 Sea level changes
readingsI know there is considerable elevation difference between
Klamath Falls and Medford (about 3,000 feet). I have been taught that a barometer and an
altimeter are the same instrument with different printed scales behind the needle. Yet we
often see similar barometric readings for both locations on the local weather feature of
our evening news. Sometimes the barometer reading in Medford is less than the reading for
Klamath Falls. I hope you can explain this discrepancy.
-- Gary S., Medford You're right about the relationship between atmospheric pressure and altitude, Gary. What's confusing you on those TV weather forecasts is the common practice of correcting atmospheric pressure to sea level. At sea level the weight of the atmosphere will support a column of mercury 29.92 inches high. Murray Orr, at the National Weather Service station in Medford, explained that the height of the column drops about 1 inch for every 1,000 feet. So in Medford (elevation 1,300 feet) the atmospheric pressure would be about 1 inch less than at sea level, and in Klamath Falls (elevation 4,300 feet) the atmospheric pressure would be 3 inches less than Medford's. If all the atmospheric pressure readings were made solely on the basis of local altitude, it would be hard to make comparisons about weather events in nearby cities, such as Medford and Klamath Falls. By adjusting each local area's atmospheric pressure to sea level, forecasters can make meaningful comparisons about pressure changes that herald changes in the weather. That's why the (corrected) barometric pressure may be lower in Medford than in Klamath Falls. (Send your questions to "Since You Asked," Mail Tribune Newsroom, P.O. Box 1108, Medford, OR 97501; or by fax to (541) 776-4376; or by e-mail to: youasked@mailtribune.comPlease include your name, address and phone number for verification.) |
