Frequently Asked Questions
1. What do the symbols on the wind forecast charts indicate?
2. Why do your wind forecasts underestimate the wind speed of hurricanes / tropical cyclones?
3. What weather data do you use to produce your weather charts?
4. When are the weather charts updated?
5. What does UTC time mean?
1. What do the symbols on the wind forecast charts indicate?

These symbols are called "wind barbs", and they indicate both the wind speed and the wind direction (at a height of 10m above sea level). The wind direction is indicated by the angle of the barb, and the wind speed is represented by the number of lines and/or flags on the tail. Each full line equals 10 knots and a smaller half-line is 5 knots. A flag (triangle) is equal to 50 knots. Every item should be added together to determine the speed of the wind.

These wind barbs show average wind speeds. Wind gusts may be up to 40% stronger.

In each of the following examples the wind is blowing from the NW:

Barb Knots Km/hr M/sec Beaufort Barb Knots Km/hr M/sec Beaufort
0
(0-2) 		0
(0-4) 		0
(0-2) 		0 / 1 35
(33-37) 		65
(52-69) 		18
(17-19) 		8
5
(3-7) 		9
(5-13) 		3
(2-4) 		2 40
(38-42) 		74
(70-78) 		20
(19-21) 		8 / 9
10
(8-12) 		19
(14-22) 		5
(4-6) 		3 45
(43-47) 		83
(79-87) 		23
(22-24) 		9
15
(13-17) 		27
(23-31) 		8
(7-9) 		4 50
(48-52) 		92
(88-97) 		26
(25-27) 		10
20
(18-22) 		37
(32-41) 		10
(9-11) 		5 55
(53-57) 		102
(98-106) 		28
(27-29) 		10 / 11
25
(23-27) 		46
(42-50) 		13
(12-14) 		6 60
(58-62) 		111
(107-115) 		30
(29-31) 		11
30
(28-32) 		56
(51-59) 		16
(15-17) 		7 65
(63-67) 		120
(116-124) 		33
(32-34) 		12
2. Why do the forecasts underestimate the wind speed of tropical storms?

The difference in wind speeds between our wind forecasts and the tropical cyclone (tropical depressions, tropical storms, hurricanes, typhoons, etc.) forecasts and warnings issued by official government agencies (such as the US National Hurricane Center) has to do with terminology and the averaging method used. Our wind charts show average wind speeds, while the tropical forecasts and warnings indicate "maximum sustained winds", which are based on maximum wind gusts, but use a shorter averaging period. These maximum sustained winds tend to be 30-40% higher than average wind speeds. Added to this is the fact that the official government agencies tend to exaggerate the intensity of tropical cyclones, to "be safe". Therefore, the maximum sustained winds from an official tropical cyclone forecast can be significantly higher than the average wind speeds that our wind forecasts indicate, sometimes as much as 100% higher.

For this reason, we highly recommend that you check the official government agency forecasts during the tropical cyclone season and when tropical cyclones are forecast. Tropical warnings are available on our main menu.

3. What weather data do you use to produce your weather charts?

The majority of our forecasts (surface wind, surface pressure, visibility, cloud cover, precipitation) are derived from the 0.25 degree GFS weather model from NOAA/NCEP.

We also provide global surface wind and surface pressure forecasts using data from the 0.25 degree ECMWF model, which is run by the European Centre for Medium-Range Weather Forecasts.

For North America, we create higher-resolution Surface Wind (10m above sea level) charts using data from the 12km (~0.12 degree) NAM model from NOAA/NCEP.

We also run higher-resolution Surface Wind forecasts for parts of Western Europe and the Mediterranean Sea, using the 18km (~0.2 degree) COAMPS model, developed by the Naval Research Laboratory.

The Global wave forecasts are derived from the WW3 wave model from NOAA/NCEP.

The Black Sea wave forecasts are derived from the WW3 wave model run by the U.S. Naval Research Laboratory.

The Gulf Stream forecasts are derived from the RTOFS model from the U.S. Naval Research Laboratory.

The Sea Surface Temperature analysis data comes from NOAA using satelite data from NASA.

4. When are the weather charts updated?
5. What does UTC time mean?

UTC or Coordinated Universal Time (Temps Universel Coordonné) is the international time standard (formerly Greenwich Mean Time, or GMT). Zero hours UTC is midnight in Greenwich, England, which is located at 0 degrees longitude. Everything east of Greenwich (up to 180 degrees) is later in time, everything west is earlier.

The current UTC time is displayed in the upper right corner of the PassageWeather website.

If using the PassageWeather forecast images when offline, the easiest way to find the UTC time is to check your GPS, which displays UTC time very accurately.