Category: "Cloud science"

A Fogbow

October 25th, 2017

A fogbow, or cloudbow (fog is a type of cloud), is a special type of rainbow. It is just white, and so not as often photographed as the full-spectrum rainbow, but it can be exciting to see nevertheless.

A Fogbow


The reason the fogbow is white is because the water droplets in fog are much smaller than raindrops. Fog droplets may vary in diameter roughly between 1 and 20 millionths of a meter (i.e., 1-20 microns), whereas raindrops are typically 1-3 thousandths of a meter, or about 500 times larger. The wavelength of visible light is only about a half a micron, so the light rays inside a fog droplet are still fairly well defined, but there is simply not enough room to separate out the colors, to state things simply.

Upon approaching a fog in the morning, look towards, but above your shadow. About 50-60 degrees from the shadow of your head is where the fogbow will sit, just as it would for a rainbow. Evenings will work too. But midday, the angles 50-60 degrees above your shadow will have you looking at the ground, so you probably won't see a fogbow there. (If you are in an outdoor shower, you might see a rainbow though.)

The above photograph shows the fogbow I saw yesterday morning, about 8:30 am, biking into a nearby park.

-- JN

How clouds form snow

January 14th, 2017

To understand snow formation, one must know a little about clouds. 

Q: What is in a cloud?

A: Air, dust, vapor, droplets, and often, ice. 

Q: How much air? How much liquid water? How much ice?

A: The answers will probably surprise you. See my short 20-min presentation below. I gave this recently to the Bellingham, WA Snow School. (23 slides, but due to file-upload-size restrictions, I had to put them into three parts below, 10 slides, 6 slides, 7 slides.)

Snow, rain, and weather affect everybody, yet how many of us learned in school even the most basic facts about precipitation in school?

Q: Who first realized how ice grew in a cloud?

How clouds form snow

As described in my presentation, he realized this by observing frost on the ground. 

Q: Who first realized how Alfred's theory was intimately connected with rainfall? 

How clouds form snow

Tor discovered this by observing fog in a mountain forest, and like Alfred, applied some of his physics knowledge. 

In my presentation, I discussed Alfred Wegener, the roles of the different cloud components, and briefly how the ice, once formed, takes on its strange shapes: 

 

First 10 slides (with blue text added to account for the things I said during the talk):

http://www.storyofsnow.com/media/blogs/a/Jan2017/snowschool_annotated1t10.pdf?mtime=1484585328

 

Next 6 slides:

http://www.storyofsnow.com/media/blogs/a/Jan2017/snowschool_annotated11t16.pdf?mtime=1484585328

 

Last 7 slides:

http://www.storyofsnow.com/media/blogs/a/Jan2017/snowschool_annotated17t23.pdf?mtime=1484585309

 

 

Later, I will show specifically how the ice gets arranged into all these strange shapes. 

- JN

Reflections off Falling Crystals

December 26th, 2009

On my morning icespotting trip the other day (12/23), I caught a glimpse of an unusual sight - a sun pillar. I thought I saw one once last winter, but this one was unmistakable. It seemed more striking even than the one in Robert Greenler's book "Rainbows, Haloes, and Glories", a classic book on atmospheric displays. A line of light above the sun forms when sunlight reflects off the bottoms of falling crystals that fall a certain way - nearly horizontal. A pillar can form from either columnar crystals, oriented like a log floating on water, or tabular crystals, oriented like a frisbee in flight. For the reflection to reach our eyes, only the crystals that appear above the sun can reflect sunlight to our eye. So we see the reflections coming from the region directly above the sun, as in the picture. The same effect can occur below the sun, when the sunlight reflects of the tops of the crystals. I took this picture on the sunset setting of my camera, which boosts the reds, but the view to my eye was, if anything, more stunning.

Reflections off Falling Crystals

-JN 

Full story »