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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 19 July 2026 Anticyclonic gloom You have probably heard all about the High-Pressure heat domes giving high temperatures over Europe this summer. And you may have puzzled about the low grey clouds found over the ocean in the high-pressure regions of the South Pacific, especially in winter. These High-pressure regions are three-dimensional. In the vertical the top part of the system contains sinking air. Just as rising air cools, so it is that sinking air warms. But the sinking air doesn't go all the way down to the surface. Instead, the bottom layer hugging the ground gets trapped ---it contains moist-laden marine air with flat grey clouds, mist and maybe fog, smog and polluted air. Anticyclonic gloom-a trapped layer of marine-moistened air. There is a distinct layer separating the warmed air aloft from the bottom trapped layer. Between the sinking warmed air and the trapped layer is a thin interface --- a lid called a subsidence inversion. It is formed by subsiding air and it inverts the normal rate of change of temperature with height. Key Characteristics . Stagnant Air: Light winds inside the high-pressure system fail to push the clouds away, meaning the gloom can linger for days . Temperature Inversion: Warm, sinking air sits above cooler, moist marine air, firmly trapping the cloud layer below . Mild & Drizzly: Despite the grey skies, temperatures usually remain mild. The moisture in the marine layer can occasionally condense into dense fog or light drizzle. The Science Behind the Gloom The mechanics rely heavily on atmospheric physics: 1. Subsidence: Descending air within an anticyclone compresses and warms, forming a stable "lid" or inversion layer. 2. Moisture Trapping: The cool ocean waters keep the lowest level of air moist. Under the lid the air is blocked by the warm air above it, so the moisture spreads out into a blanket of stratus or stratocumulus clouds. 3. Lack of Wind: Without significant wind to mix the atmosphere or break up the cloud deck, the overcast, gloomy conditions remain stationary. TROPICS . After devastating the Northern Mariana Islands, a weaker Bavi crossed the western Pacific and inundated parts of China's Zhejiang province. WEATHER ZONES The South Pacific Convergence zone remains well north of its usual position. And now it has curled around itself on its western side and produced a tropical low. I wonder if this is in response to the strong east-going current at the equator.? Here is MetService streamline analysis, showing a rotating feature very close to the equator (physically impossible to happen AT the equator) . Interesting to see a triplet of lows so close together they almost share the same streamline, with two going clockwise and the third counterclockwise. That doesn't happen often. I've named that feature L2 and it is expected to drift south into an area of strong shear and unwind near northern Vanuatu. Low L1 formed recently near New Caledonia and is expected to travel east along 20 to 30S crossing Minerva on Tuesday night, and followed by some high South to southeast swells. Avoid. After a quick front crossed North Island on Monday, High H1 should travel along 40S across Tasman Sea and NZ, very likely with some anticyclonic gloom. It then may fade but extend a nose to the northeast, weakening the trade winds between Tahiti and Tonga. Typical El Nino. From Friday a series of fronts and strong disturbed westerly winds, all on north side of L3, are expected across the Tasman Sea/NZ area. Typical El Nino. So, after several weeks with stalling systems, the weather this week transits into a mobile pattern. >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 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. >>>>>>>>>>>>>>>>

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. >>>>>>>>>>>>>>>>

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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 05 July 2026 A review of last month's weather Here is a link to a YouTube clip giving an animated loop of the isobars and streamlines For last month is at youtu.be/9vEKSmPOgQc Most of the month there where anticyclones along 30 to 40S . In New Zealand it was the warmest June on record. On June 25 a deep low crossed northern NZ The resulting southerly winds triggered an evacuation of Wellington's south coast and power cuts affecting 6500 homes around Taranaki/Whanganui and 4000 in Wellington. Sea Surface temperature anomalies from psl.noaa.gov/map/clim/sst.shtml The waters over eastern equatorial Pacific are warming --- El Nino has started. A warm pool is stretching eastwards along the Aleutians towards Alaska. There has been a cooling trend around Fiji. Average isobars for past month (From www.psl.noaa.gov/map/images/fnl/slp_30b.fnl.html) The Monsoon has arrived over southern India around normal. Not much change elsewhere. Pressure anomalies for past two months (from www.psl.noaa.gov/map/images/fnl/slp_30b.fnl.html) The anomaly pressure pattern for last month now shows a low breeding area northeast of New Zealand and another around Easter Island. The low breeding area that was over New Caledonia has now faded. The 1015 isobar has shifted north over northern Australia . TROPICS .One person was killed and three injured in eastern Japan by severe conditions created by Tropical Storm Higos and former Typhoon Mekkhala, which struck in rapid succession. . Tropical Storm Bavi formed near the Marshall Islands and is on track to strike the Northern Mariana Islands or Guam as an intense typhoon. WEATHER ZONES There has been a multi-centred low NE of New Zealand and south to southeast of southern Cooks for past few days. This system is now being pushed to the southwest and fading. An associated convergence zone is expected to travel east across the Society island early this week. Otherwise, the SPCZ is rather weak, with one section near Samoa and another section over the Solomon Islands. High H1 is slow-moving at 43S well east of NZ. A complex and muti-centred low system is forming under a cold pool over New Zealand expected to be slow-moving until mid-week then travel off to southeast followed by cold southerly flow on Thursday. The FRONT associated with this system is expected over Minerva late on local Tuesday. >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 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. >>>>>>>>>>>>>>>>