22 December 2011

Today is the southern solstice, the day of shortest sunlight in the northern hemisphere and the longest golden hours.

In photography, the golden hour is the period just after sunrise and just before sunset when the sun is low in the sky.  In that position it passes through more of the earth’s atmosphere so its light is reddish and diffuse and the shadows are long.

I learned about the golden hour when I looked up the time for sunrise and found additional information. Though there are many definitions for it the most common is that the golden hour ends when the sun is more than 6 degrees above the horizon.

Today in Pittsburgh the sun will rise at 7:40am and set at 4:57pm for 9 hours 17 minutes of daylight.  In the morning the sun will be low in the sky until 8:23am. In the afternoon it will reach the golden hour at 4:13pm for a total of 97 minutes of golden light.  This would be lovely but we’ll never see it.  The sky is overcast.

The golden hour is more pronounced the further north you go.

In Helsinki, Finland the sun rose at 9:24am and will set at 3:13pm for only 5 hours and 49 minutes of daylight.  Most of the time the sun will just skim the horizon producing two very long golden hours.  In fact they’ll have only 80 minutes of real “day” when the sun’s above 6^o.

After the solstice the days will get longer and the golden hours shorter.

Don’t miss today’s golden light.  For the best photographic effects, try Helsinki.

(photo by Hansueli Krapf  on Wikimedia Commons.  Click on the photo to see the original.)

In this day of cellphone GPS applications and vehicle navigation systems we forget that knowing exactly where you are on earth used to be a huge problem.  It was especially acute at sea where there aren’t any landmarks.

Until an accurate marine clock was invented in 1737 and became affordable in the 1780’s, seaman used the position of the sun, stars and planets to determine their location.  This worked well for north and south (latitude) but was impossible for determining east-west (longitude) because the earth rotates in that direction.

Shipwrecks occurred frequently, even under the best sea captains, and kings offered enormous prizes to the person who could solve the longitude problem.  Astronomers looked for a spot in the sky that behaved predictably and independently of the Earth’s orbit.

Galileo found an answer in Jupiter’s moons.

After he perfected the telescope in 1609, Galileo discovered the four largest moons of Jupiter.  He carefully logged their orbits and noted how often they disappeared behind the planet.  His records showed their orbits are so predictable you can tell time by them.  This was an answer to the longitude problem.

But it didn’t work at sea.  If you’ve ever viewed Jupiter through your binoculars you know that your heartbeat can make the planet jump.  No one could see the moons’ eclipses on a rolling boat deck.  However the method worked well on land with a tripod.

By 1650, the eclipses of Jupiter’s moons were so well documented that mapmakers used them to redraw the world.  Finally there were accurate land maps!  King Louis XIV of France reportedly complained that he was losing more territory to his astronomers than to his enemies (*).

Twenty-six years later Jupiter helped calculate the speed of light when Danish astronomer Ole Rømer discovered that the eclipses occurred sooner than expected when the Earth was closest to Jupiter and later than expected when Earth and Jupiter were furthest apart.  The difference is the speed of light.

Today Jupiter will rise at 2:00pm but his transit will go largely unnoticed.  His moons still keep accurate time but his role is eclipsed by our wristwatches, cellphones and satellites.

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(Composite photo of Jupiter with its Galilean moons by NASA on Wikimedia Commons.  Click on the image to read how it was constructed.)

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(*) This quote is from Dava Sobel’s book, Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time, where I learned these facts about Jupiter.  I highly recommend her book, published by Penguin in 1995.  Click on the book link to find it on Amazon.

How can a building cast a shadow on the sky?  The label on this photograph explains it:  Tyndall effect at CN Tower, Toronto.

The Tyndall effect was new to me so I looked it up.  Named for physicist John Tyndall (1820-1893), it describes how light is scattered as it passes through a colloid.

A colloid is a gas, liquid or solid that has particles microscopically and evenly dispersed within it.  Both natural and man-made colloids exist.  Some natural examples are fog, smoke, milk and gelatin.  Opals are colloids whose beauty comes from the Tyndall effect.

In this photo the air is a colloid.  Some of the particles in it are natural (water droplets and dust), some are man-made (fine particulate pollution that generates smog).  In either case the particles scatter sunlight and we can see the beam of sunlight.

Despite reading a lot about it I didn’t really understand the Tyndall effect until I watched this educational video.  The narrator first shows that a laser beam cannot be seen as it passes through plain water.  Then he puts two drops of Dettol (a cleaning product) into the water and the laser beam appears.

Pretty cool, huh?

