Published Tuesday, December 5, 2000, in the San Jose Mercury News

Atmosphere for climbing not uniform

Special to the Mercury News  

Q.  I climbed Cerro Aconcagua in Argentina in February. Nice trip. Since it is relatively close to the equator, some people say that the relative altitude
there is actually lower than at Mount McKinley in Alaska, which is close to the Arctic Circle. The elevations are 22,834 feet for Aconcagua and 20,320
for McKinley. What is the atmospheric pressure at each? I know that it took about four good breaths per step while climbing near the top. Bill Waggener - Denver

A. hat you have heard is correct, though without more specific information, such as the weather patterns at the time, I can speak only in generalities.

Roughly half the oxygen molecules in the atmosphere are below 18,000 feet. Because the atmosphere near the poles is colder than at the equator,
it is denser and therefore more compact; this means it's thicker at lower elevations and thinner at the high altitudes where mountain climbers

On a typical day the atmospheric pressure on Mount McKinley at an elevation of about 20,000 feet would be about 450 millibars, or about 45
percent of the pressure at sea level. On a peak near the equator, you would encounter roughly the same atmospheric pressure -- 430 millibars --
at a higher elevation of about 23,000 feet. 

So while these values are approximations, it would be fair to equate the amount of breathable air at 20,000 feet near the Arctic Circle to that at
23,000 feet near the equator.

Another factor to keep in mind is that the typical air temperature in the summer at 20,000 feet is about 22 degrees below zero at the Arctic Circle
and an almost balmy 14 degrees above zero near the equator. This is bound to affect the general comfort level of climbers.

Q.  I was pleased to discover your column during a recent business trip to the mid-Peninsula. I am wondering about sun dogs. Here in Houston we
see them frequently -- little patches of rainbow color to the left and right of the sun, about an hour before sundown or after dawn. I've read that they
are caused by sunlight refracting through ice crystals. If that is so, why don't they form a complete bow? David Throop - Houston

A.  It's true that sun dogs result from sunlight refracting, or bending, as it passes through ice crystals. These same ice crystals, which are in high-level cirrus clouds, can also create rainbow-like halos around the sun.

It's the shape and orientation of the crystals that make the difference. The crystals are a mix of different types. One type, shaped like a six-sided
plate, will reflect light to two spots on either side of the sun when it is oriented horizontally; this creates sun dogs. When these hexagonal crystals
are oriented in all different directions, a halo results.

Sun dogs are technically called parhelia and more informally called false, or mock, suns. For a graphic explaining this phenomenon see

Q.  What is the difference between waves, seas and swells? Ron Rose - Los Gatos

A.  All refer to the conditions of the water surface. The generic term ``wave'' refers to the undulations of the surface of a body of water, which are made up of a combination of sea and swell. It is sometimes also called ``combined seas.'' Swell results from the wind's past action on the water and has a gentler, rolling action. Even when the current wind is calm, you can see these undulations in the open water. The distance between successive crests is called the wavelength.

``Sea''' refers to the aspect of the wave that is the result of the wind blowing across the surface of the water and forming distinct features, such as crests. And surf is the interaction of the waves and the shoreline. It is influenced by the height and energy of the waves as well as the underlying topography of the coast. A beach with a gentle offshore slope will have small surf; one that drops off sharply will have larger surf. This interaction generally begins when the depth of the water is about one-half of the wavelength of the wave.

Q.  Why are precipitation amounts different in Los Angeles, Seattle, Denver and Phoenix? Carly Brown - Turlock

A.  Numerous factors influence the amount of precipitation in a location, and the four cities you mention highlight some of them. Three of the more
important ones are a city's latitude, distance from the ocean and elevation. 

In the United States most of our precipitation-producing weather systems come from the west and are carried along by undulations in the polar jet stream. The jet stream is stronger and more prevalent in the higher latitudes, thus bringing more storms to northern locales.

Western cities near the Pacific Ocean tend to get more precipitation than those inland because mountain ranges such as the Coast Range, Cascades,
Sierra Nevada and Rockies wring moisture out of the clouds as they are lifted over the mountains.

Finally, elevation determines whether precipitation will fall as rain or snow.  Average annual rainfall at the four cities is as follows: Los Angeles, 14.83
inches; Seattle, 37.92 inches; Denver, 12.55 inches; and Phoenix, 7.95 inches. The annual average at San Jose is 14.53 inches, and at San Francisco, 20.91 inches. 

Jan Null, a certified consulting meteorologist and owner of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send him questions c/o Weather Corner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone and fax them at (510) 657-2246 or e-mail them to

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