Category: "Weather and Climate"

A rare heliac arc? (plus six others)

February 5th, 2019
The day starts sunny and bright, but later you note a slight muting of the surrounding landscape. It is still bright enough, but you feel less heat bearing down. Apparently, a thin veil of high clouds has slowly and silently appeared above. When this… more »

Film frost grains and radiative cooling of the ground

December 10th, 2017
December 8th brought the first frost to the Seattle area. This doesn't mean that this is the first time this season that the ground reached 0 degrees C or lower. True, we had gotten snow in late November, though this by itself doesn't mean the ground… more »

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. [image:72] The reason the fogbow is white is… more »

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… more »

The new ice-crystal-growth apparatus

July 1st, 2014
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… more »

Halo in the sky? Uh, I don't see no halo...

April 20th, 2014
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… more »

The end of snow

March 1st, 2014
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… more »

Rime, freezing fog, and crystalline spider webs

January 22nd, 2013
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… more »

Frost Days and Ice Days: Declining Numbers over the Century

January 17th, 2013
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… more »