Can Barometer Accuratly Predict the Weather?

[danstainer]

I'm looking for a good camping watch that will tell me whether or not I'll need to pitch my tent rain fly before going to bed.

I'll be using the watch mainly in a stationary, rather than mobile, location (campsite, not on the trail).

I have a few questions:

1. How accurate is the barometer in predicting weather? Could it tell me with some degree of accuracy if a weather system is approaching?

2. How does the X6 compare with a handheld weather device, such as what Brunton offers?

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[john]

danstainer,

For what its worth, the watch will record pressure changes. How you use this to create a short term forecast is up to you.

To make an accurate forecast you will need some basic meteorological knowledge. There are a few reasons the pressure is dropping (besides you are increasing your elevation):

1. Lower pressure is approaching (as a front, low circulation, trough of low pressure)

2. An area of low pressure is not approaching but deepening (lower central pressure) and dropping the pressure where you are.

3. Or both 1 and 2, in which case an approaching and deepening low would cause a greater pressure drop ( and higher winds and heavier rain or snow). The greater the drop the greater the winds will blow (usually from the south (and warm) ahead of a cold front turning to the northwest (and cold) upon passage).

4. In the case of 3 above, the weather should rapidly deteriorate and the pressure should continue to drop rapidly until the weather hits, then rise rapidly (cold air is heavy)

Pressure may drop due to temperature changes, either at the surface or aloft. Since you are not at 10,000 feet we won't worry about what goes on above you.

1. Warm air advection (usually a southerly component wind) brings warm air (which is lighter) causing a pressure drop.

2. Moist air advection (off an ocean or large lake) causes a pressure drop (moist air is lighter than dry air).

3. Or both 1 and 2, in which case a warm, moist surface wind will drop the pressure more rapidly than either alone.

4. In this case you may have stronger southerly moist winds ahead of an approaching cold front. Expect deteriorating weather in 12-24 hours. In the meantime enjoy the warm, sunny, moist air before the rain hits.

5. The pressure also changes due to diurnal (daily) air temperature changes (from solar heating). Your local sun time (early morning) will have the coolest temps and a slightly higher pressure. The late afternoon will see a slight pressure minimum, with pressure increasing slowly as the sun sets and into the night. The effect will be lessened if the sun is not heating the air (cloud cover) and greater drops occur in the summer (warm air is lighter) and much greater rises in the winter (cold air is very dense). So don't be concerned by gradual pressure drops during the day, they are normal on a sunny day.

6. The diurnal changes may work against changes due to approaching weather systems. That is, an approaching low pressure system (cold front, low circulation, trough) would drop the pressure but the diurnal night changes raise the pressure slightly. Together you may find no change or only a slight drop. You should keep this in mind when figuring actual pressure changes (subtract it out mentally).

7. Some other temperature rules: If the ground is wet (it rained), the air will have to evaporate the water first before it can appreciably warm the air. So, don't expect real warming until the gound has dried out. Cloud cover at night traps the warmth like a blanket, expect warmer night temps (and probably no dew). Clouds in the day block the sun, expect cooler temps.

8. Fast moving low clouds indicate high winds aloft: Expect gusty winds. If the morning air is cool and calm (despite the fast moving clouds) the winds may be held aloft (due to the denser air). As the morning progresses the air warms and the inversion breaks -- around 9-10AM the high winds drop down and suddenly begin to blow. Sometimes, a sharp outline of the horns of the moon (at night) indicates high winds (-- when the inversion breaks in the morning you may get those gusty winds).

9. Near the ocean or large lakes, the wind will blow lightly from lake to land during the day and from land to lake at night. This is called a sea breeze. Hot air rises over land and is replaced by the cooler water air, reversing at night when the land cools off and the water remains warmer. Got it?

