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Name: Terri W.
Status: educator
Age: 50s
Location: N/A
Country: N/A
Date: 1/10/2004

Do weather patterns (eg. dust storms) move from west to east on Mars as they do on Earth?

Terri W.,

The following information has been excerpted from Microsoft Encarta. I hope that it helps.

Although the atmosphere of Mars is very thin (1% of the Earth's atmosphere), extensive dust storms occur and are responsible for the color changes observed from Earth-based telescopes. The elliptical orbit of Mars causes the planet to have seasons of unequal lengths. For example, the southern hemisphere's summer on Mars is about 25 days shorter than the northern summer. The intensity of sunlight also changes substantially during the Martian year. Solar heating during the summer when Mars is closer to the Sun, is 40 percent more intense than in the northern summer. During the warmer southern Martian spring and summer, great dust storms have sometimes been observed through telescopes as bright yellow clouds. The largest of these storms can cover the entire planet and last for months. An unusually large dust storm covered the planet in 2001 and was the largest storm seen since 1971. Smaller local and regional dust storms can occur any time during the Martian year. There appear to be longer-term trends in the Martian climate, but scientists are only now beginning to untangle the complexities required to understand and someday maybe predict climate changes on Mars.


Bob Trach

D. B. -

I hope your friend's altimeter reads 10,000 feet at Leadville. If it does not he needs to get it fixed.

An altimeter is a pressure instrument just like a barometer, but it is calibrated in feet of altitude for a standard day. Yes, the air pressure does change with altitude. As you move up through the atmosphere the pressure decreases about one inch of mercury for every 1000 feet. That number is rather accurate up to an altitude of 18,000 feet.

The unit "inch of mercury" is used on barometers and refers to the weight of a column of mercury. Standard barometric pressure at sea level is 29.92 inches of mercury. With this pressure on a standard day the altimeter reads zero at sea level. (Your number of 20.50 in Hg at 10,000 ft is close. Using this formula, it would be 19.92 in hg.)

The altimeter in an aircraft has a correction window - the Kollsman Window - which allows the pilot to correct for non-standard pressure at sea level. (Even if you are at Leadville the sea level pressure can be calculated.) The number placed in the Kollsman Window is the present pressure that would exist at sea level. Pilots call this the "altimeter setting" and can obtain this number from FAA facilities allowing the correction even during flight.

The airport I fly from is 880 feet above sea level. If I have the correct altimeter setting in the Kollsman Window my altimeter indicates 880 feet when I am on the ground.

Larry Krengel

D. B.,

You are right and your pilot friend is incorrect.

The altimeter portion of his instrumentation indicates his altitude in the plane; this is usually determined from an equation based on the standard atmosphere adopted by the International Commission for Air Navigation. The equation gives an altitude based on actual air pressure (what meteorologists like myself call "station ressure").

The barometer portion of the altimeter may be set up to read pressure adjusted to sea level. At 10,000 feet the standard atmosphere station pressure would normally be around 20.5 inches of mercury, as you state. The barometer may use the altitude determined from the altimeter to adjust the pressure reading to sea level pressure. Your pilot friend does not see a difference in pressure at different altitudes if the barometer is making this adjustment. All this time he has been making an assumption (that there is not a radical difference in pressure with altitude) because of what his barometer corrected to sea level has displayed.

David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory

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