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Old 14-10-2005, 13:10   #1
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Weather Basics

The current issue of Good Old Boat magazine
November/December 2005 (#45)
has several interesting & informative articles, including:
Weather Basics
Going cruising ? Here’s how to read weather maps
by yours truly - I know, I’m truly shameless

BTW: You can get a FREE sample issue of “Good Old Boat” magazine at:
http://goodoldboat.no-ip.com:8080/GOBWeb/GOBSamplers/

Hope it’s helpful,
Gord
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Old 15-10-2005, 12:10   #2
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Timely article. Sent you a PM.
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Old 16-10-2005, 06:59   #3
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correction / clarification

October 16/05

Letter to the editor (Karen Larson)

Weather Basics - Correction re: Continuity, Persistence, & Trends

Karen:

I’ve just reviewed my article on ‘Reading Weather Maps’ (GOB Issue 45 - Nov/Dec ‘05), and note that I misused & failed to explain the term the “principle of continuity”* (following ‘Isobars’ page 36), wherein I said:

“... If previous surface charts are available for the last day or two, you will be able to predict the movement of weather systems over time, based upon the principle of continuity ...”

One of the most important techniques for making a forecast is to use weather maps to estimate the speed of movement of air masses, fronts, and high and low pressure systems. There are a variety of simple forecasting techniques, the easiest ones being “Persistence” and “Trend”.

PERSISTENCE suggests that tomorrow’s weather will be same as today’s weather
"What you see, is what you get."
Persistence forecasts are generally good only for short periods of a few hours, and become less accurate as the time period lengthens.
In the tropics, especially near islands, where day after day the weather is basically the same, (because the location is affected by the same air mass, with no passages of fronts), a persistence forecast that tomorrow is going to be the same as today ,is usually fairly accurate.

A TREND describes a phenomenon in a steady state, or one that’s moving at a constant or predictable speed. So, if the distance traversed by front is known, over a given time period, it's position can be extrapolated in time, by the formula:
Rate x Time = Distance

The Trend forecast is based on the assumption that changes will continue at the same rate they have been occurring. Thus, if a cold front, located 450 - 500 nm to your West, is approaching your location at 20 kts, then it will continue to move at 20kts in the same direction - so, you might extrapolate it’s arrival in about 24 hours.
Similarly, if a cold air mass is moving toward the station and temperatures at stations within the air mass are dropping at one Fahrenheit degree per hour, then temperatures at your location may drop at one Fahrenheit degree per hour.

Warm fronts usually move at an average speed of about 10 knots (or 240 nm/day).
Cold fronts usually move at an average speed of between 15 - 20 knots (or 360-480 nm/day).

Analyzing previous weather maps, you can adjust the averages to more
closely reflect current (& likely future) frontal speeds - and determine a likely time of arrival at your location.

Because weather patterns (in the Northern Mid-Latitudes**) generally move from West to East, storm systems generally approach from the west. Tomorrow’s weather is possibly between 200 - 500 nm West of you right now.

** Winds in the tropics & semi-tropics generally blow from East to West.

Stationary fronts will either dissipate after several days, or change into a cold or warm front.

* The “principle of continuity” (law of the mass continuity - empty spaces are not tolerated in fluids) is actually a restatement of the principle of Conservation of Mass, as applied to fluid dynamics in the atmosphere.

Remember to adjust your forecast for differences in latitude, possible acceleration/deceleration, or intensification/deintensification of storm systems, and local effects such as topography, bodies of water, and the land-mass heat-sink effect.

Northern sailors - look to the West for tomorrow's weather, and finally , don’t forget to look outside.

I hope this helps clear up any confusion I may have inadvertently created by my misused terminology, and failure to more fully explain my intent.

Best regards,
Gord May
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Old 02-11-2005, 02:16   #4
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Weather Lessons at Bluewater Books & Charts (Ft. Lauderdale):
Bluewater gives you the opportunity to learn about weather in the Bahamas from the very best: Chris Parker*. He'll be in town in December, while you're waiting for a
weather window to cross the Gulf Stream.
When:
Friday, December 16, 2005
6:00 PM to 7:30 PM
Where:
Bluewater Books & Charts
1811 Cordova Road
Ft. Lauderdale, FL 33316
Info Line: (954) 763-6533, ex 228
http://www.bluewaterweb.com/newslett...05_weather.asp

* Chris Parker is a noted weather expert & author of "Coastal and Offshore Weather, Southeast US and Bahamas" http://www.mwxc.com/
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Old 02-11-2005, 05:11   #5
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What is “Z” Time?
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Old 02-11-2005, 05:22   #6
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“Z” TIME:

Greenwich Mean Time, or Z (Zulu) Time, refers to a universal 24-hour clock which runs from 0000Z (at Greenwich Observatory in England) to 2359Z.

