Barometric pressure data is important because it gives the hiker an idea of how the weather is developing and changing. Monitoring barometer using GPS allows you to know more about your weather in the back country My personal choice for a GPS is the Garmin GPSMap60CS receiver, and here’s what I do.
In the back country, keeping yourself aware of pressure changes gives you more time to plan, improvise and re-evaluate your situation. Today, I continue to keep a sharp eye on the weather before an outing and while in the field. At home I frequently check my internet sources such as Wunderground.com and the National Weather Service’s site and watch the weather reports each morning. At a base camp I have a radio capable of receiving the NOAA broadcasts.
My “go to” reference is Northwest Mountain Weather by Jeff Renner (published by the Mountaineers). Renner is a professional meteorologist and broadcaster, 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 provides 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 to provide a Pressure Plot. (Select and push the menu button one time on a 60CS/CSx.) 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. A key point to remember: 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 watching out for. Remember, the time frame is a three hour observation period.
Watching barometric pressure is just part of the weather puzzle. Begin by planning at home and listening to the NOAA and TV broadcasts. In the field, the GPS receiver’s display, coupled with observations of cloud type, temperature and wind helps the hiker prepare for changes in the atmosphere.
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