The electronics now available for outdoor use have increased the number of safety tools in our kit and improved the potential for good weather decision-making. It’s tempting to focus on such electronics and related ‘apps’ to the exclusion of more traditional tools such as the barometer. I strongly advise against that temptation. In this blog, I’d like to focus on the importance of considering changes in the air pressure.
Such pressure changes can offer us valuable guidance in making good decisions-whether we’re on the water or in the mountains. Examples would include anticipating large scale trends; the approach of high pressure, typically followed by clearing skies and decreased precipitation, or the approach of low pressure with increased clouds, wind and the onset of precipitation. Simply put-weather you may not want to experience in the outdoors!
Some boats, such as the sailboat I race on (a J-120), have traditional aneroid barometers-the big mounted brass dials. However, with some good apps available for smartphones, you can always get air pressure information-on the water, in the field or in the mountains.
There are no absolutes as to which sustained pressure drops are significant, but the following guidelines have proven useful in my experience, both in the forecast office and outside. Let me add that checking your barometer every 3 hours will usually be enough to allow you to catch an approaching disturbance while you can still take action.
3 hour Pressure Change (inches of mercury/millibars): 0.02 to .04”/.6 to 1.2 mb.
Possible Conditions: Fair Weather Likely
Suggested Action: Monitor sky, wind speed & direction. Normal vigilance.
3 hour Pressure Change (inches of mercury/millibars): 0.04 to .06”/ 1.2 to 1.8 mb.
Possible Conditions: Potential approach of front or disturbance
Suggested Action: Look for lowering/thickening clouds, increasing winds, wind shift to the east or southeast
3 hour Pressure Change (inches of mercury/millibars): 0.06 to .08”/1.8 to 2.4 mb.
Possible Conditions: Likely approach of disturbance, possible Small Craft Advisories: winds up to 33 knots/38 mph/61 kph.
Suggested Action: Begin checking barometer, air pressure changes hourly. Continue to look at changes in cloud cover, wind speed and direction.
3 hour Pressure Change (inches of mercury/millibars): 0.08 to .19”/2.4 to 5.9 mb
Possible Conditions: Gale Force winds likely: 34-47 knots/39-54 mph/62-87 kph.
Suggested Action: Seek safe harbor immediately.
3 hour Pressure Change (inches of mercury/millibars): 0.20”+/6.0 mb.+
Possible Conditions: Storm Force winds likely: 48 knots+/55 mph+/88 kph+
Suggested Action: Seek safe harbor immediately!
I can’t overstate the value of tracking pressure changes. Fast moving weather systems won’t always give you a lengthy warning in the form of lowering and thickening clouds, but there will almost always be a significant and sustained change in air pressure.