Ask A Scientist Weather Archive

name         Terri W.
status       educator
age          50s

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

Sincerely,

Bob Trach
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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
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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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