The Bounty, Sandy and Storm Avoidance Techniques. The NTSB Report

Bounty (Wikipedia photo)

The National Transportation Safety Board has released it's report on the sinking of the Bounty. (here pdf).  There were lots of questionable practices with regard to maintenance, training and procedures  however with regards to the actual navigation in relationship  to the storm  there has been questions about the decision to pass west of the storm. There is a good graphic of the track of both the ship and the storm here.

Possibly the captain was using traditional low-tech  hurricane avoidance techniques that were not applicable to the situation.

From the  NTSB report:

 It is possible that the captain may have focused too narrowly on the position of the storm’s eye instead of on Sandy’s total expanse (winds associated with the storm spanned more than 1,000 miles in diameter, and the area into which the Bounty was heading was already under tropical storm warnings, with conditions  forecasted to worsen).Still, the captain seemed to believe that he could outrace the storm.

A traditional method of hurricane avoidance  is to stay a safe  distance from the eye on a track that passes the hurricane using  the so-called  navigable  semi-circle.

 On the navigable side,(the west side in the case of Sandy) as the storm approaches,  the winds and seas will be astern (important for a square rig sailing vessel) while also  pushing the vessel out of the path of the storm. Also the wind speeds are less because the winds are the cyclonic so the total wind speed is the  winds minus the storm direction.   On the dangerous semi-circle side (east side) the winds and sea tend to push the vessel into the path of the storm and also the cyclonic winds are added to the storm direction.

It seems plausible that Capt. Walbridge would have had high confidence in traditional low-tech avoidance methods.

The problem is that in the case of Sandy at the time of the encounter  the concepts of navigable/dangerous semi-circle were invalid.

This is because  when the Bounty encountered  Sandy it was transiting from a tropical cyclone(warm core) to an  extratropical cyclone (cold core). The meteorology is complex but the take-away for the mariner is that the wind field will expand and the field will no longer have the same characteristics.

Here is  tropical cyclone Katrina:

Katina (From NASA)

The dark colors represent the area of highest winds.  The strongest winds are NE of the center, the so-called dangerous semi-circle.

From NASA: 

Katrina was a textbook tropical cyclone, with a compact, symmetrical wind field that whipped around a circular low-pressure center.

Sandy, at about the time of the encounter on the other hand looks quite different:

Sandy (from NASA)

  In Sandy  the strongest winds are in south and west of the center and further from the center. Tragically this is where the Bounty was when it encountered Sandy.

 So when tropical cyclones become extratropical, their wind and cloud fields expand dramatically. Their strongest winds generally weaken during this process, but occasionally a transitioning storm retains hurricane force winds, as was the case with Sandy.

 From the NHC on the 28th.

THE MOST RECENT AIR FORCE HURRICANE HUNTER AIRCRAFT MISSION DID NOT
FIND WINDS OF HURRICANE FORCE NEAR THE CENTER OF SANDY.
HOWEVER...EARLIER DROPSONDE DATA INDICATED THAT WINDS TO HURRICANE
STRENGTH WERE OCCURRING WELL TO THE SOUTH AND SOUTHWEST OF THE
CENTER.

What  the Bounty encountered may have  the highest wind speeds in the system. The Bounty  was about 100 miles from the center in what would be considered (in a hurricane) the navigable  semi-circle. What was encountered instead was  not only the highest wind speeds but also the roughest seas, made short and steep  by the contrary (north bound) Gulf Stream current.

In spite of the complex situation the models used by meteorologist  for the Sandy forecast were  accurate.  There is a good discussion of the forecast here.   Had the captain used the NHC (National Hurricane Center) forecasts he may have been able to avoid the worse of the storm. Of course with few hurricane  encounters under his belt the inexperienced  Walbridge likely had more confidence in his own skills then the NHC forecasts.  Walbridge's confidence was misplaced.

A better approach is to set wind speed  and/or sea height  limits before the voyage and to use forecasts to ensure those limits are not exceeded. 

