Map, Compass & GPS

Map, Compass & GPS
Wild flowers along Fall Creek on the way to the Green Lakes - Oregon

Tuesday, January 24, 2017

Monitoring Backcountry Weather - Part 2

I use my GPS receiver to provide barometric pressure information while hiking in the backcountry.  Barometric pressure data gives the hiker an idea of how the weather is developing and changing.

My weather training began while serving as a deck officer aboard navy destroyers in the 1970’s.  I was specifically directed to monitor the atmospheric pressure while at sea.  In the days before satellite imagery and modern meteorological equipment, barometric pressure was recorded and monitored hourly with weather reports sent to the fleet forecasting center approximately every four hours.   Aboard the ship, the Captain’s standing orders required that I notify him if the barometric pressure dropped more than .04 inches of mercury in four hours; this was a big deal.

As an outdoorsman, I continue to keep a sharp eye on the weather while afield.  At home I frequently check my internet sources such as 
WeatherUnderGround  and watch the weather reports each morning.  In the camp I have a radio with the NOAA broadcasts. 

I also enjoy reading information about the weather.  An excellent reference is Northwest Mountain Weather by Jeff Renner (published by the Mountaineers).  Though out of print, copies are available online through at $0.99 each (not including shipping.)  Renner is professional meteorologist and broadcaster.  He is an outdoorsman and flight instructor.

Renner’s book provides a superb overview on how “the weather works” in the Pacific Northwest.  Uniquely focused to this region, this book provides an overview on the climate and weather, local weather patterns, snow and avalanche conditions and many charts and data sources. 

My personal favorite is Chapter Seven’s “Field Forecasting Guidelines.” This chapter identifies how to watch for and monitor weather system changes. In the subsection “Clues from Pressure Changes” Renner states:

“Remember that a pocket altimeter can give excellent indications of an approaching weather system.  An altimeter that registers an increase in altitude, even though none has taken place, is actually reporting a drop in air pressure.  Changes in pressure create changes in wind and are often related to approaching fronts that may bring precipitation.”

The comments regarding the pocket altimeter apply equally to a GPS receiver equipped with an altimeter.

Following this discussion is a short table regarding pressure drop.  The following is an excerpt and is what I keep an eye on:
Many of the newer Garmin receivers have barometric altimeters.  The Altimeter display can be adjusted provide a Pressure Plot.  An example is shown below.

The green plot illustrates the trend of pressure change over time.  This plot was developed over a 40 hour period for illustrative purposes.  The receiver remained powered up the entire time.  (Turning the receiver off or changing batteries erases the historical data.)

Pressure measurements need not be accurate, it is the trend of information that the hiker is interested in.  A near vertical drop in a short period of time is what we are looking for but hope to avoid: time to look for shelter.

Coupled with observations of the cloud type, temperature and wind, the barometric pressure plot will give the hiker information about how the weather is changing.

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