Last weekend on PABIRDS, Bill Drolsbaugh reported seeing light pillars at night in Sharon, PA.

He wrote:

Around 3 a.m.there were ice columns or pillars suspended above strong lights in Sharon Pa Quaker Steak and lube parking lots to the East [and]  NLMK steel plant on the horizon about a mile away to the s.w.    They are vertical refractions that can be seen in very cold temps and suspended fine snow above lights and have tall vertical rainbow-like qualities.  One of them was floating in front of my window about 20 ft out suspended in mid air like a Winter Wraith.

I’ve never seen light pillars so I looked for photos online and found this one taken in Laramie, Wyoming.

In the photo, the pillars look as if they shine straight up from each streetlight but as Les Cowley explains on his Atmospheric Optics website, they’re caused by reflections from millions of flat plate-like ice crystals between the light source and the observer.  This explains why Bill saw one floating 20 feet outside his window.

Click on Les’ diagram below to see it full size and read more about this optical phenomenon.

As winter gives way to spring there will be fewer opportunities to witness these icy phenomena.  Given the choice I think we’d rather have warm weather than light pillars.

(photo of light pillars from Wikimedia Commons. Diagram of light pillars by Les Cowley at Atmospheric Optics.  Click on the images to see the originals.)

28 January 2014

The weather was weird yesterday but it made something beautiful.

In Pennsylvania and Ohio people looked outdoors to find thousands of large snowballs dotting hillsides and open fields.  The snow rollers resembled hay bales, jellyrolls or the unstacked segments of snowmen and were so unusual that they became online sensations in social media.  They were made by the wind.

I didn’t know they’d happened until Marianne Atkinson sent me photos from her backyard in Clearfield County, PA.  I’d seen the wind make little snowballs in the Laurel Highlands so I thought I knew what she was talking about.  But no, these are special.  They’re a foot across!

Snow rollers are pretty rare but yesterday morning produced the perfect weather mix…

• With an icy layer on top of the snow that new snow couldn’t stick to …
• Wet, loose snow fell on top of the icy layer.
• The temperature was near the melting point and…
• The wind blew at just the right speed to start the balls rolling without destroying them.
• The rollers stopped when they became too heavy for the wind to move them.  Even so they’re often hollow and too fragile to pick up.

Look quickly! The weather will warm up soon. These delicate wonders will collapse as they melt.

(photos by Marianne Atkinson)

Here’s some old news to some of you… but new to me.

Launched in 2002, the twin Gravity Recovery and Climate Experiment (GRACE) satellites have collected data for 12 years and are already in a decaying orbit that will suck them down to Earth in the next year or two.  During their run they’ve carefully measured Earth’s gravitational pull and contributed to our knowledge of aquifers, ice sheets, magma and earthquakes — all because of gravity.

Did you know that gravity is uneven around the world and can vary in the same location during the year?  Interestingly, water has a lot to do with it.  Gravity is determined by mass so an increase in groundwater causes a higher gravitational pull.  Since the GRACE satellites measure gravity, they find groundwater.  It’s as if they were dowsing (finding water using a forked stick) from outer space.

Here’s how it works.  Skimming like hockey pucks in their pole-to-pole orbit, the GRACE satellites maintain a 140-mile distance between each other which they measure constantly. Their microwave ranging system is so accurate it can record a 10 micrometre change in separation (1/10 the width of a human hair)!  When the lead satellite first encounters a stronger gravitational pull, gravity makes it speed ahead, increasing the distance between them.  When the second satellite encounters the same “bump” it speeds up too and their separation decreases.  When they’ve both passed the “bump” they both slow down.

Round and round since 2002 they measure the distance between themselves and report back to Earth.  NASA’s computers crunch the ebb and flow of gravity and create gravitational anomaly maps.  Click here to see a selection of them.

The maps help scientists understand changes in aquifers and improve groundwater management.  You can see the greatest anomalies in the tropics where there are dramatic wet and dry seasons.  On this map(*) the Amazon basin is routinely blue (decreasing gravitational pull) in October at the end of the dry season and routinely red (higher gravity) in April at the end of the rainy season.  Bangladesh’s color cycle is the opposite because its wet-dry cycle occurs during the other half of the year.

Until gravity pulls them down and ends their mission the GRACE satellites travel above us, dowsing from outer space.

(image of the GRACE satellites and photo of George Casely dowsing on his farm from Wikimedia Commons. Click on the images to see the originals)

p.s.  Click here for more news from the GRACE satellites.

(*) Note that on this particular animated map the colors are “backwards:” red=wet, blue=dry

17 December 2013

When I wrote about the lawn sprinkler asteroid on November 11, I was intrigued by this sentence in the news article:

“The asteroid could possibly have been spun up if the pressure of sunlight exerted a torque on the body.”

The pressure of sunlight?

Apparently small bodies in outer space — from dust particles to 10km wide asteroids — are affected by the relentless though tiny touch of photons.  They reflect or absorb the photons’ energy and emit what they don’t retain.  The emissions become a tiny propulsion force. However, dust and asteroids have irregular shapes and surfaces so they reflect, absorb and emit unevenly.  This affects their rotation and flight path.

There’s a lot of fancy physics that predicts what a small irregular body will do under the pressure of sunlight.  I read about the Yarkovsky effect, the YORP effect and the Poynting-Robertson effect until I got confused.  Then I googled for a simple description and found …

The United Nations’ Space Generation Advisory Council holds an annual contest to solve the problem of deflecting a killer asteroid on a collision course with Earth.  In 2012 the winning solution of the Move an Asteroid Competition was to bombard it with white paintballs.

The reason this would work is due to the Yarkovsky effect (I think).  A dirt-colored rotating asteroid absorbs photons and heats up on its daylight side, then releases energy when that side turns to night.  In a steady state the asteroid would stay on course and hit the Earth but if it’s painted white it will absorb less and emit less — and this will alter its course.

All we need for deflecting a dangerous asteroid is a 20-year lead time, a rocket, a lot of white paintballs and very good aim.

Watch the video to see how we’d paint an asteroid.  Click here to read how it works in MIT News.

Paintballs to the rescue.

(video from MIT News on YouTube)

There are a lot of iridescent things in nature:  birds, beetles, seashells, fish, minerals and clouds.

Yesterday, after a snowy start (and really bad traffic!) the wind swung around to the west and the sky cleared with a few fast-moving clouds.  At lunchtime I looked up while standing in a building’s shadow and saw a thin, beautiful, iridescent cloud blowing past the sun.

Thin is important.  Iridescence occurs when sunlight diffracts through a thin layer of water droplets (or ice crystals) of uniform size and orientation.  The glowing colors are named for the Greek goddess Iris, the personification of rainbows.

Pittsburgh’s iridescent clouds aren’t nearly as cool as the nacreous clouds in Antarctica, but we don’t have the super-cold stratospheric temperatures that cause those clouds. For which I am grateful!

(photo by “not on your nelly” on Flickr, Creative Commons license. Click on the image to see the original)