Even better then dodging bad weather  of course to find a good hurricane hole, put out extra lines and stay put till the storm passes.

K.C.


========================================================================

The NWS warns that the 1-2-3 Rule  "does not account for the typical expansion of the wind field as a system transitions from hurricane to extratropical gale/storm."  Nonetheless the rule holds up quite well.

Sandy and the 1-2-3 Rule from David Burch Navigation

Source for the info on tropical system and extratopical is from: Comparing the Winds of Sandy and Katria

Comments on the Mariners Danger Rule for Hurricane Avoidance

 
 The original 1-2-3 and 34 kt Rule at the Mariner's Weather Log is here (pdf file).


The Mariners Danger  Rule is actully two rules - The 1-2-3 Rule and the 34 kts Rule. The 34 kt Rule is the recommendation that deep-sea ocean going ships stay outside the 34 knot wind field.  The 1-2-3  Rule is a method for plotting a danger area using a margin for forecast errors. The Mariners Dangers Rule  allows mariners to plot the danger area based upon forecasts.

Bill Bishop at The Marine Installers Rant put up an interesting post this past June - 123 TD Andrea about the Mariners 1-2-3 with links to the National Hurricane Center's National Hurricane Center Forecast Verification  with some charts showing the errors in hurricane forecasts.

Bill makes some good points about average errors  compared to the errors in the forecasts of individual hurricanes. 

  The 24 hour position forecast is pretty much on the money, the 48 hour position isn't quite as good, and at 72 hours you can see the outliers really start to appear, and by 120 hours the data permutations are biting hard. It's pretty much hit or miss, with a lot of misses. The intensity forecasts that impact the size of 34 kt wind fields are even more difficult to get right.

Bill makes another point about being inside the  35 knot field as too much. If 35 kts is too much just adjust accordingly. However besides the issue of the accuracy and reliability of the track forecast there is the issue of the forecast wind intensity and variations in wind field size. . For me  if I'm in 35 kts it is not usually an issue but if it was I'd add some distance from the system.

. From the NWS:

The 1-2-3 rule establishes a minimum recommended distance to maintain from a hurricane in the Atlantic. Larger buffer zones should be established in situations with higher forecast uncertainty, limited crew experience, decreased vessel handling, or other factors set by the vessel master. The rule does not account for sudden & rapid intensification of hurricanes that could result in an outward expansion of the 34 KT wind field. Also, the rule does not account for the typical expansion of the wind field as a system transitions from hurricane to extratropical gale/storm.

There is also a post at the Art of Dredging about the  1-2-3 rule which makes remarks about the dangers of using average tracks for hurricane avoidance.  That's a good point, using average tracks is a very bad idea,  but I don't agree with this:

The 1-2-3 system assumes that the cyclone will follow the textbooks.

Here is something new: tropical cyclones do not read textbooks.

And the chances you take (by assuming that cyclones stick to the rules) are huge.

Guessing that cyclones will follow the rules is close to a game of Russian roulette.

That's incorrect, no assumptions about cyclones following textbooks are made, the 1-2-3 Rule uses the forecast track with a margin of error. There is an important difference between the average track and the forecast track.

 In most cases the forecast is going to be the best information you have. Staying out of the area where the hurricane is forecast to go seems smart to me. The post mentions "modern techniques" but doesn't say what they are.

  This is what the original  article says in the case your vessel is in the danger area:

when a hurricane’s track is plotted, a 100-mile error for each 24-hour period must be applied and a vessel within this adjusted area must take action as if a hurricane were bearing directly toward them, which may become the case  

Forecasts are issued every six hours and the danger area can be replotted each time a forecast is received. The idea that the 1-2-3 rule uses textbook assumptions is nonsense.

All rules, including the Mariners Danger Rule are simplifications of more complex realities - knowing the rule alone may not be sufficient in all cases. When  there is any elevated risk or a complex situation  I get advice  from professional weather routers. Information about model agreement and confidence in the predicted track could be critical.  It's all about using more expertise if required to stay safe.

