Fundamentals of Atmospheric Science

5.1 Do you recognize these clouds, drops, and snowflakes?

Clouds have fascinated people for millennia, but it wasn’t until 1802 that Luke Howard first classified clouds with the terms that are used today. His classification scheme was formalized later in the 19th century and has 10 basic cloud types with many minor variations (see figure below).

chart of cloud types
Genus classification of clouds by altitude of occurrence.
Credit: Wikipedia


This website contains a nice overview of the cloud types with descriptions and accompanying images. Check out some of their amazing photos!

NOAA and NASA put together this thorough Sky Watcher Chart that describes a wide variety of cloud formations.

Cloud physics goes beyond the classification of clouds to determine the actual physical and chemical mechanisms that create clouds and cause their evolution over time. There are two aspects of cloud physics. One is the physics on the cloud scale, which is tens to hundreds of meters in size. This physics is driven in part by behavior in the cloud’s environment, such as the wind shear or the location of a front, and determines the evolution of the cloud and the cloud’s size and shape. All of this action, however, is not possible without the physics that is occurring on the microscale, which is less than a few centimeters in size.

This lesson deals mostly with the physics that occurs on the microscale and is often called cloud microphysics. Now that you are familiar with the concepts of thermodynamics and water vapor, we are ready to look at the fundamentals of cloud microphysics. To understand cloud-scale physics will require an understanding of atmospheric dynamics and turbulence, which are introduced in later lessons of this course.

A cloud is defined as a (visible) suspension of small particles in the atmosphere. For a water cloud, there are a number of types of particles that we are interested in.

Cloud drop sizes. See text below image

Cloud drop sizes and characteristics. D is the typical diameter; n is the typical number per volume of air. Sizes are almost but not quite to scale.
Credit: W. Brune (after Lamb and Verlinde)

Note the wide range in size, volume, and number of particles in the figure above. The smallest, the cloud condensation nuclei (CCN), can have rather little water vapor and are made up of substances to which water can attach (called hydrophilic, water loving). The other particles grow by adding water molecules but still contain the original CCN upon which they formed.

We can specify the amount of water that is in liquid form by using the liquid water content (LWC). The liquid water content can be defined as:

LWC= ω L = massofliquidwater volumeofair ,units=g m 3 This equation is not rendering properly due to an incompatible browser. See Technical Requirements in the Orientation for a list of compatible browsers.


Typical LWC are 0.1- 0.9 g m-3, but a few g m-3 are possible for wetter conditions.

Check Your Understanding

A cloud drop is typically 10 µm in diameter, while a raindrop, which comes from a collection of cloud drops, is typically 1 mm (1000 µm) in diameter. How many cloud drops does it take to make a raindrop?

Click for answer.

ANSWER: Find the volume of the cloud drop and the volume of the raindrop and then find out how many times bigger the raindrop is. The answer is the number of cloud drops it takes to make a raindrop.

n cloud V cloud = V rain n cloud = V rain V cloud = 4 3 π ( r rain ) 3 4 3 π ( r cloud ) 3 = ( r rain r cloud ) 3 = ( 1000 10 ) 3 = 10 6 This equation is not rendering properly due to an incompatible browser. See Technical Requirements in the Orientation for a list of compatible browsers.

So we see that it takes about a million cloud drops to make one raindrop. Thus 109 cloud drops per m3 of cloud should make about 103 raindrops per m3 of cloud. This is about the number per m3 that are observed.

Some clouds exist in regions where the temperature is below 0oC and thus can be made of ice. Ice is simply water in an organized crystalline form. While on the molecular scale the arrangement of water molecules in the ice matrix is the same, the visible shape can vary dramatically. In fact, certain shapes are favored in different temperature regimes (see figure below). Excess vapor density is simply the amount of water vapor (in terms of milligram per cubic meter of air) in excess of ice saturation, not liquid water saturation, which is shown as the dashed line. At temperatures above -3oC, simple hexagonal plates occur. Between -3oC and -8oC, columns form. Between -8oC and -22oC, plates are formed, with the more complex plates occurring with a greater excess of water vapor. At temperatures below -22oC, columns once again dominate and go from simpler to more complex as the excess water vapor increases.

Ice crystal shape for different temperatures and different excess water density levels as described in the text above
Ice crystal shape for different temperatures and different excess water density levels. Excess water vapor is the amount of water vapor above the saturation water vapor amount. This situation can occur as air is lifted and the temperature (and thus saturation vapor pressure) drops faster than the water vapor can deposit on the ice.
Credit: W. Brune (after Lamb and Verlinde)

Ice crystal habits as a function of temperature and excess water vapor (i.e., water vapor greater than saturation water vapor).

close up view of a snowflake
A snowflake. Its shape with dendrites indicates that it formed with a lot of excess water vapor and a temperature of about -16oC.
Credit: bkaree1 via flickr

The next time it snows, catch snowflakes on a cold surface and take a good look at them. Their shape will tell you a lot about the environment in which they were formed. In State College, we often see plates with broad branches and sometimes we see dendrites, telling us that the snowflakes were formed at altitudes in the cloud where the temperature was between -22oC and -8oC and the excess water vapor was large.

The following video (3:52) entitled "Snowflake Safari" gives a simple explanation of snowflake formation and shows some nice pictures of different snowflake shapes.

Click for Transcript of Snowflake Safari

Discussion Activity: Cloud Identification

(3 discussion points)

It's time to look up at the sky to observe the clouds. During the next week, take pictures of clouds and identify the clouds in the pictures. Try to focus on just one cloud type per image. Submit an image that depicts at least one cloud type.

You will upload each image in its own post. You should include the following in your post:

  • your name
  • the picture's location
  • the picture's date and time
  • your identification of the cloud
  • your reasoning for the identification in a short sentence or phrase.

Copy and paste your picture into the post box. Instructions for how to embed an image in your post can be accessed here.

  1. You can access the Cloud Identification Discussion Forum in Canvas.
  2. Post your pictures of clouds with their identification using the format described above.
  3. Keep the conversation going! Comment on at least one other person's post. Your comment should include follow-up questions and/or reasoning for an alternate identification of the clouds in the post.

This discussion will be worth 3 discussion points. I will use the following rubric to grade your participation:

Discussion Activity Grading Rubric
Evaluation Explanation Available Points
Not Completed Student did not complete the assignment by the due date. 0
Student completed the activity with adequate thoroughness. Posting answers the discussion question in a thoughtful manner, including some integration of course material. 1
Student completed the activity with additional attention to defending his/her position. Posting thoroughly answers the discussion question and is backed up by references to course content as well as outside sources. 2
Student completed a well-defended presentation of his/her position, and provided thoughtful analysis of at least one other student’s post. In addition to a well-crafted and defended post, the student has also engaged in thoughtful analysis/commentary on at least one other student’s post as well. 3