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Indonesia Climate
The main variable of Indonesia's climate is not
temperature or
air pressure, but rainfall. The almost uniformly warm
waters that
make up 81 percent of Indonesia's area ensure that
temperatures on
land remain fairly constant (see
table 2, Appendix). Split
by the
equator, the archipelago is almost entirely tropical in
climate,
with the coastal plains averaging 28°C, the inland and
mountain
areas averaging 26°C, and the higher mountain regions,
23°C. The
area's relative humidity ranges between 70 and 90 percent.
Winds
are moderate and generally predictable, with monsoons
usually
blowing in from the south and east in June through
September and
from the northwest in December through March. Typhoons and
largescale storms pose little hazard to mariners in Indonesia
waters;
the major danger comes from swift currents in channels,
such as the
Lombok and Sape straits.
The extreme variations in rainfall are linked with the
monsoons. Generally speaking, there is a dry season (June
to
September), influenced by the Australian continental air
masses,
and a rainy season (December to March) that is the result
of
mainland Asia and Pacific Ocean air masses. Local wind
patterns,
however, can greatly modify these general wind patterns,
especially
in the islands of central Maluku--Seram, Ambon, and Buru.
This
oscillating seasonal pattern of wind and rain is related
to
Indonesia's geographical location as an archipelago
between two
large continents. In July and August, high pressure over
the
Australian desert moves winds from that continent toward
the
northwest. As the winds reach the equator, the earth's
rotation
causes them to veer off their original course in a
northeasterly
direction toward the Southeast Asian mainland. During
January and
February, a corresponding high pressure system over the
Asian
mainland causes the pattern to reverse. The resultant
monsoon is
augmented by humid breezes from the Indian Ocean,
producing
significant amounts of rain throughout many parts of the
archipelago.
Prevailing wind patterns interact with local
topographic
conditions to produce significant variations in rainfall
throughout
the archipelago. In general, western and northern parts of
Indonesia experience the most precipitation, since the
north- and
westward-moving monsoon clouds are heavy with moisture by
the time
they reach these more distant regions. Western Sumatra,
Java, Bali,
the interiors of Kalimantan, Sulawesi, and Irian Jaya are
the most
predictably damp regions of Indonesia, with rainfall
measuring more
than 2,000 millimeters per year. In part, this moisture
originates
on strategically located high mountain peaks that trap
damp air.
The city of Bogor, near Jakarta, lays claim to having to
world's
highest number of rainstorms per year--322. On the other
hand, the
islands closest to Australia--including Nusa Tenggara and
the
eastern tip of Java--tend to be dry, with some areas
experiencing
less than 1,000 millimeters per year. To complicate the
situation,
some of the islands of the southern Malukus experience
highly
unpredictable rainfall patterns, depending on local wind
currents.
Although air temperature changes little from season to
season
or from one region to the next, cooler temperatures
prevail at
higher elevations. In general, temperatures drop
approximately 1°
per 90 meters increase in elevation from sea level with
some highaltitude interior mountain regions experiencing night
frosts. The
highest mountain ranges in Irian Jaya are permanently
capped with
snow.
Located on the equator, the archipelago experiences
relatively
little change in the length of daylight hours from one
season to
the next; the difference between the longest day and the
shortest
day of the year is only forty-eight minutes. The
archipelago
stretches across three time zones: Western Indonesian
Time--seven
hours in advance of Greenwich Mean Time (GMT)--includes
Sumatra,
Java, and eastern Kalimantan; Central Indonesian
Time--eight hours
head of GMT--includes western Kalimantan, Nusa Tenggara,
and
Sulawesi; and Eastern Indonesian Time--nine hours ahead of
GMT--
includes the Malukus and Irian Jaya. The boundary between
the
western and central time zones--established in 1988--is a
line
running north between Java and Bali through the center of
Kalimantan. The border between central and eastern time
zones runs
north from the eastern tip of Timor to the eastern tip of
Sulawesi.
Data as of November 1992
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