My post: Guidelines for Avoiding Hurricanes at Sea is here and How Accurate are Weather Forecasts? is here 

The NWS National Hurricane Center Marine Safety  is here. 

Mariner's Guide For Hurricane Awareness
in the North Atlantic Basin

 K.C.

Twilight Mersin Turkey

Twilight Mersin Turkey

Our stern ramp is aft on the starboard side so we don't often tie up port side to. I  walked the length of the ship with my camera  to record any new damage and I snapped this picture when  got to the bow. (no new damage)

K.C.

Accident Defense - The Costa Corcordia

Costa Concordia lays on the rocks Isola del Giglio

The Costa Concordia sank about a year ago, there have been numerous reports and commentaries  I thought the his one:   Costa Concordia   Anatomy of an organizational accident was interesting.

The report listed 6 errors which lead to the grounding. The errors were referred to as breaches of a defense. Defenses against incidents can be compared to a defense in a game such as soccer  against the opposing team scoring a goal.

In the case of a soccer team the defense is also in layers.

The first layer  is a good offense. If your team has possession of the ball and is in scoring position there is little chance the opposing team can score.

The second layer is keeping the opposition out of scoring position. The last layer of defense is the goalie.  If at all possible the goal of the defense is to avoid allowing shots on goal.

Looking at the 6 errors;

 This was where the captain allowed the opposition to have possession of the ball.

 The first error was made by the Captain, when he decided to change his original voyage plan without the agreement of the Company and local authorities

On the second and third errors  the opposition is moving the ball down the field.

The second error was a shortfall in voyage planning. According to the Safety Officer8 - who arrived on the bridge just after the impact with the rock – only the original route was drawn on the paper chart, a few miles off the island.

The third error relates to route monitoring, a specific task of the Officer Of the Watch (OOW). The bridge team was composed of a Senior OOW, a Junior OOW, a deck cadet, and a seaman with either lookout or helmsman functions. The SOOW was in charge of the conduct of navigation, with regards to conning orders, collision avoidance and route monitoring on the INS. The JOOW was assisting him fixing the ship’s position on paper charts, which has to be considered with priority over the INS route monitoring. The JOOW could not correctly monitor the approach to Giglio Island, firstly because there were no routes drawn on larger scale charts than 1:100.000, and secondly because she left the chart table to assist the helmsman when the Captain took the command of navigation

Now the opposition is moving the ball into scoring position

The fourth error also relates to route monitoring. In contrast to the third error, this involved the INS, to be officially used only as an aid to navigation. The equipment was operated by the SOOW, who used the radar distances and the electronic chart overlay to monitor the approach to the island. The error consisted in assessing the distance from the furthermost radar echo of Le Scole rocks, and not from the limit of the no-go area, that is the 10 meters bathymetric line.

 The fifth error is in the area of Bridge Resource Management (BRM), and it can be attributed to the Captain, as team leader. Indeed the effectiveness of BRM practices - essentially aiming at optimizing team work – depends heavily on the leadership skills of the Captain. That night, since the Captain arrived on the bridge at about 5 miles from Giglio Island, a series of erroneous BRM practices can be extracted from the depositions. These shortfalls are of a non-technical nature, involving mainly lack of team briefing, and lack of formal handover. In short, the Captain did not share intentions and expected outcomes of the decisions made, both before and during the manoeuvre.

 On the sixth and last error nothing stands between the opposition and a goal except the goalie.

 The sixth error is of a shiphandling nature. It was the one breaching the last available defence: the human expertise. The error consisted in failing to maintain the newly established safety margin of about 0.25 nautical miles.

The captain worked himself into a position where he had only his shiphandling skills to prevent disaster.

A good defense in sports reduces the number of shots on goal rather then depending upon the goalie to make saves. Careful voyage planning and good bridge team management likewise reduces the number of "saves", situations which in which the crew has little room for error.

