Surf Forecasts and Marine Weather - No Hype - Just the Facts!
Late Blooming El Nino Feeds the Storm Machine! - Video Forecast HERE (11/27)
Buoys | Buoy Forecast | Bulletins | Models: Wave - Weather - Surf - Altimetry - Snow | Pacific Forecast | QuikCAST | El Nino | Tutorials | Great Circles | Video


Stormsurf Mobile App

Create Your Own Surf Forecast
Swell Calculator
Swell Decay Tables
Sea Height Tables
Swell Category Table
Convert from GMT:
 to timezone:

Pacific Storm and Surf Forecast
Updated: June 5, 2011 12:26 PM
Buoys: Northern CA - Southern CA - Hawaii - Gulf of Alaska - Pacific Northwest
Buoy Forecast:
Northern CA - Southern CA - Hawaii - Gulf of Alaska - Pacific Northwest
Pacific Links:  Atmospheric Models - Buoy Data - Current Weather - Wave Models
Forecast Archives: Enter Here
A chronology of recent Mavericks Underground forecasts. Once you enter, just click on the HTML file forecast you want to review (e.g. 073199.html equals July 31, 1999). To view the maps that correspond to that forecast date, select the html file labeled 073199 maps.html
Swell Potential Rating = 4.1 - California & 4.0 - Hawaii
Using the 'Summer' Scale
(See Swell Category Table link at bottom of page)
Probability for presence of largest swells in near-shore waters of NCal, SCal or Hawaii.    
Issued for Week of Monday 6/6 thru Sun 6/12
Swell Potential Rating Categories
5 = Good probability for 3 or more days of Significant swell
4 = Good probability for 1-2 days of Significant swell
3 = Good probability for 3 or more days of Intermediate/Advanced swell
2 = Good probability for  1-2 days of
Intermediate/Advanced swell
1 = Good probability for 3 or more days of Impulse or Windswell
0 = Low probability for 1-2 days of Impulse or Windswell   

Minor Swell #3S Hits Hawaii
Heading Towards California


New Swell Classification Guidelines (Winter)

Significant: Winter - Swell 8 ft @ 14 secs or greater (11+ ft faces) for 8+ hours (greater than double overhead).
- Head high or better.
Advanced: Winter - Swell and period combination capable of generating faces 1.5 times overhead to double overhead (7-10 ft)
Summer - Chest to head high.
Intermediate/Utility Class: Winter - Swell and period combination generating faces at head high to 1.5 times overhead (4-7 ft).
- Waist to chest high.
Impulse/Windswell: Winter - Swell and period combination generating faces up to head high (1-4 ft) or anything with a period less than 11 secs.
- up to waist high swell. Also called 'Background' swell.


Current Conditions
On Sunday (6/5) North and Central California was seeing waist high or so south local windswell with light south winds and almost clean conditions at select breaks.  
Southern California had chest high lined up windswell up north and fairly clean early. Down south the same northwest windswell was about chest high and pretty lined up with pristine clean conditions and even some sun early. Hawaii's North Shore was getting some decent windswell producing waves to near head high and clean with light trades in effect early. The East Shore had head high easterly tradewind generated windswell and chopped. The South Shore was getting Swell #3S with waves shoulder to head high with a few bigger sets and calm and clean conditions.

See QuikCASTs for the 5 day surf overview or read below for the detailed view

Meteorological Overview
The North Pacific is moving quickly into hibernation mode. One small gale is forecast just west of the dateline on Tues (6/7) producing 24 hours of 19 ft seas maybe good for a tiny pulse of background swell for Hawaii, but that's it. Down south Storm #3S pushed under New Zealand on Sat-Mon (5/30) with up to 46 ft seas but all aimed well to the east. Some decent but modest sized southwest swell (barely significant class) is expected for the bulk of the workweek. Beyond a gale continues to be forecast tracking due north through the Southeast Pacific Tuesday into early Wednesday (5/8) resulting in up to 40 ft seas pushing pretty well towards the US West coast, then the gale is to turn east and regenerate, providing another shot of 42 ft seas aimed at Chile and in fairly close proximity. Another pulse of very southerly angled swell looks possible. Nothing else on the charts behind though.  


