Saturday, June 14, 2008

Weather Quiz - Significant Wave Height

Another Quiz:

The forecast is for 10 foot seas answer the following:

What average wave height do they expect you will encounter?

a. 10 feet
b. 8 feet
c. Just over 6 feet.

With the same forecast of 10 feet how high a wave will be most frequently encountered?

a. 10 feet
b. 8 feet
c. 5 feet

Again with seas forecast for 10 feet what is the expected average of the highest 10 % of the waves?

a. About 10 feet
b. 15 feet
c. 13 feet

With a forecast of 10 feet what is the highest wave expected to be encountered?
a. 10 feet
b. 12 feet
c. 16 - 20 feet

The correct answer to each of the questions above is c.

a forecast of 10-foot seas in open waters means a mariner should expect to encounter a wave spectrum with many waves between 6 and 10 feet along with a small percentage of waves up to 16 feet and possibly even as large as 20 feet!
from NWS Significant Wave Heights

From Significant Wave Height by Tom Ainsworth, NWS Juneau, Alaska

The diagram above shows the distribution of wave heights at sea. Confusingly it looks somewhat like a wave but each dot represents a single wave so the y axis (labeled N) is the number of waves and the x axis (labeled H) is the height. The point H(s) is the value used in forecasts.

I recommend you also read this article Expecting the Unexpected Wave: How the National Weather Service Marine Forecasts Compare to Observed Seas as well from Robin Storm, same info but together give a good picture of the wave spectrum

It is important to note that the same relationship between the significant wave height, average wave height and so on, including the highest wave expected to be encountered does not change with wave height. Consider what that means when the forecast is for 35 foot seas.

No comments: