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Breathing down deep, freezing up high, blinking in the dark 

Q: When SCUBA diving at 33-foot (10 m) depth, air pressure in your lungs is about twice that at sea level, which means your lungs have twice the amount of oxygen. Why can't you hold your breath twice as long? Mike, Benton, Tennessee

Scuba diver photographing the coral reefs of St. Croix, US Virgin Islands [OAR/National Undersea Research Program (NURP)]

A: It’s true that a scuba diver has twice the amount of oxygen as at sea level. You can’t hold your breath twice as long, though, because you didn’t evolve under that much water (at least not in recent eras). It’s the carbon dioxide that does you in.

The diver breathes from a tank that’s highly pressurized (200 times sea-level pressure) and through a regulator that allows her to get air into her lungs at the same pressure as the water around her. Thirty-three feet down, the pressure is twice the pressure on the surface. So she breathes in air that has twice as much oxygen as surface air. Her lungs stay about the same size as they were at the surface, though, because the surrounding water pushes in with the same pressure as lung air pushes out.

Our diver has twice the number of oxygen molecules as she would at the surface. Now, she holds her breath as a test of how long she can do it. She "runs out of breath" at about the same time as she does at the surface even though she has twice the oxygen. She starts breathing again from the compressed air tank. (That’s important because holding her breath while rising to the surface would rupture her lungs.)

Sure, her body has plenty of oxygen to do more work when she can’t hold her breath any longer. Oxygen, however, isn’t the trigger. It’s the partial pressure of the carbon dioxide in her blood that stimulates breathing. When that waste gas builds up too much (twice the pressure that it normally is on the surface), the central nervous system alarms the body.

She feels like she’s suffocating and, eventually, her diaphragm convulses. Living on the surface set the value that triggers all this. So the set value is the same (0.08 atmospheres) whether you hold your breath on the surface or down 33 feet. You can’t hold your breath longer scuba diving. Your body’s convinced it’s in trouble before it really is.

Further Surfing:

Diving Medicine Online

Freediving Team of Finland: Breathing and blood circulation

Scuba diving explained by Lawrence Martin

Dive Fort Young: Air pressure

Freezing up high

Q: Why is it colder in the mountains than in the valley? The mountains are closer to the sun. Marissa, Mountain Home, Idaho

A: The Sun’s rays pass right through the atmosphere. Earth’s surface absorbs them and re-radiates the energy in the form of heat that air can absorb. Thus, the Sun warms the air by warming the ground. Air closest to the Earth’s surface is, in general, warmest. Air as high as a mountain is poorly warmed and, therefore, cold.

Mountains stick up like islands in the cold atmosphere [Sean Linehan, NOAA]

Air temperature normally drops 1.8 to 3.6 degrees Fahrenheit (1 to 2 degrees Celsius) for each 1000 feet (300 m) of altitude.

Mountains poke up through the atmosphere like islands above a sea. The temperature of the enveloping atmosphere cools mountain ground even though — you’re right — the Sun warms the high ground. The influence of atmosphere temperature on a mountain is similar to that of the sea’s temperature on an island. The higher and the more isolated the mountain, the closer its temperature is to the cold air around.

Mountain weather, however, is fickle — changing like a shifting kaleidoscope with every passing cloud or gust of wind. The soil surface on a mountain can get hotter than the valley soil surface because of the Sun’s greater intensity up high where the air is thin and clean.

Mountain soil can be as hot as desert dirt. In the Alps at 6800 feet (2070 m), the soil temperature one day shot up to 180 degrees F (80 degrees C) on a dark humus slope near the timberline. The slope faced southwest at a gradient of 35 degrees. Dark humus soil absorbs the Sun’s energy better than light sandy soil. A southwest-facing slope of 35 degrees in the Alps receives the Sun’s rays more directly than does flat ground.

A mere five feet above the warm surface, however, was frigid, cooled by the surrounding atmosphere. Alpine plants grow low to the ground — to stay warm.

Further Surfing:

University of Colorado: Mountain climate by A. Bach

Blinking in the dark

Q: What is the name of the brightness star? Dwight, Warren, Ohio

A: Sol at -26.74 apparent magnitude (Definition: http://www.wonderquest.com/000-definition-apparent-magnitude.htm) is the brightest star as seen from Earth. The next brightest in apparent magnitude (-1.46) is Sirius A, which is the seventh nearest star to us. (Proxima Centauri is the nearest, not counting Sol [4.22 light years].)

Sirius A is the bright star in the middle of the image. On the right, slightly below Sirius A, is the optically faint white dwarf star, Sirius B. [McDonald Observatory]

Sirius A is a bright white star (twice as massive as the Sun) that’s part of the constellation, Canis Major. Sirius lies in a descending line from Orion’s Belt.

The dog days of summer get their name from Sirius, the Dog Star. In the summertime, Sirius appears closest to the Sun. "Ancient skywatchers thought the heat from Sirius and the Sun combined to produce the year’s hottest weather," reports the StarDate radio program of McDonald Observatory.

By the way, Sirius A has a dim companion star, Sirius B, which, in 1862, was the first white dwarf discovered. Sirius B eluded detection because the two stars are so close together. The white dwarf (B) is as massive as the Sun but only 90 % the size of Earth, with a gravity 400,000 times our planet’s. Imagine a 180-pound Earthling on Sirius B. He would weigh 72 million pounds (23 million kg).

Further Surfing:

McDonald Observatory: All about Sirius

(Answered Feb. 27, 2004)