Current marine weather and wave analysis.cgius forecast conditions for the next 72 hours

North Pacific

On Sunday (6/5) the jetstream was meandering off Central Japan dipping into a small trough on the dateline but winds there only 100 kts, then ridging northeastward while tracking over the Gulf of Alaska pushing into British Columbia. A cutoff trough was stationary just off Central CA providing some support for low pressure there. Otherwise there was no support for gale development. Over the next 72 hours the trough on the dateline is to track northeast and dissipate, providing no real support for anything more than weak low pressure development down at the oceans surface. The flow back tot he west is to get a bit more cohesive with winds up to 110-120 kts, but with no troughs forming. Beyond 72 hours something that almost looks like a trough is to organize on the dateline Wed-Thurs (6/9) offering some support for low pressure development there, but lifting northeast and flattening out. By the end of the forecast period (Sun - 6/12) a flat flow is projected running over the 40N latitude with winds 110-130 kts (strongest off Japan) and not looking too bad, but not offering much in terms of support for low pressure development either with no troughs indicated.

At the surface on Sunday (6/5) a very weak pressure pattern was indicated. A series of three small and weak low pressure systems were in.cgiace, one just off Central CA, the second north of Hawaii and a third off Japan. Each had a small area of 25 kt winds, good for local windswell production only. In between weak high pressure was in.cgiace. A very.cgiacid pattern. Light trades were blowing at almost 15 kts over the Hawaiian Islands. Over the next 72 hours the low just off CEntral CA is to evaporate Sunday evening, with high pressure at 1032 mbs building in behind it and holding to at least Wed (6/8) producing northwest winds along the entire US West Coast building to 20 kts or so. Also the low that is currently off Japan is to build in strength and areal coverage while moving northeast setting up a good fetch of 30 kt west winds just west of the dateline Tues-Wed (6/8) resulting in a decent patch of 19 ft seas on Wednesday AM at 44N 173E perhaps good for some windswell pushing down towards Hawaii for the weekend. Size to be minimal though. Otherwise no low pressure of interest is forecast. Trades to remain light (15 kts or less) over the Islands.


North Pacific Animations: Jetstream - Surface Pressure/Wind - Sea Height - Surf Height


No tropical storm formation of interest is occurring or forecast for the next 72 hours.  

California Nearshore Forecast
On Sunday AM (6/5) weak low pressure remained stationary off the Central CA coast with light south winds in effect and a chance of rain falling mainly over interior locations. By Monday the low is to be dissolving off Pt Conception with high pressure building into behind providing a light northwest flow forecast mainly in the afternoon from Morro Bay northward then increasing to 15 kts by Tuesday AM (6/7) from Pt Conception northward and getting stronger from there. A summer-like gradient is forecast for the Cape Mendo area by Wednesday with north winds there to 25 kts but generally staying away from the coast from Pt Reyes southward then fading some Thursday (6/9) but moving closer to the coast with 15 kts winds forecast all locations other than SCal. More of the same is expected on Friday before high pressure gets a firmer grip on Saturday with 20 kt northwest winds forecast for the entire North and Central Coast. A lighter northwest winds pattern is forecast for Sunday (6/12) with winds down to 10-15 kts, but still not anything near calm.   


South Pacific

On Sunday (6/5) high pressure at 1032 mbs was anchored east of northern New Zealand reaching down to 55S and pretty much eliminating odds for gale development in the West and Central Pacific. Weak low pressure was tracking east to southeast under the high, but of now use yet. Over the next 72 hours
remnants of a cutoff low that previously was south of Tahiti is to start developing in the deep Southeast Pacific Monday AM (5/6) resulting in a solid fetch of 40 kt southwest winds at 62S 142W and starting to take aimed more due north. By evening south winds at 40-45 kts are forecast lifting to 59S 140W with seas starting to build from 28 ft in that area. Tuesday AM (6/7) a small fetch of 45 kt south winds is forecast lifting north at 57S 130W resulting in 32 ft seas at 56S 133W. By evening that fetch is to intensify with 45-50 kt south winds forecast at 50S 127W resulting in 38 ft seas at 50S 128W pushing up the 186 degree path to Central CA and the 188 degree path to Southern CA. That fetch is to race north and start fading Wed AM (6/8) from 45 kts resulting in 38-40 ft seas up at 46S 124W (182 degs NCal/184 SCal) while a secondary fetch of 45 kt south winds builds under it. By evening the fetch is to start wrapping into the northern quadrant of the storm all aimed to the Northeast and east (Peru-Chile) and moving out of the CA swell window. a small area of 38 ft seas are forecast at 45S 119-115W. Maybe some more swell to be pushing up the 180 degree path to SCal, with not much for Central CA (178 degs). Thursday AM (6/9) 45 kts fetch is to be pushing due east towards chile with 42 ft seas at 45S 110W, totally outside the CA swell window and effectively only aimed at Peru southward. This system, if it forms, has good chances of again generating a small significant class swell pushing up into CA on down into mainland Mexico, with better odds for moderate to large swell targeting Chile and Peru. But as of now, no fetch is even occurring yet, so it's all just a guess by the models.

