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Old 05-13-2001
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Michael Carr is on a distinguished road
Monitoring Weather

In the Northern hemisphere, without the benefit of instruments, how can we be relatively certain that the winds we are currently in are the result of a high-pressure system and not the leading edge of a low-pressure system? 
Michael Carr responds:
Great question! It's interesting that you should ask because this is something that always comes up in mariner weather courses, in fact it's a topic that I covered recently in a seminar on Using Ocean Currents and Weather to Win.
Here is how you tell the difference between winds associated with low pressure or high pressure:
1. Monitor surface pressure readings. Normal sea level pressure is 1013 mb. If pressure is lower than 1013 and falling, a low is approaching; if it's above 1013 and rising then a high is approaching.
2. Cloud cover. Clouds thicken and lower with an approaching low, while they dissipate and provide clear skies with high pressure.
3. Relate the present weather you're seeing to the weather pattern. If you have been under the influence of  good weather, i.e. high pressure, for the past several days and winds have been SW (we are talking about any venue on the US East Coast), then a shift of winds to NE, accompanied by increasing cirrus clouds, a lowering barometer and increasing winds, is no doubt a developing area of low pressure. The reverse is true of a high-pressure system approaching. If the conditions are clearing and you experience a dissipation of clouds and a shift in the wind to a northerly direction, that shows the arrival of high pressure from the west.
The very best way to keep tuned in and maintain good situational awareness of the weather is to look at the Marine Prediction Center surface and 500-mb charts each day. These are available via weatherfax and the Internet. 
Without some instruments you will always have a difficult, if not impossible time sorting our the differences between local weather (sea breezes, etc.) and larger (synoptic scale) weather events. At a minimum you need a reliable and calibrated barometer, and at the high end you need satellite imagery, and easy access to charts via weatherfax or satcom systems. I hope this helps.
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