K.C.

Susan Clark, Ship Captain, Pilot, Crosses Final Bar.

Capt. Susan Clark (Bangor  Daily News Photo)

Capt. Susan J. Clark, Portland Harbor pilot passed away 6 Sept.  Susan was born and raised in Skowhegan Maine, a mill town on the Kennebec River.

Susan finished first in her class at Skowhegan High School, and first at Maine Maritime. She was Exxon's first women ship captain. She Obtained her law degree and became a Portland Harbor Pilot.

Condolences to her family and friends.

Bangor Daily News obituary here.

 

Kennebec River at Skowhegan


K.C.

Debby - When Forecasts are not Accurate

Possible paths for Debby (from The Original Weather Blog)

After I posted "How Accurate are Weather Forecasts?" I encountered Tropical Storm Debby this past June  as it formed  in the Gulf of Mexico.  We didn't experience much besides some rain and 25 kts or so of wind. 

The interesting thing about Debby was the  large error in the forecast which seems to contradict my post in which I linked to the post  Extraordinary Skill for Extended Weather Forecasts  from Cliff Mass Weather Blog

This was an early  forecast from the NWS:

(from The Original Weather Blog)

This is a latter  forecast which is much closer to the actual track.

(from The Original Weather Blog)

In fact forecasters were well aware of the uncertainty in the forecast, here is a chart showing what the various models were showing:

(from The Original Weather Blog)

 The question is; how does the mariner at sea know when forecasters are not confident in the forecast?

 If access to the internet available mariners can dig down deeper by reading the discussion of the forecast which is available on weather web sites including the NWS.

Without internet access it's a little more difficult. The key is to use more then one source when available, in addition to national weather service forecasts  I use a weather routing service if I am going to be in the vicinity of heavy weather. The operations department also sends additional weather if  I request it.  A couple of times I've had my wife email me weather info.

Don't bet too heavy using a single source. As before, with the weather forecast, trust, but verify.

K.C.

A good discussion of Debby at The Original Weather Blog here
Tropical Storm Debby Forms in the Gulf... Now What?

Running the Route with the St. Louis

Loading at Dundalk Terminal Baltimore the ship in front of us pulled out first, I snapped a couple  photos as she slid by.

American flagged Alliance Saint Louis

Here she is sliding past using two tugs.

Sliding by

Here is the same same ship a few days later as we are both southbound in  the Gulf of Suez after leaving the canal.

It's common to see the same ship during a voyage. Often the same ships will be ahead or behind you for the entire voyage. It's like driving a bus on a bus route.



K.C.

Risk Managment - Failure to Rescue

One of the concepts in the book Managing the Unexpected is "Weak Signals of Failure". There is a lot to this, in operations small things are always going wrong, this is the "Unexpected" in the title. Risk management is not just about reducing or avoiding risk but also having a plan for coping when things go wrong.

In medicine when poor outcomes result from the failure to recognize and  properly deal with weak signals of failure a term used is "failure to rescue"

From this  post at The Epicurean Dealmaker - 50 Ways to Leave Your Lover.

The point of risk management is not to prevent failure, for that is impossible. The point is to have a plan ready to manage and control failure when it inevitably comes.

The  post links to a commencement speech by Atul Gawande . The speech uses a medical example of a women who had surgery for one problem but had a second hidden, unexpected problem which was discovered by investigation of a weak signal of failure.

From the speech -

This may in fact be the real story of human and societal improvement. We talk a lot about “risk management”—a nice hygienic phrase. But in the end, risk is necessary. Things can and will go wrong. Yet some have a better capacity to prepare for the possibility, to limit the damage, and to sometimes even retrieve success from failure. 

When things go wrong, there seem to be three main pitfalls to avoid, three ways to fail to rescue. You could choose a wrong plan, an inadequate plan, or no plan at all. Say you’re cooking and you inadvertently set a grease pan on fire. Throwing gasoline on the fire would be a completely wrong plan. Trying to blow the fire out would be inadequate. And ignoring it—“Fire? What fire?”—would be no plan at all.