Minimal Storm #3S
On Saturday AM (5/28) a new co.cgiex gale started organizing under New Zealand. It had 36 ft seas from fetch the previous evening at 45 kts. Those seas were positioned at 53S 174E aimed pretty well up the 215 degree path to CA and unshadowed and well east of the 199 degree path to Hawaii. Some swell likely pushing towards both locales. That fetch was fading out Saturday evening with residual 30 ft seas at 54S 176W. 

Of more interest was a new fetch building directly behind with 55 kt southwest winds at 55S 164E (216 degs NCal but shadowed by New Zealand relative to HI).  By Sunday AM (5/29) a tiny area of 55-60 kt southwest fetch was moving into exposed waters generating up to 44 ft seas at 57S 175E (210 degs NCal and moving into the Tahiti Swell Shadow/196 degs HI unshadowed) and building. In the evening 50 kt west fetch was holding with 46 ft seas peaking at 58S 172W pushing due east (not good). That's 40 degrees east of the 205 degree path to NCal and in the middle of the Tahitian swell shadow, 207 degs relative to SCal and moving out of the core of the shadow, and 60 degs east of the 189 degree path to Hawaii. Some swell possibly moving towards all locations but favoring California and points south of there. Also swell pushing 40 degree east of the 203 degree path to Tahiti.  This storm was fading on Monday AM (5/30) with 40 kt west winds dropping and seas fading from 40 ft at 55S 165W (203 degs relative to NCal and out of the core of the Tahitian swell shadow, 205 degrees SCal and out of the heart of the shadow and pushing 65 degree east of the 183 degree path to HI. Secondary 40 kt southwest fetch held into the evening with seas fading from 40 ft at 50S 165W.   

This system was on a very direct west to east track with all fetch aimed due east, limiting the amount of swell that will radiate north. Still, with seas forecast to nearly 47 ft, some degree of energy is expected to push up into Hawaii and CA.

Hawaii:  Swell to hold early Monday (6/6) with swell 2.6 ft @ 17 secs (4.5 ft with sets to near 6 ft) and period dropping to 16 secs later in the day. Swell to hold at 2.6 ft @ 15-16 secs (4 ft faces with sets to 5 ft) on Tuesday (6/7).  15 secs residuals expected to be fading on Wednesday (6/8).  Swell Direction: 192-199 degrees

Southern CA:  Expect swell arrival on Tuesday (6/7) near 6 AM with period 20 secs and size tiny but building.  Swell up to 2.6 ft @ 19 secs near sunset (5 ft faces with sets to 6.5 ft). Swell to continue upward with period 18 secs Wednesday AM (6/8) and peaking near noon with pure swell 3.0-3.3 ft @ 17-18 secs (5.1-6.0 ft with sets to 7.5 ft) and holding through sunset. Swell to continue solid at 3.0-3.3 ft @ 16 secs (4.8-5.3 ft with sets to 6.5 ft) through the day Thursday (6/9), with  period dropping towards 15 secs at sunset.  14-15 secs residuals expected on Friday (6/10). Swell Direction: 207-218 degrees

Northern CA:  Expect swell arrival on Tuesday (6/7) near 8 AM with period 20 secs and size tiny but building.  Swell up to 2 ft @ 19 secs near sunset. Swell to continue upward with period 18 secs Wednesday AM (6/8) and peaking near 2 PM with pure swell 3.0-3.3 ft @ 17-18 secs (5.1-6.0 ft with sets to 7.5 ft) and holding through sunset. Swell to continue solid at 3.0-3.3 ft @ 16 secs (4.8-5.3 ft with sets to 6.5 ft)  through the day Thursday (6/9), with  period dropping towards 15 secs just after sunset.  14-15 secs residuals expected on Friday (6/10). Swell Direction: 205-216 degrees 


