Tuesday 27 December 2022

Rough Guide to Monsoon Seasons, AU Top End & SE Asia

Weather, Almost everyone we talked to when we went into SE-Asia were a little confused when trying to work out the reasons for the two differing wind directions for the one monsoon season. The Intertropical Convergence Zone, or ITCZ, is the region that circles the Earth, near the equator, where the trade winds of the Northern and Southern Hemispheres come together. Sailing up in this area can be difficult at times, however if you can get a handle on the basics it really helps especially as some voyages will start in one hemisphere and end in another. 

Indonesia spans across the equator and you need to take this into account when you are reading about which weather patterns fall in which month. We didn't one time and as it turns out were not pleasantly surprised. Indonesia doesn't get a summer or winter its either the wet season or dry season the notation on the list below is more to denote the hemisphere season. Also of note is the dry/wet is not the whole of Indonesia in the same season.

Southern Hemisphere Indonesia                              Northern Hemisphere Indonesia
N-West monsoon Dec-Jan-Feb   (S Summer)          N-East monsoon Dec-Jan-Feb     (N Winter)
First transition Mar-Apr-May                                  First transition Mar-Apr-May
S-East monsoon Jun-Jul-Aug     (S Winter)             S-West monsoon Jun-Jul-Aug     (N Summer)
Second transition Sep-Oct-Nov                                 Second transition Sep-Oct-Nov

Bruce's rough guide to the monsoon seasons on both side of the equator. 

Winds change through the year.

During December and January, the Southern Hemisphere is heated more strongly by the Sun than the Northern Hemisphere, so the hottest air — the air that rises in the ITCZ — is found south of the equator. Winds from the Northern Hemisphere blow across the equator toward the ITCZ.

During June and July, the Northern Hemisphere is heated more strongly by the Sun, so the ITCZ and its rising hot air lie north of the equator and winds blow from the Southern Hemisphere across the equator to reach the ITCZ in the Northern Hemisphere.

As the ITCZ changes location through the year, the winds and rains and the location of monsoon wet weather changes, too.


Remember that the Coriolis force changes direction on the equator: It turns winds toward the right in the Northern Hemisphere (so a SE in the Southern Hemisphere turn SW in the northern Hemisphere) Wind turns toward the left in the Southern Hemisphere (so a NE in the Northern Hemisphere becomes North West in the Southern Hemisphere).

So when air crosses the equator as it flows from the cold winter hemisphere toward the ITCZ in the summer hemisphere, it experiences a change in the Coriolis force. This causes the trade winds to reverse direction and blow toward the west in the winter hemisphere and to the east in the summer hemisphere.

This seasonal reversal of the winds was historically very important for trade between Africa and Asia. Ships would sail from Asia to Africa in winter and then undertake their return voyage when the summer monsoon changed the wind from westward to eastward.

Winds within a couple of degrees of the equator are generally “light and variable” all the time. Weather forecasts are rarely accurate in these areas. Make sure you have a cruising spinnaker and as a back up, an engine. Winds are spasmodic and currents are/can be quite strong depending where you are.


SE / SW Monsoon Season

While the drawing I have done is a little rough I hope the fundamental idea makes sense. 

During the months of June-September dry season trade winds blow consistently from the SE and ESE throughout the Southern Islands of Indonesia. Trade winds are stronger in Eastern Indonesia and areas around Timor and the Arafura Sea than they are further to the West near Lombok, Bali, Java and Sumatra.

Further North as SE trade winds approach the equator they lose considerable strength and veer from SE to a very light S. Then, once across the equator and into the northern hemisphere, due to the Coriolis effect, these light S winds again veer from S to SW and slightly increase in strength. It is for this reason that we sometimes refer to this season as the SE/SW monsoon season.

Precipitation on both sides of the equator is at odds. While Bali and Java experience dry season trade winds out of the SE, the areas of northern Sumatra, Kalimantan, Sulawesi, Morotai and Thailand will be in their SW monsoon wet season.

NE / NW Monsoon Season

During the months of December-February NE winds blow in the northern hemisphere and across the top of Indonesia. As these NE winds approach the equator they lose strength and back to the N. Once across the equator, because of the Coriolis effect, winds back further and increase in strength to become the NW monsoon wet-season for much of Southern Indonesia. Southern Sumatra, Southern Sulawesi, Java, Bali, Flores, Timor, and Rote will all experience strong westerly winds and rainy conditions for much of this period with the height of the wet season falling in February.

Clear sky's and calms are often experienced in between passing storm systems.

Cyclones can develop in the Timor, Coral and Arafura Seas and this area should be monitored if passing near Timor and Rote Islands during the NE/NW monsoon season.