Also mentioned however was the Deepwater Horizon disaster. -

In the BP oil disaster in the Gulf of Mexico two years ago, all of these elements came into play, leading to the death of eleven men and the spillage of five million barrels of oil over three months. According to the official investigation, there had been early signs that the drill pipe was having problems and was improperly designed, but the companies involved did nothing. Then, on the evening of April 20, 2010, during a routine test of the well, the rig crew detected a serious abnormality in the pressure in the drill pipe. They watched it and took more measurements, which revealed a number of other abnormalities that signal a “kick”—an undetected pressure buildup. But it was two hours before they recognized the seriousness of the situation—two hours without a plan of action. Then, when they did recognize the trouble, they sent the flow through a piece of equipment that can’t handle such pressures. The kick escalated to a blowout, and the mud-gas mix exploded. At that point, emergency crews went into action. But for twelve minutes, no one sounded a general alarm to abandon the rig, leading directly to the loss of eleven lives in a second explosion.

Failure to Rescue - Weak Signals of Failure, same concepts.

 I highly recommend Managing the Unexpected, it's a good solid book. I like being able to recognize these concepts when I run into them elsewhere.

K.C.

I posted about Managing the Unexpected Thinking Like a Mariner - Managing the Unexpected

and here At Sea

A good review from Harvard Business Review here.

For terminology this is a good site (pdf) Normal Accident Theory from NASA

How Accurate are Weather Forecasts?

GOES Eastern US SECTOR Infrared Image

How much trust can a mariner place in a weather forecast?

Aboard ship this question is sometimes a matter of considerable importance.

 I've run across a couple items recently that address this question.

The first is from Cliff Mass Weather Blog -  the post is Extraordinary Skill for Extended Weather Forecasts

(found via Channel 14, Seattle Traffic)

 In the early days of forecasting, when our forecast models were crude and our observational resources were few, weather prediction could not get close to the theoretical limits.  But during the last decade or so, with the availability of satellite data, fast computers, and far better computer models, prediction skill has extended further and further out in time.   In the 60s-80s we were lucky to skillfully forecast out 2-3 days.  In the 90s 3-4 days.  And in the 2000s, 4-5 day forecasts were often quite skillful.  But recently, we have seen highly skillful forecasts consistently at 5-7 days, and occasionally approaching two weeks.

Here is the bottom line

So when people ask you how far into the future meteorologists can predict the weather, a good answer is:

2-3 days with excellent skill

3-4 days with moderate, but useful skill

5-6 days with marginal skill

..and occasionally skill extending out 7-10 days.

Yes...sometimes forecasts go wrong in less than a day...but is considerably less frequent than a decade ago.

The second post is from gcaptain - What is the Mariner’s 1-2-3 Rule and Should it Be Updated? - By Fred Pickhardt,

I'm not going to get into details here as I've posted on the 1-2-3 Rule before here: Guidelines For Avoiding Hurricanes At Sea

 Basically the argument is that the accuracy of hurricane track forecasting has improved and the safety margins used in the old 1-2-3 rules can, in certain circumstance, be reduced. Better to get the info  from the horse's mouth - Fred Pickhardt's post is here 


Aboard ship it is important to  monitor the accuracy of the forecasts, We don't dispose of the weather until the voyage is completed without incident. If you check  the 96hr, the 48 hr and the 24 hr forecasts against the actual  weather you can develop a sense of how accurate they are.

My experience is  that the NWS  (National Weather Service) forecasts and some other agencies  are much better then many people believe but  also it is  easy to fall into the trap of accepting them uncritically out of habit. Much like monitoring the position  of the ship to ensure it stays in safe water you should check from time to time to ensure the forecast is "on track".  How often you check depends upon the situation.

