This morning it was extra cold (15 degrees F!).  It would have been cold anyway because an arctic air mass arrived over the weekend, but it was extra cold because the sky was mostly clear last night.  If we’d had lots of cloud cover we’d have been a little warmer.

The reason for this is not what you’d expect.  Traditionally we’ve heard that cloud cover acts like a blanket to hold the heat in.  The illustration above plays to that notion by showing heat arrows bouncing off the clouds.  But it ain’t exactly so.  Believe it or not this illustration is wrong.

The truth is that we’re warmer under cloud cover because the clouds radiate their own heat which warms the air below them.  You’ve seen this principle in action if you’ve parked your car under a leafy tree on a frosty night and found your windshield frost-free the next morning though the open ground has frost.  The tree radiated heat to keep your car just a little warmer than the open air.

I learned this from Dan Satterfield’s blog, Dan’s Wild Wild Science Journal, where I found this illustration.  In “Scientific Facts That Aren’t True” Dan writes:

The clouds do not “hold the heat in”. They absorb the heat, and radiate their own heat in all directions.  …If you’re camping, and you sleep under a tree, you will escape most of the dew compared to your buddies, who slept right out under the stars. The tree did not catch the dew, it just radiated energy to the ground around you, and kept it warmer. Warmer ground, less dew!

Click on the image to read more of Dan’s Scientific Facts That Aren’t True.

Clouds may blanket us but they aren’t blankets.

(traditional image of heat bouncing off clouds from Dan’s Wild Wild Science Journal: Scientific Facts That Aren’t True by Dan Satterfield)

Back in September an amazing asteroid flew by in outer space.

It first appeared as a fuzzy dot, seen by a PanSTARRS Survey telescope in Hawaii.  Wondering what it was, astronomers directed the Hubble Space Telescope to take a look.  Boy, were they surprised.  It has six tails!

This is not a normal asteroid.  Asteroids are very tiny planets and — until now — they don’t have tails.  This one is only 700 feet across and is traveling around the sun in the asteroid belt between Mars and Jupiter.  Like it’s traveling companions in the Flora asteroid family, its probably a chunk left over from a planetary collision.

So why does it have tails?  Comets have tails because they are made of ice, dust and small rocks.  When they get near the sun the ice evaporates, causing a long streamer of debris.  But this asteroid has no ice.  It must be streaming dust.  Lots of it.

Scientists named it P/2013 P5 and ran its behavior through modelling software at the Max Planck Institute for Solar System Research.  The model showed this asteroid is spinning so fast that anything loose on the surface (dust) is traveling toward its equator.  There it accumulates and episodically escapes the asteroid’s weak gravity, arcing into outer space. Yow!  Six tails!

Why is it spinning so fast?  Scientists theorize that the pressure of sunlight could have pushed P/2013 P5 into a tail spin.

Photos, above, from the Hubble Space Telescope show it spinning like a lawn sprinkler in the sky.

Read more here at NASA’s Hubble website.

(images of Asteroid P/2013 P5 from the Hubble Space Telescope, courtesy of NASA)

I love it when the sky does this.

These pretty clouds won’t rain … yet.  They drift by in thin shields with gaps between and when they thicken they look like buttermilk (click here to see).

These are altocumulus clouds that form in the mid-level of the cloud deck, between 6,500 and 20,000 feet.  Their thicker version is called a mackerel sky because the effect resembles the scales on a king mackerel.

Weather sayings confuse me about the message these clouds are bringing.  One poem says, “Mackerel sky, mackerel sky – never long wet, never long dry.”   Worst case:  These are overtaken six to eight hours later by different clouds that bring rain.

On the day I took this photo I was outdoors for six hours and yes, these happy clouds were followed by thick, potential rain clouds.

But it didn’t rain until I got home.

(photo by Kate St. John)

16 February 2013

There we were, focusing our attention on an asteroid that was going to miss the Earth when Bang!  A real live meteor zipped low over Russia yesterday morning, 15 February 2013.

The meteor taught me a lot more than the asteroid.  After it lit the sky, made an explosive boom, blew out windows, and injured more than 1,000 people I learned from NASA:

• Its light was brighter than the sun.
• Its contrail was 300 miles long. (That’s the distance from Pittsburgh to Philadelphia).
• Eyewitnesses said the sonic boom lagged by three minutes … just long enough for everyone to go to their windows to watch.
• The meteor was about the size of a bus (55 feet) but it weighed 10,000 tons –> 1,400 times heavier than a bus.
• The atmosphere really did help after all.  When the meteor exploded it was still 12-15 miles up.  At least twice as high up as a jetliner.
• If it was only the size of a bus and 2 to 4 times higher up than a jet, why did it cause such a problem?   Well, it was traveling at 40,000 mph!

So, hold onto your hats.  It’s the stuff we aren’t worried about that gets us.

This video from The Telegraph UK shows dashcam video and explains what happened.

p.s.  16 Feb 2013: The meteor also taught me two things about Russian culture:  (1) Russians have dashboard cameras in their cars to protect against corrupt policemen and disputed traffic accidents, and (2) They have already made a joke about it, quoted from the Houston Chronicle: “The meteorite was supposed to fall on Dec. 21, 2012 — when many believed the Mayan calendar predicted the end of the world — but was delivered late by Russia’s notoriously inefficient postal service.”

(photo of the Chelyabinsk meteor’s trace from Wikimedia Commons; click on the caption to see the original)

Even when scientists develop an answer to why something happened, they still test the idea to make sure they’re right.

That’s what happened with the Great Arctic Cyclone of August 2012.

Last August a rare, massive cyclone formed in Siberia and swirled out over the Arctic Ocean for days.  During its transit the sea ice disappeared faster than anyone had ever seen.  (See the swirl here.)

By September Arctic sea ice was at an all time low.  Some said the cyclone caused the lowest ice extent since record-keeping began.  Did it?  Or would the ice have melted anyway due to warm temperatures?

Scientists at the University of Washington’s Applied Physics Laboratory ran two computer simulations of last summer’s Arctic weather.  One matched the actual weather.  The other included everything except the cyclone.

The result showed that yes, “the effect is huge in the immediate aftermath of the cyclone, but after about two weeks the effect gets smaller.  By September, most of the ice that melted would have melted with or without the cyclone,” said lead author Jinlun Zhang.

Why?  Because of mixing.

Back in September most thought that the wind broke up the thin ice or pushed it into a warmer part of the ocean.  Since then scientists have learned that the ocean underneath the ice is like a layered parfait.  Just below thin ice is a layer of ice-cold fresh water.  About 65 feet down is a layer of salty water warmed by the sun.  The cyclone stirred the parfait.  The ice was exposed to the warm water beneath and it melted.

The cyclone did cause the ice to melt 10 days sooner, but in the end it made less than 5% difference in the ice extent.

So yes, the sea ice melted because it was hot last year.

Click on the photo to read more about the study in Science Daily.

(photo courtesy University of Washington)