South Pacific Animations: Jetstream - Surface Pressure/Wind - Sea Height - Surf Height




Marine weather and forecast conditions 3-10 days into the future

North Pacific

Beyond 72 hrs moderate high pressure is to dominate the East Pacific keeping a steady northwest flow pushing down the US West Coast at 15 kts, but nothing extraordinary. Trades to become a bit more cohesive over Hawaii by Thursday (6/9) at 15 kts solid and moving towards the 20 kt range by the weekend as high pressure falls south closer to Hawaii. Northwest winds over Cape Mendocino to build to the 20 kt range too, maybe good for some local windswell for Central CA. Otherwise no activity of interest is forecast over the West Pacific.

As of Sunday (6/5) the Southern Oscillation Index (SOI) was holding in dead neutral territory. The daily SOI was up slightly to -0.70. The 30 day average was up to 1.76 with the 90 day average down some to 13.34. This was the lowest the monthly SOI has been since March 1, 2010, the start of La Nina, but well above the whole of 2009 (and El Nino year).

Wind anomalies as of Saturday (6/4) at the 850 mb level (approx 5000 ft up) as defined by models indicated a dead neutral pattern in control of the entire Pacific Basin suggestive of neither the Inactive nor the Active Phase of the MJO. A neutral pattern is forecast to hold if not get more entrenched through 6/24, even over the Indian Ocean.

Sea Surface Temp anomaly data (6/2) is unchanged and continues to indicate that cooler waters (-1 C degs) had a grip on the equator covering from a point south of Hawaii to the dateline but steadily loosing coverage. the larger issue was cooler than normal waters present in feeder bands originating off the US West Coast (not as strong as earlier in the Winter) and somewhat colder ones off Chile sweeping fully to the intersection of the dateline and the equator, serving to continue the existing La Nina pattern. These colder waters are a reflection of stronger than normal high pressure that built in over both hemispheres in the winter causing upwelling in the Gulf of Alaska and off South America, though it looks like the upwelling effect was stronger in the southern hemi than in the north. Regardless, the cooler waters in the North Pacific continue to slowly relent in spurts as are the cool temps over the equator. Warmer than normal waters are building over the Galapagos Islands and over a very narrow band west of there and continue slowly increasing in coverage in fits and spurts. And tongues of warmer water are positioned in the both the Northwest and Southwest Hemispheres trying to make inroads to the east but not very effectively. Still the big picture still looks like a La Nina setup (though fading in intensity). 

Below the surface on the equator there had previously been indications of Kevin Wave activity.  Colder than normal water that had been locked all winter southeast of Hawaii under the equator evaporated in late February and moved to dead neutral presumably from the effects of two consecutive bouts of the Active Phase of the MJO which in turn might have driven a Kelvin Wave. In parallel warmer than normal water had edged east from the West Pacific, previously up +2 degrees above normal and positioned due south of Hawaii (150W) under the equator through 3/22. But an impenetrable wall at 140W separating the warm anomalies, and cool anomalies east of there was blocking any eastward progress of warmer subsurface water. Then on 4/4, it appeared that that wall was fading if not gone entirely by 4/7 and by 4/19 a small but steady finger of normal to slightly warmer (0 to +1 deg C) water started flowing east making it to the equatorial East Pacific up at 100-150 meters and building some.  Almost +1 degrees anomalies tracked from the West Pacific to the East Pacific short of one small break at 160W as of 5/1. On 5/7 a small pool of negative temperature water started to make a faint showing at 140W and was holding through 6/5, presumably driven by the previous Inactive Phase of the MJO. On 5/26 it appeared more warm water was pushing through the subsurface current heading towards Central America, possible a new Kelvin Wave and the likely result of the latest Active Phase of the MJO. +1 degree anomalies covered the entire subsurface current other than one little break at 140-150W with up to 2 degree anomalies embedded in the larger flow. It would be best to see warm anomalies down to 200 meters in the east, but the current state is the best it's been in 9 months and suggestive of a near normal subsurface thermocline, and continuing to get better by the day.  The thought is this normalization of the subsurface flow will eventually affect water temps at the surface and then the atmosphere above it (6 months later).  So all this is a step in the right direction though slow evolving.  