So when you see a shadow on the sky, you know there’s something in the air.

(photo by Wladyslaw, a featured picture on Wikimedia Commons. Click on the photo to see the original.  Video posted by ksvsrao on YouTube.)

10 September 2011

Today, September 10, is the statistical peak of the North Atlantic hurricane season that runs annually from June 1 to November 30. 

This year my friends and family have had more than their usual share of destruction from the few hurricanes that hit the United States.  Hurricane Irene and the remnants of Lee have been headline news for weeks and Hurricane Katia, who’s missing the U.S. entirely, is affecting friends on a cruise in Greenland.   

So I wondered… how and why do hurricanes form?  I did a little research and found that even the basic facts are fascinating.

Hurricanes are tropical cyclones, complex dangerous storms that occur around the world. In the North Atlantic and on the eastern side of the Pacific we call them hurricanes.  In the northwest Pacific they’re called typhoons. 

Tropical cyclones are not completely understood but scientists know that six ingredients are required for a hurricane to form.  The ingredients, listed at NOAA, are:

1. A warm ocean surface of at least 79.7^oF to a depth of 160 feet.   To get an idea of how warm this is, the water temperature at the Eastern Maine Shelf this morning is 54-52^o from the surface to 160 feet.
2. Rapid cooling of the air as it moves upward, causing condensation and thunderstorms which release heat to drive the storm. 
3. High humidity in the mid troposphere 3 miles (15,800 feet) above the ocean.  If you were on a trans-Atlantic airplane, you’d be flying high above it.
4. The Coriolis effect must cause the storm to spin.  There is no Coriolis effect at the equator so the storms cannot form at less than 5⁰ of latitude (345 miles) from the equator. 
5. A pre-existing disturbed weather system near the ocean surface which provides the nascent storm with something to organize around.
6. Low wind shear where the storm is forming.  Wind shear is an abrupt difference in wind speed and direction and can break up a cyclone before it gets going. 

In August and September hurricanes often form off the Cape Verde Islands near the north coast of Africa.  They are then carried by the trade winds across the Atlantic and sweep over the Caribbean islands and sometimes the U.S. or Central America.  Right now Tropical Storm Maria is heading for Puerto Rico and Tropical Storm Nate is about to hit Mexico. 

Thank heaven we’re over the hump of hurricane season for 2011.  We’ve certainly seen enough of them this year.

Learn more about hurricanes at NOAA’s Hurricane FAQ page.

(satellite image of Tropical Storm Katia from NASA’s MODIS Rapid Response System on 31 Aug 2011.  Click on the image to see the original at Wikimedia Commons)

Last Friday afternoon our basement floor was briefly lower than the water table.

At 3:45pm Friday a violent thunderstorm moved so slowly over Pittsburgh that it dropped as much as 2″ of rain within half an hour.  Flash floods filled the streets, stranded motorists, and drowned four people on Washington Boulevard (tragic and awful!).

But I didn’t know that at the time.  I was at work, watching a huge muddy lake form over Central Catholic’s parking lot and football field.  Definitely not a good sign.  I decided to stay at my desk.

By 5:00pm the lake was gone so I went home.  On the way there I thought about the basement.

Our 104-year-old house is built into the side of a hill with the back of the basement below ground and the front above.  Though it’s not prone to flooding, very heavy rain (Hurricane Ivan) leaks in from above by following the electric and air conditioning chaseways.  When I got home I took a look.

It’s a good thing we’ve never “finished” the basement.  Its concrete floor was poured after the house was built and is uneven.  A minor flood had gone down the drain but I was able to mop up a gallon from the low spots.  Then I set up fans to dry the floor.

On Saturday morning I examined my handiwork.  The floor was dry but the base of the back wall was still damp.  Aha!  Some of the water was behind that wall.  How did I not notice this before?

I wasn’t paying attention.  The water table is the top of the zone where the subsurface is saturated with groundwater.  It rises and falls as groundwater is added — or not added — by rainfall and snow melt.  When it rises above the low spots on the surface, streams and rivers form.  On Friday afternoon before I got home a little stream was in my basement.

I’m glad I wasn’t there when it was happening.  I’m glad I wasn’t on the road either.

It’s not good to be in the zone of intermittent saturation.

(image from Wikimedia Commons. Click on the image to see the original.)

Ladies of my grandmothers’ generation never felt hot and sweaty.  They may have perspired but they never called it sweat.  Instead they “felt dewy.”

