The SE monsoon prevails from April to September but does not attain any great degree of constancy until August; it is particularly unsteady in the daytime, and variable winds are common. In August and September the monsoon blows stronger and steadier, generally from SE and SSE, and particularly off the Java coast at night when the land breeze reinforces the monsoon. The NW monsoon arrives at the end of November. This results in WSW winds in December, W winds in January and WNW winds in February. Clouded skies and SW squalls are frequent, the latter occurring generally during the NW monsoon and accompanied by the thunder.

The rainfall corresponds with the monsoon, the NW monsoon month being the Rainer. February has the greatest rainfall, with September the least. The state of the sea is generally slight to moderate but can be rather rough; heavy swells are not uncommon, particularly during the stronger period of the NW monsoon, when the current is running counter to the prevailing wind. As a rule, the sea is calmest in March, July and November.  Weather Information (klik)

The tidal rise and fall is small with 1.5 - 3 meters maximum between maximum high and low water and is usually semi diurnal.

Currents are variable but can be quite strong.

Tide

HWSL                                                    + 3.00 M               (High Water Springs Level)

MWSL                                                   + 1.50 M               (Mean Water Springs Level)

LWSL                                                    + 0.00 M               (Low Water Springs Level)

Platform/Jetty Head + 5.50 M               ( Low Water Springs Level )

The tidal current velocity is 0.1 m/sec without negative influence on ship approach as well as on ship berthing.

Controlling Depth

The maximum permitted draught alongside The Siam Maspion Terminal Jetty, including an allowance for under keel clearance (UKC) of 10% of vessel loaded draught, is currently -10.5 meter LWS. Chart depths alongside and immediately off the jetty are based on a chart datum of Low Lowest Water Springs (LLWS).

The Surabaya Port Authority do permit deeper draught vessels to transit the North Channel based on maximum draught allowed 8.0 meter plus height of tide, less 10 % of vessel summer draught of UKC.