Expert: Ned Dolan - 2/17/2004

Question
A friend and I have an argument.  I claim that commercial planes must fly lower in cloudy / stormy weather because I notice them more when the weather is this way.  I know really nothing about flight, but imagine that these planes are trying to get under the clouds in order to better see the landscape and the upcoming airstrip.

He's never been a pilot but was an air traffic controller for the U.S. air force in Africa in the 1950's.  He says that, especially nowadays with modern instrumentation, planes have no need to be physically lower in order to better see the landing strip or anything else -- everything is essentially done by the instruments.  I must be experiencing an unusual visual / auditory effect.

In retrospect I can agree with him somewhat.  I noticed once that an overflying jet's sound volume went up and down suddenly and dramatically -- and it seemed to me in that case to be due not to engine revving but rather to the fact that the clouds at that point were patchy.  I can imagine that moist/cloudy air might amplify sound and that may account for why the planes sound lower.

But, when I can glimpse the plane, the LOOK larger (closer) than usual.  I feel confident that planes I see overhead are consistently lower when it is cloudy / rainy than when it's clear.  There could be a visual effect here, that cloud makes the plane seem bigger and thus closer, but I feel it couldn't account for how much closer they actually seem to me.

I have notice this effect where I live -- Berkeley, California (about 10 miles north of Oakland International and 15-20 miles northeast of SFO) but also in Walnut Creek, California (farther away, about 15-20 miles west of Oakland and 30-35 miles west of SFO).

Do they fly lower or is this all an effect?  Or, possibly, do they always fly that low, but change their flight paths to fly over Berkeley and Walnut Creek during storms?  I have heard that yes, airliners will get down under cloud cover before landing, but only RIGHT before landing.

Help!

Answer
Tim, sorry, but I can't back up your theory. As a general rule commerical aircraft fly a 3 to 1 descent profile as they approach an airport. That is, they will be at 9000 feet when 27 miles from the airport and 6000 feet when 18 miles out etc. This can vary of course depending upon the requirements of the air traffic situation. ie: several airports in close proximity.

As we start what is called an approach (usually within 5 miles) we are at 1500 feet when 5 miles out and 900 feet when 3 miles out. This descent profile during approach is very standard.

It may be that the very humid air found on cloudy days is more conductive of sound waves than is dry air, but our descent profiles don't in any way depend on the presence of absence of clouds except in very unusual cicumstances which would not occur in SFO or OAK, but only in very undeveloped airports.

Hope this helps.  Ned