Over the entire Equatorial Pacific trades were blowing all the way to the Philippines and beyond. From a historical perspective these easterly winds were 'normal' with only light easterly anomalies persisting in the far Western Pacific.  

We did some analysis on ocean currents on the Pacific equator this year an found anomalies developed flowing from west to east starting in February and were continuing through May 2011.  We oft look at such symptoms as an El Nino indicator, but that does not seem likely given all the other data.  But that co.cgied with a falling SOI at least it depicts a tendency towards normal conditions.  Will monitor. Historically it is very unlikely if not impossible to have an El Nino form directly behind a La Nina.  More typical is several years of a slow buildup before an actual El Nino event occurs.  

Remnants of what was a moderate.cgius strength La Nina Pattern (where the Inactive Phase takes control) are still evident and momentum from this La Nina event are expected to hold well into the Fall of 2011 (and likely to early 2012) in the upper atmosphere regardless of how quickly La Nina's demise occurs in the ocean. In short, it's going to be tough for surfers on west facing shores in the Eastern Pacific and Eastern Atlantic, though east facing shores of the West Pacific and Atlantic might do well from the Inactive Phase's dominance, especially in summer months.  That is not to say there will be no storms, in fact, there could be short periods of intense activity when the Active Phase gets an opportunity to come to fruition, but that will be the exception rather than the rule, with the Inactive Phase trying to keep a cap on storm activity. Best bet's at this time are for an enhanced tropical season in the Atlantic (2011).

One note of interest, The National Weather Service long term models are now predicting slight warming of East Pacific Equatorial waters in the Summer and Fall of 2011 (about + 0.2-0.3 degs C). This would not qualify as an El Nino, but it is much better than what one would expected given being right behind a solid La Nina. Something to monitor and it almost gives on the sense something unusual is developing.   

See more details in the   El Nino update.


South Pacific

Beyond 72 hours high pressure that was just east of New Zealand is to be pushing further east reaching down to 53S and generally pushing the storm track to the south and the fetch angle from west to east, bypassing any routes up into our forecast area. But with the eastward movement of the high, this opens up the favored Southwest Pacific by next weekend (6/11). A series of two gales are forecast tracking under New Zealand with decent fetch and seas exceeding the 30 ft threshold. It's too early to say with any certainty whether they will materialize, but odds are looking more favorable. Something to monitor.

Details to follow...


External Reference Material: El Nino Southern Oscillation (ENSO), Madden Julian Oscillation (MJO), Pacific Decadal Oscillation (PDO), Southern Oscillation Index (SOI), Kelvin Wave

Add a STORMSURF Buoy Forecast to your Google Homepage. Click Here: Add to Google
Then open your Google homepage, hit 'edit' button (top right near graph), and select your location

TheJay@Mavericks MAVFILM MAVSURFER SURFPULSE Inside Mavericks Randy Cone Surfboards

Local Interest

Chasing the Swell has been nominated for a Webby Award. See details of this great piece of video journalism below. Some say this is the "Oscars" of online awards.One of the awards is voter based. If you have a moment,.cgiease cast your ballot by going to:, register, then click on the "Get Voting" tab and then to the "Online Film and Video" > "Sports" category and vote for "Chasing the Swell".

Timmy Reyes - Curt Myers from Powerlines Productions found this little gem with Timmy Reyes providing a brief statement about which sites he uses for swell chasing. Thought we'd pass it on. Enjoy:

Buell Wetsuits - When surfing in Santa Cruz, we've been seeing a new wetsuit in the line-up worn by many top flight surfers. They're getting good traction and are well respected. Take a look:

Stormsurf Mobile App (1/9/11) We are proud to announce the official public release of our smartphone mobile app. It provides access to our most popular and commonly used products, optimized for use on the road, on the beach or anywhere you don't have a desktop or laptop.  With a smart phone and signal, you will have access to our data. And we're not talking just a few teaser products - We're talking full feature wave models, weather models, real-time buoy data, manually built forecasts and hundreds of spot wave and wind forecasts enabling you to construct a surf forecast for any location on the.cgianet, all from your cell phone and all for free.  No subscription required and no hidden fees. And better yet, there's a few new things sprinkled in that are not yet available even on our full-featured web site. From your smart phones browser just navigate to: 