You can convert Standard/ Daylight Savings Time to “Zulu” (GMT) by first converting your local time to 24-hour time by adding 12 hours, ie. 3:00pm=1500. Then add the appropriate number of hours as shown below (North America):

Time Zone: STANDARD TIME (OCT-APR) ~or~ DAYLIGHT SAVINGS (APR-OCT)
Eastern Time: EST add 5 hours ~or~ EDT add 4 hours
Central Time: CST add 6 hours ~or~ CDT add 5 hours
Mountain Time: MST add 7 hours ~or~ MDT add 6 hours
Pacific Time: PST add 8 hours ~or!~ PDT add 7 hours

ie: If Local Eastern Standard Time is 6:00pm, convert this to 1800 on the 24-hour clock, then add 5 hours = 2300 Zulu Time (2300GMT).

HTH,
Gord
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Old 07-12-2005, 05:31   #7
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WEATHER LINKS:

WORLD WEATHER

Weather Organizer: http://www.weather.org
(An easy to use world view of weather information of all types)

INDIAN OCEAN & SOUTH ATLANTIC
Cruising Connections: http://www.cruisingconnections.co.za/weather.htm

U.S. NATIONAL WEATHER

Interactive Weather National Weather Service:
http://iwin.nws.noaa.gov/iwin/graphicsversion/main.html

Weather Site, Inc:
http://www.marineweather.com/

CNN - Weather:
http://www.cnn.com/WEATHER/index.html

NOAA WEATHER SERVICES

NOAA Weather Services: http://www.noaa.gov/wx.html

About NOAA Weather Radio: http://205.156.54.206/om/marine/wxradio.htm

High Seas Forecasts and Charts: http://www.nhc.noaa.gov/forecast.html

NOAA Data Buoy Center: http://www.ndbc.noaa.gov/index.html

NATIONAL WEATHER SERVICE

NWS Home Page: http://www.nws.noaa.gov

NWS High Seas and Offshore Forecasts, Western Atlantic and Caribbean:
http://www.nws.noaa.gov/om/marine/atlantic.htm

NWS Radiofax: http://weather.noaa.gov/fax/marine.shtml

NATIONAL OCEAN SERVICE

NOS Home Page: http://wave.nos.noaa.gov

HURRICANE PREDICTION / TRACKING


Tropical Prediction Center/National Hurricane Center: http://www.nhc.noaa.gov/

T-Storm Terry’s Tropical Weather Page http://www.moreweather.com/tropics/
(A good local alternative to the NOAA site)

Hurricane Watch Net: http://www.hwn.org/index.html

Atlantic Seasonal Forecast / W.M. Gray and Team:
http://typhoon.atmos.colostate.edu/forecasts/

NAVIGATION

USCG Navigation Center: http://www.navcen.uscg.mil

ATOMIC CLOCK

Atomic Clock Time-Boulder, Department of Commerce:
http://www.bldrdoc.gov/doc-tour/atomic_clock.html

Atomic Clock Time (Java Script) http://www.time.gov

Philex Atomic Web Clock Program http://www.philex.net/clock/#download
(Coordinate your computer with the Atomic Clock - free software download)

Weather Fax Schedules:
Remember that frequencies advertised often need to have 1.9khz subtracted to get correct reception of weather fax signals.

NOAA Fax list: http://www.nws.noaa.gov/om/marine/rfax.pdf
A worldwide list of schedules by NOAA. A PDF file

Australia: http://www.bom.gov.au/nmoc/rad_sch/
Australian schedules provided by Australian Met Service

Australia Info: http://www.bom.gov.au/marine/marine_weather_radio.shtml
Australia reception info provided by Australian Met Service

Northwood UK: http://www.franksingleton.clara.net/...sked.html#sked
(Updated July 2004) UK Northwood schedule and frequencies

South Africa: http://www.weathersa.co.za/Marine/Fr...CBroadcast.jsp
Schedule for South Africa by SA Weather Service

Yachtcom: http://www.yachtcom.co.uk/comms/Frequencies.htm
A very comprehensive site with many frequencies for all radio applications

Other Weatherfax Resources:

Weatherfax on the Cheap: http://home.no.net/naomij/files/weatherfax.pdf

Mariners’ Guide to Tropical Storms: http://www.nhc.noaa.gov/marinersguide.pdf
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Old 07-12-2005, 08:02   #8
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Weather link for INDIAN OCEAN & SOUTH ATLANTIC

Very comprehensive weather information for the above regions:

Cruising Connections
http://www.cruisingconnections.co.za/weather.htm
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