Most mariners are aware that the longer the range the forecast is the less accurate it is. The 24 hr forecast can be relied  upon more then the 48 hr and so on.  Beyond that  my rule of thumb  regarding the trustworthiness of  the forecast is it depends on the issuing agency and how complex the weather situation is. Another important considerations is how much is at stake if the forecast is inaccurate.

 I have higher trust in forecasts from the United States, Japan and Europe then some other places. I've had  the most experience in those areas and have had many opportunities to verify the forecasts. I have more trust in simple situations then complex ones, for example if there are lots of low pressure areas about I am more wary then the simpler case of a single big high pressure system.


An important question  is how much is at stake? If you lose the bet will it result in a  delay in operations or are you risking the safety of ship and crew?  In cases where you have lots of sea room it may matter little if the forecast is inaccurate as you can adjust your track as needed.  On the other hand if you are coastwise or in port your options are likely far fewer. You need to think what is the worse case scenario  and is a plan B practical?

 Plan B developed for unfavorable weather is a subject in and of  itself.

The bottom line is trust but verify - make sure you monitor the weather on a continuous basis, keep the latest forecast posted - the watch mates  should know when the next forecast is due in. Keep the old forecasts and compare them to the actual weather.

Finally, keep an eye on the barometer and the sky. - Look not to leeward for fine weather.

K.C.

UPDATE: I've tinkered with this post a couple of time and reserve the right to continue editing it in the future.

My Post Capt McWhirr on Weather Routing.

 Here is a somewhat  inane post from Freakonomics How Valid Are T.V. Weather Forecasts? The conclusion is the NWS does better then the TV weather man and that short term forecasts are more accurate then long term (no surprise there)

Lee Chesneau's Marine Weather is here - A few years back Lee wouldn't leave me alone until he was sure I understood a critical element of the 500 mb chart

Book Recomend - Thinking Fast and Slow

 The first GPS I ever encountered had a quirk. If it stopped receiving a signal it would, without any indication or alarm of any kind, switch over to navigation by dead reckoning. It would always give you a position that looked accurate out to three places - even if it was not receiving a signal at all.

 (The cruise ship Royal Majesty went aground for this reason)

 Turns out, the human mind works much the same way. Here is an example from the book

 Likely you read the left box as A B C and the right box as 12 13 14 but A 13 C or 12 B 14 would have been equally valid. Your mind resolved the ambiguity without effort but like the GPS switching to DR it did so with out notification.

People have a strong bias for plausible  narratives and prefer them to more probably  scenario. From chapter 9:

A remarkable aspect of your mental life is that you are rarely stumped......The normal state of your mind is that you have intuitive feelings and opinions about almost everything that comes your way.

The  mind uses a trick the author calls Substituting Questions. Rather then answer a hard question the mind substitutes a simple problems and answers that one instead. Ironically the less information we have about a question the more certain we are that we have found the correct answer.


There is far more in this book then the above. The best review I've seen is this one from the New York Time Book Review. 

Wikipedia has a good summary here. 

 Thinking Fast and Slow has five parts:

Part 1 Two Systems
Part 2 Heuristics and Biases
Part 3 Overconfidence
Part 4 Choices
Part 5 Two Selves


 The difficulty I had with this book is it seems too long. I found parts 1 through 3 to be fascinating but started bogging down a little on parts 4 and 5. When I hit the section about Bernoulli's Errors and  the fourfold pattern  I felt the author was starting to torment me.

 At one point when I came to problems in the book like "Bet A 11/36 to win $160, 25/36 to lose $15 or Bet B 35/36 to win $40, 1/36 to lose $10"  I had to skim, I understand that I don't understand.

 This problem is a little more up my alley - I got it wrong but it illustrates the point.

A bat and ball cost $1.10.
The bat costs one dollar more than the ball.
How much does the ball cost?

Most people choose 10 cents - the correct answer  is 5 cents: an explanation is here.

