Bobgram

Bob McDavitt's ideas for sailing around the South Pacific. Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world. Compiled 12 July 2026 Wave height Marine forecasts give wave heights in "significant metres" Significant wave height (often abbreviated as Hs or Hsig is a statistical measure defined as the average height of the highest one-third (33%) of all waves occurring in a given period. The parameter aligns very closely with what a human observer visually estimates when looking at a body of water. It is the standard metric used by meteorologists and marine agencies for ocean and surf forecasts. Why "Significant" Doesn't Mean "Maximum" Because ocean waves are highly irregular and vary widely in size, Hs provides a more practical representation of the overall sea state than a simple average. It is important to remember the following realities: . The tallest waves: Statistically, individual waves can be significantly taller than the significant wave height. A wave roughly twice (x1.8) Hs can be expected to occur. . Calculation: If an observer measures 100 consecutive waves, the 33 tallest waves are selected and mathematically averaged to find the significant wave height. WALTER MUNK (1917-2018) was one of the first scientists to bring statistical methods to the analysis of oceanographic data. World war II interrupted his doctoral studies at Scripps Institution of Oceanography (Scripps) at La Jolla and led to his participation in U.S. military research efforts. Munk developed methods for forecasting wave conditions which were used in support of beach landings such as on D Day. Munk in 2010 In 1932, Munk was performing poorly in school because he was spending too much time skiing, so his family sent him to a boys' preparatory school in upper New York state. In 1939 he took a summer job at Scripps, and this led to taking on a Master of Science degree at Caltech in 1940. In 1940, Munk enlisted in the US Army then in 1941, a week before the attack on Pearl Harbor, he was assigned to US Navy Radio and Sound Lab at Scripps. Soon he and Sverdrup looked for ways to measure, report and predict wave height (https://commons.wikimedia.org/wiki/File:Animation_for_a_deep_water_wave.gif ) . In 1944 Munk wrote Proposed uniform procedure for observing waves and interpreting instrument records. In this he introduced the significant wave height which was intended to mathematically express the height estimated by a "trained observer". The thing is that because an observer's view of the waves is such that the taller waves block out the smaller, it seems that an observer's "average" is the average of the top third Generally, a set of waves follows the Rayleigh distribution and the whole field can thus be determined from just the significant wave height Hs Note that the "maximum wave height" is close to TWICE the significant wave height (one such wave can be expected once a day). . 1 wave in 7 is higher than Hs. And that occurs around once a minute for waves with period around 8 to 9 seconds) Hence the surfer's rule of thumb to "wait for the 7thwave". Another way of looking at wave height is to rate it according to its energy, or "sea state" E This is from http://www.seafriends.org.nz/oceano/waves.htm This graph uses sea state E to look at relative wave heights (h/SQRT(E)) The graph shows that 80% of the waves are higher than a relative wave height of 1. 50% of the waves are higher than a relative wave height of 1.77 The significant height Hs is at relative wave height of 2.83, and 15% of the waves ( 1 in 7) are higher. Here is an interesting website for anyone wishing to experiment with interference and diffraction patterns play: the ripple simulator at www.falstad.com/ripple/ (simulates sound, radio and microwaves but not sea waves). Click and drag the mouse to make waves). TROPICS .Category-5 Typhoon Bavi caused catastrophic damage on Rota, in the Northern Mariana Islands. China evacuated nearly 2 million people as BAVI made landfall. . Typhoon Maysak unleashed deadly flooding across southern China, trapping residents on rooftops. . Minimal Tropical Storm Douglas formed briefly between Hawaii and Mexico. WEATHER ZONES The South Pacific Convergence zone remains well north of its usual position. Akso there is a very strong east going surface drift along the equator possibly liked to BAVI There is still a quasi-stationary low east of New Zealand near 40S 140W this week, with southwest winds which are tonight affecting Tahiti - these should swing back to be from the SSE in a few days. A High H1 has been over NZ this weekend and most of it is pulling off to the south of L1 next few days. A front is expected to cross South Island on Monday and then fade away over North Island on Tuesday. Then the remains of H1 should stay slow-moving just NE of New Zealand. This allows troughs to form in the Tasman Sea. The isobars marking the Easterly flow over New Caledonia are expected to curl up and bunch together as an upper cold pool forms and that should result in a surface low near Norfolk Island early next week. Labelled as L2 in my map. HIGH H2 is expected to travel east steadily along 35S across the south of Australia until mid-week, and then the slow to a crawl late in the week. >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> If you would like more details about your voyage, check metbob.com Or Facebook at /www.facebook.com/metbobnz/ Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom). Weathergram archive (with translator) is at weathergram.blogspot.co.nz. Contact is bob@metbob.com or text 64277762212. >>>>>>>>>>>>>>>>