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A wrist-top barometer is one of the most useful features you can get on an outdoor sport watch. While the specific measurement of air pressure may not be especially useful to you while you're hiking, a barometer is needed for altimeter functions, and for forecasting the weather.
Altimeter...
When you are hiking, your watch's barometer reading will constantly change. The change may be due to two factors: weather change, or change in altitude. If the barometer reading falls, ie pressure drop, this indicates a rise in altitude (the air gets thinner the higher you go), or the approach of a low-pressure system which could mean bad weather is coming. So which is it? This is when calibration of your watch's barometer & altimeter is so important, combined with a good topo map.
Before you leave on your trip, consult your local weather office for the exact barometric pressure reading, and set your watch accordingly. Your altitude can also be obtained from local city data. Check online. For short hikes, you can typically trust that any change in barometer reading is due to altitude change (unless you are hiking on flat terrain of course). Your watch will display the barometer change, and will also show your new altitude.
For longer hikes, you will need to use your map's topographical information to continually keep your watch calibrated. Be sure to take a notebook with you so that you can record readings of both altitude & barometric pressure. For example, if the weather changes overnight, your watch will assume that the change was due to a change in altitude, and this will show on the altimeter. But you can correct for this by setting your altitude to the same reading you had the night before (or that your map shows for your position).
If all this seems a little confusing, consult your watch manual, and be sure to take the manual with you when you practice using the watch.
Thermometer Readings
Take yourself out of the equation...
Your body temperature is 37C (98F), and even on your wrist, this can affect the temperature reading of your watch. So be sure to take your watch off for a few minutes for the most accurate reading.
Compass Watches
An excellent quick reference...
Let's face it, it would be difficult for a watch's digital compass to match the accuracy of a good map compass. So the practical application for digital watch compasses is as a quick reference tool. Rather than drag your map & compass out, or waste valuable GPS battery life, simple hold your wrist flat & press the button to make sure you are still on course.
Atomic Timekeeping
Nothing radioactive about it...
The US department of commerce broadcasts a radio signal out of Fort Collins, CO that contains a time & date signal. The exact time broadcast is based on an atomic clock that is accurate to billionts of a second. Clocks and watches that are able to receive this time signal automatically synchronize themselves and therefore never need to be set.
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