University of Washington
Atmospheric pressure
J. E. Tillman & N. C. Johnson

• Each pressure data point is a 1-minute average of 30 values ( 2 second sample interval, resolved to 0.01 millibars). The top frame contains the last 240 points of data taken. This is normally 4 hours of one-minute averages. If these points include gaps of any kind due to program failure, the time span will be greater than 4 hours.
• The second frame is always 1 week in length and the last day is always the current day.
• The third frame is a plot of a 60 point running mean of slopes of the pressure time series, d(Pressure)/d(Time).
• The abscissa times are Seattle local time and no adjustment to sea level pressure has been made. The approximate adjustment to obtain SLP is to add 4 millibars (note: this varies with atmospheric conditions).
• The range of the ordinate scale for the first and second plots is a flexible quantity that varies with the range of the pressure data. It changes at midnight when appropriate and whenever the data approaches the upper or lower limits. The ordinate range of the third frame is coded to be 2, 5, or 10 millibars.
• Continuous operation of this sensor began May 21, 1993. It has been measured in its current location, approximately 33 meters above sea level since July 1993.
• It was moved approximately 3 meters to a new location in the same room, maintaining the same height to within 1 cm, on 29 July, 1996.
• The two Vaisala barometers were moved again, maintaining the same height to well within 1 cm, on November 1, 2006.
• The Vaisala instrument directly produces ASCII in CGS units or TTL output.
• Sampling, averaging, and other operations utilize a Campbell Scientific, Inc., 21X programmable data logger.
• These plots were created using gnuplot, courtesy of the Free Software Foundation.
• Meteorological analyses and pressure sensor intercomparisons are routinely (see weekly and annual plots) carried out using the statistical software Splus, courtesy of Statistical Sciences, Inc. a division of MathSoft.
• This page provides dynamic refresh, which will include the most recent pressure, via a client pull every 5 minutes to compatible browsers.