Category: "Weather and Climate"
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?
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?
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):
Next 6 slides:
Last 7 slides:
Later, I will show specifically how the ice gets arranged into all these strange shapes.
After a few years in the making, our new device for growing single ice crystals in a well-controlled laboratory environment is nearly ready. We are just adding a few small accessory pieces to allow us to start testing. I was to describe the apparatus at the cloud-physics conference this month in Boston, and made a poster to present, but decided to opt out. But, having spent a few days making the poster, I present it below.
As with all images here, click on image to see large-scale view.
And below is the same, but in blog format.
After a few days of fine bright spring weather, the barometer falls and a south wind begins to blow. High clouds, fragile and feathery, rise out of the west, the sky gradually becomes milky white, made opalescent by veils of cirro-stratus. The sun seems to shine through ground glass, its outline no longer sharp, but merging into its surroundings. There is a peculiar, uncertain light over the landscape; I 'feel' that there must be a halo round the sun!
And as a rule, I am right.
The quote, from Minneart* describes a common ice-related atmospheric apparition. It appears in skies all over the world far more often than the rainbow, yet few notice it. As a graduate student, I read about halos and often looked for one, but didn't notice it myself until someone else pointed it out. As a post-doc in Boulder, I was out walking with Charles Knight, and I mentioned my lack of success. He glanced up near the sun, pointed, and said “why there's one right now”.
What I had missed in my readings had been the fact that most halos are rather indistinct and often incomplete circles. Indeed, now when I point out the most common one (the 22-degree halo) to someone nearby, they often don't see it. But occasionally, it is sharp enough (and colored) to the extent that anyone will see it if they bother to look up and glance toward the sun. And often it occurs with other ice-crystal apparitions that are even more obvious.
Last fall, while perched high on a rock face, belaying my partner up**, I saw such a vivid display.
The bright spot is called a “sun dog”, “mock sun”, or “parhelia”. They, one on either side of the sun, usually appear together with the 22 degree halo. Indeed, the sun dogs very nearly mark the spots where that halo intersects another arc called the parahelic circle. Their cause: horizontally oriented, tabular ice crystals.
This recent front-page article caught my eye:
The writer is an avid skier-snowboarder, and thus concerned about the future of his sport. The facts he relates paints a grim picture:
- In the past 47 years, a million square miles of spring snowcover has disappeared from the Northern Hemisphere.
- Since 1970, the winter warming-rate has been triple the rate of the previous 75 years.
- The Alps are warming 2-3 times faster than the world-wide average.
- Potential Winter Olympic venues shrinks from 19 (now) to 6 by 2100.
and many other facts. Avid skiers like spring skiing. For March, the data (from Rutgers University Global Snow Lab) show a decline in area covered by snow.
The Pacific Northwest has been foggy a lot lately, but the fog droplets have been subzero, or supercooled. When such fog droplets hit an object, they almost always freeze. The resulting frozen aggregate is called rime. Freezing fogs make rime.
The resulting rime formations may look like hoar frost, or even snow, from a distance. But close up the special features of rime become apparent.
David Easterling recently reported in BAMS** that the number of frost days per year is decreasing over the US. A frost day is a day in which the minimum temperature goes below the melting temperature of ice (32 F or 0 C). This doesn't sound good for a frost observer like me. Moreover, the largest decrease, a value of 2.6 fewer days per year per decade, is in my area, the Pacific Northwest. Will frost soon be a threatened species?
His analysis and presentation was based on 1948-1999 data, and moreover, he averaged over multi-state regions. I'm more interested in my local area, King County, Washington, and would like to see both the longer-term trend and variability. I quickly sent him an email, requesting more info. But I was impatient and sought the raw data myself. Following the info in his article, I found the data online***. An added plus: at many stations, the data now goes back another 50 years. With a few simple commands typed into an Excel spreadsheet, I determined the longer-term trend for a station near the University of Washington stadium in Seattle (the only such site in Seattle).