you hold your hand out to the sky an individual altocumulus cloud is about the size of your thumb-tip. The puffs can be all jumbled together or line up in rows to form stripes.


2.2 a. Altocumulus as stripes.	b. Altocumulus in uniform puffs.

3. High Level Clouds:

CIRRUS:

On sunny days you can often see white wispy clouds that look high in the sky. These are cirrus clouds. Their wispiness is caused by the fact that when their ice crystals get spread out by the wind they don’t evaporate right away. We see the trails of ice crystals as streaks of white in the sky. Because of this wispiness cirrus clouds are sometimes called “mares’ tails.”

3.1 Cirrus clouds on a sunny day

CIRROCUMULUS:

Cirrocumulus is similar to altocumulus, however the puffs are higher and appear smaller. If you reach your hand out towards the sky a single cirrocumulus cloud will be about the size of your pinky-tip. Like altocumulus these clouds can form a uniform pattern or rows. Since these clouds can look like fish scales a cirrocumulus covering is sometimes called a “mackerel sky.”

3.2 An example of a “mackerel sky”

CIRROSTRATUS:

Cirrostratus is a high, thin layer of ice crystals which causes a light, milky, gauze-like covering of the sky.

A single ice crystal is hexagonal and very small—about 10 micrometers in diameter. The arrangement of these ice crystal and their interactions with sunlight can produce halos, sundogs, and sunpillars.
	3.2 an ice crystal

Halos are produced when sunlight passes through randomly oriented ice crystals. If you extend your arm and put your thumb on the sun, the halo will most likely be at your pinky- tip.

When light passes through ice crystals that are uniformly oriented, sundogs can form. These are colorations about a thumb-to-pinky distance on either side of the sun.

Sometimes when the sun is low in the sky a sunpillar can be seen. A sun pillar appears as a shaft of light extending above or below the sun. It occurs when light is reflected off uniformly oriented ice crystals which tilt as they fall.

4. Nimbus Clouds:

NIMBOSTRATUS:

Nimbostratus clouds are low-lying clouds with no clear base. They produce continual precipitation, usually drizzle or snow.

CUMULONIMBUS:

These clouds are associated with heavy rain, lightning, and thunder. They are also called anvil clouds or thunderheads. Believe it or not, such storm clouds begin as fair weather cumulus.


4.2 a. Towering Cumulus: Due to continuous warming and accelerated rising, fair weather cumulus grows taller and becomes towering cumulus clouds.	b. Developing Cumulonimbus: The updrafts become stronger and the clouds grow darker and larger.	c. Mature Stage: In the mature stage of a thunderstorm water droplets grow and fall as precipitation. There are also violent updrafts and downdrafts throughout the thunderhead.

On the note of rain…where do rainbows come from?

To see a rainbow you need to stand facing rain with the sun at your back. Here is what
happens:
- White light from the sun enters a water droplet.
- The water bends the light. It turns out that different colors of light are bent differently. Thus, the colors are spread out.
- The separated colors reflect off the back of the droplet and return to your eye.
- You only see one color per droplet because of the angle of the light leaving the raindrop. Thus, it is seeing the light from many droplets simultaneously which creates the display of the rainbow.

4.3 The path of light as it passes through water droplets to create a rainbow. The yellow arrows represent white light from the sun. Red and purple arrows represent the light after it has been bent and the colors spread out.

Double rainbows occur when there are two reflections within the same water droplet.

4.4 Double Rainbow