One question we hear a lot from people who buy a heart-rate monitor like Suunto's t6 is, "Why the hell do I need a barometer on this thing?" Well, it's there—among other reasons—because it could save your hide from a torrential afternoon downpour.

Think about it: If you're on an all-day bike ride, hike, or multi-hour training run, you'd like some advance warning that the darkening skies overhead are indeed going to dump instead of pass on. That way, you can seek shelter or head home sooner rather than when it's too late. That's where the barometer comes in.

By checking the barometric pressure (a number usually between 28.00 and 31.00 Hg, or inches of mercury) throughout your workout, you can get a good sense of how the weather's going to play out. According to Dennis Feltgen, a meteorologist with the National Oceanic and Atmospheric Administration (NOAA), a rapid fall in pressure—up to one tenth of an inch in less than an hour—may signify the impending approach of a strong to severe thunderstorm. If the Hg reading holds steady or rises, then the weather is getting better, and you may be alright.

As another reference, NOAA still refers to the rough observations from the British Rev. Dr. Brewer's 1848 tome, A Guide to the Scientific Knowledge of Things Familiar, which states the following:

• In very hot weather, a falling barometer denotes thunder or high winds.
• In sub-freezing weather, a falling barometer denotes thaw.
• In already wet weather, a falling barometer means much more wet.
• In fair weather, a barometer that falls and remains low means much wet in a few days.
• In sub-freezing weather, the rise of a barometer presages snow.
• In a dry period right after rain, if the pressure rises suddenly, fine weather will not last long.

To give you an idea of the extreme ends of the Hg band, check out these records listed by NOAA: The lowest recorded Hg reading was 25.69 Hg found in the eye of a typhoon over the Pacific Ocean. The highest recorded reading was found in Siberia at 32.06 Hg. So what pressure is "just right"? Well, that would be the mean sea-level pressure, which is a constant of 29.92.

Train right with tips and tricks and of the trade from Chris Carmichael and Carmichael Training Systems, at www.trainright.com.