Mavericks Surf Shop Grand OpeningSunday, December 19 2:00 - 6:00 p.m. rain or shine!  Check out the new home of Jeff Clark's Mavericks Surf Shop, now located at 25 Johnson Pier in Pillar Point Harbor.  The shop features much of Clark's surfing memorabilia, classic boards and photos, as well as an entirely new line of Jeff Clark original Mavericks clothing, accessories and surfboards. The shop has been open in the new location since December 8, and the Grand Opening party is set for this coming Sunday, just in time for Christmas.  The party starts at 2 p.m., with live music, food and drinks. Jeff Clark and many Mavericks surfers will be there to meet the public. Local restaurants Ketch Joanne's and Princeton Seafood will serve up delicious food, while San Francisco Wine Trading Company is providing the beverages.  The shop will be open all weekend, from 9 a.m. to 8 p.m. on Saturday and Sunday.

Stormsurf Maintenance Upgrades: Buoy 46059 and 46012 were r.cgiaced a month or so ago. Totally new buoys were installed. Here on Stormsurf we had to reset the algorithms used to calculate 'pure swell' for them. That was acco.cgiished on 11/13. Pure swell numbers are now correct. Links: 46012, 46059

Also since we moved to the new weather model server last month we discovered that our Longrange Precipitation Models ceased to display frozen precipitation (as they once did). Some of our scripts did not get installed on the new server. That has been fixed (11/13) and now snow is again viewable worldwide. Here the new North America sa.cgie.

Chasing The Swell: Sachi Cunningham from the LA Times spent the entirety of last winter chasing surfers and swells around the North Pacific with her high def video cam. Her timing couldn't have been any better with the project exactly coinciding with the strongest El Nino in 12 years resulting in the best big wave season in a decade. And being an acco.cgiished surfer herself helped her to bring a poignant and accurate account of the what it's like to ride big waves and the new (and some not so new) personalities that are revitalizing the sport. This is must-see material for any surfer or weather enthusiast. Check it out here:

New Weather Models With the activation of our new server we have now released a new block of weather models including North America jetstream, wind and precipitation, local coastal wind forecasts in 1 hr increments and snow and mountain wind forecasts in both 1 and 3 hours increments. The new animations can be found here (look for those items tagged with the New! icon):

New Weather Model Server Stormsurf has installed another weather model production server. This has enabled us to spread the load across more servers allowing us to post both wave and weather model updates much quicker.  Also we are testing new content (like North America jetstream, winds and precipitation, local wind forecasts in 1 hr increments and snow and mountain wind forecasts in both 1 and 3 hours increments). The model menus will be updated shortly with these new links.   

Click here to learn more about Casa Noble Tequila! Casa Noble Tequila If you are looking for an exquisite experience in fine tequila tasting, one we highly recommend, try Case Noble. Consistently rated the best tequila when compared to any other. Available at BevMo (in California). Read more here:

New Wave Model Facts: Click HERE to read more about the new wave models. Important info.

Time Zone Converter By popular demand we've built and easy to use time convert that transposes GMT time to whatever time zone you are located. It's ion left hand column on every page on the site near the link to the swell calculator.

Stormsurf Google Gadget - Want Stormsurf content on your Google Homepage? It's si.cgie and free. If you have Google set as your default Internet E.cgiorer Homepage, just click the link below and a buoy forecast will be added to your Google homepage. Defaults to Half Moon Bay CA. If you want to select a different location, just click on the word 'edit', and a list of alternate available locations appears. Pick the one of your choice. Content updates 4 times daily. A great way to see what waves are coming your way! .xml

Free Stormsurf Stickers - Get your free stickers! - More details Here

Read all the latest news and happenings on our News Page here

Surf Height-Swell Height Correlation Table


Contact | About | Disclaimer | Privacy
Advertise/Content | Links
Visit Mark Sponsler on Facebook Visit Stormsurf on Instagram Visit Stormsurf on YouTube
Copyright © 2023 STORMSURF - All Rights Reserved
This page cannot be duplicated, reused or framed in another window without express written permission.
But links are always welcome.
Buoys | Buoy Forecast | Bulletins | Models: Wave - Weather - Surf - Altimetry - Snow | Pacific Forecast | QuikCAST | El Nino | Tutorials | Great Circles | Calculator