Transitional Seasons

Some nice cruising can be had during the changing of the seasons. Winds are often light to non-existent with only short duration squalls passing through. Some of the more exposed and windy areas of Indonesia are often calm and pleasant and open to exploration at this time. With the sun directly over the equator and calms likely, conditions can be very hot. The ITCZ can move by as much as a thousand miles overnight or not move at all and sometimes will appear in two places at once. With such changing conditions occasional late or early season weather does develop.


We have found that neither the GFS nor the ECMWF do a good job of forecasting winds in Indonesia. Generally they get the direction right, but wind strength is hit and miss. Weather forecasts (GRIB files) in the Malacca Strait never seem to be right either, perhaps this is just our observation because, we possibly never got far enough off shore to get away from its effects.

Here is a reasonable explanation of the monsoon seasons:


Malacca Strait

The weather description below is for the Malacca Strait (Selat Melaka), being a passage between Peninsular Malaysia and Indonesian Sumatra there are a couple of anomalies. When using any of the Island anchorages, be prepared to get underway at a moments notice. After being caught, we would work out an action plan to get to the other side quickly. If we were lucky, once the front passed there would be a calm for a couple of hours after the storm, before the wind returns to its original direction.

Northern Hemisphere Melaka Strait

Northeast Monsoon - November to March

This is the dry season. The winds are predominantly from the NE from 10 to 20 knots, with occasional winds up to 30 knots. There are frequent calms lasting from a few hours (often in the evenings and early morning) to whole days or a few days duration. Rain is very rare and the days are fine and relatively cool as the sun is at its furthest south at this time (in December the sun is only 60 degrees above the horizon at midday) - Very pleasant cruising weather and because the wind is coming straight off the Malaysian peninsular there is no swell and any seas that do come up during local strong winds drop quickly when the wind drops.

Transition period - April and May

This is the height of the transition period between the NE and SW monsoons and is marked by many calms, very light winds and flat seas. When there is wind it can come from any direction and vary in strength and direction, but is not common from the East through to South or from the North. There is the odd rain squall that can last from five minutes to an hour or two with winds sometimes up to 25 knots under them.

Very little chance of a "Sumatra" (see SW Monsoon). Hot, sultry weather.

Southwest monsoon - June to September (Some years into late October)

This is the wet season. Hot, sultry weather as the sun is in the North and the humidity is high. The winds are predominantly from the SW from.10 to 25 knots, with frequent calms and frequent rain squalls. The rain squalls last from a few minutes to a few hours but typically last for about 15 minutes with very heavy rain and either no wind or, more frequently, winds up to 25 knots. These winds change strength and direction depending on your position relative to the centre of the squall.

Once they have passed over, the skies clear and the wind returns to its condition before the squall arrived. A squall can usually be seen well before it arrives as an isolated large, dark, towering cloud and, should you be close to land or rocks, gives you plenty of time to either anchor or head for an open area before the heavy rain causes a complete whiteout.

This is the season for "Sumatra" winds.

"SUMATRA" winds are a different matter altogether. In the day they can easily be identified as a long rolling bank of very dark cloud extending roughly North and South from horizon to horizon and coming from the West from Sumatra, thus their name. At night, when they are most common, the first warning is when they hit. They are most common in the South Malacca Straits, but do occur occasionally around Penang ,and Langkawi, though they are rarer around Phuket. Typically there will be two to four during this season in Penang and one or Penang and one or two in Phuket. There is no avoiding the very strong (up to 60 knot)\ Westerly quarter wind that just precedes the cloud bank and very heavy rain. They can last for five or more hours, though more commonly last for from about 20 minutes to one hour and then end as quickly as they began.

Transition period - October

Same as the Transition period from April to May.

This is the height of the transition period between the SW and NE monsoons and is marked by many calms, very light winds and flat seas. When there is wind it can come from any direction and vary in strength and direction, but is not common from the East through to South or from the North. There is the odd rain squall that can last from five minutes to an hour or two with winds sometimes up to 25 knots under them.

Very little chance of a "Sumatra" (see SW Monsoon). Hot, sultry weather.


Red sky at night sailors delight; red sky in the morning sailors take warning.”

There is a simple explanation for this. When the sun is on the horizon, its light, shining at an angle, must pass through more atmosphere.

Red is the colour of the spectrum with the longest wavelengths and can therefore reach all the way through to where water vapour and dust is denser. Here it reflects off the vapour and dust projecting a red light to our eye. In mid latitudes storms predominantly move from west to east.

Red sky at night sailors delight" So when the sunset in the west reflects off particles of a low pressure in the east, creating the red lit sky, the weather has past making way for improved (delightful) conditions.

"Red sky in the morning sailors take warning" So then if the sunrise in the east reflects off particles of a low pressure in the west, making the red lit sky, we can assume that storms are approaching, sailors take warning the weather should worsen.

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