We’ve all been feeling dewy lately.  The humidity has been so thick in Pittsburgh that we’ve been cloaked in haze for days.  When we step out the front door at dawn it feels hot even though it’s only 73^oF.

Temperature has been no guide when choosing what to wear.  73^oF outdoors is oppressive; 73^oF on an air conditioned bus is freezing!

My niece Kelley, who grew up in south Florida, gave me a tip on what to listen for in the weather report:  Dewpoint is the number to watch.  If it’s 70^oF or more you’re going to feel hot.

The dew point is the saturation temperature, the point at which dew will form if the air pressure remains constant.

Our bodies cool by evaporating perspiration from our skin.  When the dewpoint is over 70^oF we perspire but the dampness doesn’t evaporate and we still feel hot.  The weather’s oppressive even though the temperature sounds comfortable.

Since Kelley told me about dewpoint I’ve been paying attention to it.  This morning it was 69^oF.

Dress accordingly!

(photo by Sam Leinhardt)

29 July 2011

Monday morning a storm was brewing when I looked out the window and saw a roofer walking on Central Catholic’s steep slate roof.  He and his crew had come to replace a few bad slates.

I have a healthy fear of heights and lightning so I was morbidly fascinated.  Would they be macho about the approaching storm or would they leave?

Two of them were up there when a brief downpour swept by.  The roof became slippery.  They sat down.

The next time I looked the crew was off the main roof waiting on a lower level while the boss walked the ridgepole.  He examined the approaching black cloud with professional interest.  There was lightning in it.  He pointed out the cloud’s leading edge to the crew as it slowly moved south.  Would he get off that roof?!?!

I know enough about lightning that I didn’t want to see what might happen.  I stopped watching.

In most years lightning kills more people than tornadoes and hurricanes combined.  (This year’s Joplin tornado turned that statistic on its head.)   Most people survive lightning strikes but have lifelong health problems afterward.  Most people are hit by lightning when it’s not raining — probably because they don’t take shelter unless it rains.

I avoid lightning and have learned that…

• The safest place to be is in an enclosed building that has plumbing or wiring (or lightning rods!) or in a car with a metal roof (not a convertible).
• Lightning hits tall objects (don’t stand under something tall; don’t be tall yourself) and it travels through the ground (don’t lie flat; don’t stand near metal fences).
• If I’m outdoors far from shelter I try to crouch in this position to protect myself. I’ve only done it once.  It’s so hard to do that it took my mind off of being scared.  (By the way, NWS stopped recommending the lightning crouch in 2008 because it doesn’t help.)

A friend once told me a harrowing story of being in a rustic cabin in Canada during a nighttime thunderstorm.  Lightning repeatedly hit the ground and came up through the floor.  It made the bedsprings glow.  Yikes!

My worst scare was the time I parked at the top of Laurel Mountain and hiked down the Tebolt Trail at Quebec Run Wild Area.  Thunderstorms were predicted to arrive at 2:00pm but I lost track of time.  At 1:00pm I heard thunder and knew it would take at least an hour to walk back up the mountain no matter how fast I went.  I started to walk and run uphill.  By the time I reached the top of the mountain the storm was quite close and I was praying and bargaining, “Not now! Please wait!”  I made it to the car, slammed the door and “BOOM!”  The lightning didn’t hit me but my heart sure beat fast!

I could never be a roofer.

And, yes, the roofers left before the storm.

p.s. Do you have any close calls with lightning?  As I said, I’m morbidly fascinated.

(photo by “jcpjr” from Shutterstock.com)

The weather has moderated a little, but it’s still hot and humid.

I’d like to go hiking but southwestern Pennsylvania has a 50% chance of thunderstorms today and I won’t go out there in lightning if I can avoid it.

Now it looks unavoidable.  The sky has become ominously dark as I write.

If the weather was merely hot I’d visit Cucumber Falls, pictured above.  The falls are part of Cucumber Run in Ohiopyle State Park and they’re easy to get to.  There’s a parking lot near the falls for a quick visit, or you can get a better look at Cucumber Run by hiking the Great Gorge Trail.  Hike upstream to feel the cool air in the creek valley or walk downstream to the Youghiogheny River where you can watch the rapids.  Here’s a map of the park.   (The map takes a while to download.  If it looks black, change the zoom and the map will appear.)

Unfortunately Ohiopyle is a 90 minute drive from Pittsburgh and I can’t see driving that far to wait in the car for a storm to pass.

I guess I’ll have to stay close to home and merely think cool thoughts.

(photo of Cucumber Falls by Caleb Foster from Shutterstock.com)