10. In the morning the wind may blow gently up a mountain until about 10AM. In this case the sun is warming the mountain top and air is rising and being replaced by the valley air. At night the mountain top air will cool first and drain downhill creating a slight nighttime breeze into the valley. Don't be fooled by these local winds (9 & 10) when predicting weather changes.


To use the watches pressure graph accurately you'd need to weigh other weather parameters into the equation (wind & clouds). Here are some basic rules:

1. Winds switching to the south - temperature will rise (warm air advection). North winds, temperature will fall.

2. Falling pressure with thickening clouds and wind shifts to south or southeast and with temperature rises: Expect rain in 12-24 hours (may not have all these parameters, the more the better).

3. Rising pressure with breaking clouds and wind shifts to west or northwest: Fair weather next 24 hours, probably cooler.

4. Steady pressure, west wind, morning fog breaks early, frost or dew at night: Continued Fair weather.

Other empirical weather rules:

1. Stand with your back to the wind: The low (bad) weather will be to your left and the high (good) weather will be to your right.

2. Stand with your back to the wind. If low clouds (not cirrus) move from your left the weather will deteriorate. If low clouds come from your right weather will improve. If clouds move with the surface wind direction (either way) expect no change for next 12 hours.

3. At night, if you see halo (rings) around the moon (caused by ice crystal cirrostratus), expect rain in 24-48 hours.

4. If you see cirrocumulus (i.e., Mackeral sky: looks like tiny puffs of very high cloud) the air aloft is unstable and bad weather is approaching (next 24 hours expect rain).

5. If you see altocumulus castellanus (white puffy mid-level cloud that looks like small towers in a row), the air is unstable. In the summer expect thunderstorms by afternoon. You may also see towering cumulus (puffy low cloud with vertical extent), unstable air is causing the air to rise, expect thunderstorms in the afternoon.

6. Cumulus that has dark, raggedy edges is dissipating (usually with sunset). Cumulus that is white with rounded edges is building (watch for thunderstorm development in the afternoon).

7. You can get the distance to a thunderstorm -- count the seconds from lightning flash to boom -- divide by 5 to get distance in statute miles. The average speed of a thunderstorm is about 30 mph. Thus, at 0.5 miles per minute, expect your average approaching thunderstorm to reach you in double its distance. That is, a thunderstorm 10 miles away (50 secs between flash and boom) will reach you in about 20 minutes (10 miles doubled = 20 minutes). Of course some thunderstorms move slower (and some faster) so watch out. Generally, thunderstorms from NW-W-SW will come close or hit but those east or south will pass by or move away. You may be hit by strong outflow winds ahead of the storm even if the thunderstorm misses (the storm may be many miles away, this is especially true in the hot, dry Southwest US areas where thunderstorm bases are high and cool outflow drops out as the storm dissipates in the afternoon).

8. A green hue in thunderstorm clouds indicate extreme vertical height (refracting light downward) and are linked to hail and extreme wind gusts (downburst winds).

9. To test your watch: Count cricket chirps in 15 seconds. Add 39 to number to get air temperature (at cricket level) in Farenheit. It works if you have crickets available (i.e., not in the winter).

10. One of the basic forecasting techniques is persistence. Use persistence: If the weather was good yesterday and the wind, sky, and pressure haven't appreciably changed: Forecast continued good weather for the next 12-24 hours. Don't forecast change without a good cause -- that is the rule of persistence.

If you get really good, you can toss dirt in the air and point in the direction where a F5 tornado will form (if you believe that you watch too much TV).

PS. I've been a meteoroligist for 24 years -- most of these rules will work if you get good at interpreting the weather signs correctly. Most of the skill is in pattern recognition for your area, anyone who is aware of their surroundings can make a good 12-24 hour forecast. The watch is an important part, but consider all the evidence and trends (clouds, pressure, winds, time of year) for the best forecast. Or just watch the NWS forecast on TV, it is probably no better than yours ... forecasting is hard beyond 48 hours even for the experts.

Hope this helps you,

john

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