 

 A  point of interest to mariners was the section on feedback and practice in the chapter on Expert Intuition. The author compares driving a car which we can master quickly compared to a harbor pilot maneuvering a large ship. Ship handling takes  much longer to learn because of the long delay between  the action and the outcome. The quality and speed of the feedback is much better in the case of the car.

I highly recommend this book if you make high-stakes decisions, every master does. I've read other books on cognitive bias and so forth but this book goes both deeper and covers more ground then any other book I've read. It is the "big picture"  The going gets a bit heavy in places but overall well worth reading.

K.C.

Reduce Work Load - Add Expertise, Not Crew

There is no question in my mind that many mariners  are between the rock and  a hard place when it comes to workload and crewing levels.

Three maritime officers unions issued a statement (at gcaptain here)

Cuts in manning levels and burgeoning paperwork loads are increasing the risk of maritime accidents worldwide, says a group of U.S. maritime unions.

and

Maritime officers are calling on Congress to direct the U.S. Coast Guard to conduct an assessment of fatigue and crewing levels based on the recommendations of independent professionals experienced in workplace fatigue.

I don't agree that manning levels are necessarily  too low.The problem is not lack of man-hours but  a lack of expertise.

What needs to happen is ships need more officers and fewer unlicensed. Many American ships now carry six ABs and three watchstanding  officers. Three of the ABs need to upgrade to officer level.

 In the wheelhouse at sea  the watch can be stood  a single crew but  usually there are  two crew members, an officer who acts as  mate on watch and the AB acting as helmsman /  lookout.

On a modern ship however  a helmsman is rarely needed, sophisticated auto-pilots can be programed to turn the ship as required.  Instead of an watchstanding officer and an AB the bridge team could consist of two officers, a senior officer of the watch and a junior.

During times when the workload on the bridge is low, one mate could navigate and watch forward while the other mate could  perform clerical work. When the work load increases both mates could cooperate using the principles of Bridge Resource Management (BRM) saving having to call the master when assistance is required.

Work loads aboard ship are high and increasing, however rather  then adding crew a better response is   adding to the crew's capabilities.

K.C.

Name Rectification: Monsoon

Photo by Vinoth Chandar

For navigators, the term Monsoon refers to a seasonal wind.

This usage is reflected in the roots. According to Wordnik: ", monsoon comes from the  Arabic  word " mawsim" which means season. Obsolete Dutch monssoen, from Portuguese monção, from Arabic mawsim, season, from wasama, to mark; see wsm in Semitic roots."

Essentially monsoons are a large scale, continental version of  sea and land breezes which occur on the coast. The driving force behind both monsoons and sea breezes  is the different rates at which sea and land heat and cool. Land heats and cools at a faster rate  then the sea. Warm air rises and is replaced by cooler air. Thus the daily (in the case of sea breezes) and seasonal (in the case of monsoons) reversals of wind direction.


The most well known Monsoon is the Indian Monsoon. From May to September, the winds are SW,  from sea to land. From December to February winds are NE, from land to sea.


The summer Monsoon famously  brings moisture from the sea  to the Indian sub-continent which results in (sometimes heavy) rain. This accounts for fact that the term Monsoon is often  used to refer to the rainy season only (or sometime  just to refer to heavy rain).

For navigators, the less well know, but  equally important  winter Monsoon brings NE winds.

 In practical terms the Monsoons influence the weather over a very large area. On a voyage from the Suez Canal to Japan, the effects of the SW monsoon are first felt in the southern Red Sea but more dramatically upon leaving the Gulf of Aden and entering the Arabian sea. There, exposed to  long fetch of the Indian Ocean,   the SW monsoon winds sometimes result in a  heavy swell. Entering the Straits of Malacca the weather is dominated by local conditions but after leaving Singapore Straits and proceeding northward in the South China Sea the monsoon reasserts itself. 


The term monsoon is  used to refer to any seasonal reversal of winds  -  here is an article about the Arizona or Mexican Monsoon.

K.C.