Weather characteristics are described in unusual detail in this essay
because of the influence  rainfall, wind velocity, humidity, cloud cover
and temperature have on a wide variety of activities. For example,
structures should be built to withstand high winds. This natural element is
also a factor to consider relative to lagoon fishing and other recreational
activities. Precipitation influences farming and potable water supplies,
particularly ground water sources and residential catchment systems and
increases road maintenance costs when replacing or regrading washed out
roadways.
  Humidity is frequently a factor to consider in insulating certain
electrical devices.  Cloud cover not only influences electric generating
solar power cells and temperature, but is also a comfort factor since it
can influence the efficiency and cost of operating air conditioning
equipment.
   Overall, climate is an important determinant in establishing an area as
a tourist resort destination. With mariculture activities, particularly
marine shrimp, the salinity of the water in grow-out ponds can be lowered
by heavy rainfall and additional pumping from the natural water source will
be required. Weather can, and does, have a direct influence on a wide
variety of activities in the Northern Marianas.
    The Mariana Islands enjoy a tropical oceanic climate characterized by
relative high and uniform temperatures. The annual mean temperature is 83
degrees Fahrenheit (F.). The seasonal variation in mean monthly temperature
is less than 3.5 degrees F. However, temperature is affected by elevation,
hence, the islands of Saipan and Rota show considerably greater temperature
variations between the coastal and mountainous areas. The Guinness Book Of
World Records lists Saipan as having "the most equable temperature" with a
low of 67.3 degrees Fahrenheit and a high of 88.5 F., with the range being
21.2 degrees. While the barometer pressure is uniform there are double
diurnal maxima and minima. Humidity is very high with monthly averages
between 79 and 86 percent but fresh breezes provide a measure of
comfortable relief.  The lowest relative humidity occurs in the early
afternoon when the air is heated faster than evaporation can compensate and
the highest just before dawn when temperatures and the capacity of the air
to hold water vapor are at their minimum.
   The months of greatest humidity are July to November. The mean annual
rainfall is approximately 83.8 inches, but rainfall varies from year to
year. Sometimes the islands experience droughts generally during the period
from December through June. Some rain does occur during the dry season.
The wet season is from July through October.
   In 1983 half of the monthly rainfall totals recorded were the lowest on
record for the period from January to May (dry season). Total rainfall west
of 155 degrees east  longitude for this period was 27-29 percent of the
normal rainfall. At locations east of the above longitude, 12-13 percent of
the normal rainfall was experienced.  The recurrence intervals for these
rainfall conditions have been estimated by the U.S. Corps of Engineers to
be 125 years.  The drought , believed to be a result of the phenomenon
known as El Nino, may now be over as groundwater levels are responding to
recharge from rainfall.
   Approximately 69 billion gallons of rain falls on Saipan in an average
year but it  is not uniformly distributed throughout the year.
   There is considerable cloudiness in the Marianas and on a scale of 10
(ranging from 0 for a cloudless sky to 10 representing a completely
overcast sky) the annual mean cloudiness averages 6.7.  Cloudiness is
greatest from July to September when the average number of cloudy days per
month is about 19.
  The Marianas experience three wind patterns.These are: trade
winds,doldrums and typhoons. The islands lie near the border between the
Asiatic monsoon and the belt of northeast trade-winds. At Saipan, easterly
winds prevail about 45 percent of the time. From November until March or
April winds tend to be easterly and northeasterly. Average annual wind
velocity  is 10.5 mph.
   Two principal types of storms influence the climatic character of the
Northern Marianas: small-scale storms, consisting of thunderstorms and
squalls, and large systems of tropical storms and typhoons which can
dominate an area as large as 300,000 square miles and persist for a week or
more.
   Light thunderstorms occur occasionally throughout the year, particularly
during the months of June to October. The season of most serious storms is
from August to mid-December. This is the period of the most frequent
occurrence of tropical disturbances. While typhoons do not occur every
year, when they do pass over the islands winds of 115 miles per hour with
gusts of 160 miles per hour can be experienced. The islands have a very
efficient system of monitoring tropical storms and ample time is provided
to prepare for such disturbances.
El Nino
   The oceanic and atmospheric event which can change weather patterns
within the Pacific and along its eastern coastlines in both the Northern
and Southern Hemispheres is known as El Nino, (named the "Christ Child"
because it occurs near Christmas). It is believed by some scientists to be
related to a reversal of the equatorial undercurrent in the western
Pacific. Presently the cause of the start and end in this change of
direction of the current is unknown. The phenomenon appears to run in
cycles which recur  every four to seven years. It warms the waters of the
eastern Pacific and causes unusually heavy rain while having a cooling
effect on the waters in the Indonesian region and produces droughts
throughout the area including the Pacific islands.
    This equatorial undercurrent is about 275 miles wide and extends across
the Pacific flowing eastward at the equator at about one mile per hour,
however, at times it has been measured flowing in the opposite direction.
Scientists theorize that the reversal of this current may precipitate the
El Nino event which can have a devastating effect on the ecology,
particularly fisheries. Those who study the phenomenon  think that between
periods of El Ninos there occur La Nines, periods of one to two years when
the surface water of the equatorial Pacific becomes cooler and flows
westward. This can be equally influential in affecting the climate of
distant places.
   Sea surface temperatures in the area are influenced by air  temperatures
and ocean currents.
Global Warming
     One hears a great deal about global warming as a result of a rapid
increase in carbon dioxide (CO2) in the atmosphere which traps heat within
the atmosphere thus resulting in a "greenhouse" effect. Within the
classification of Pacific islands ranging from category "A" or those areas
likely to sustain the most severe impact to category "D" evaluated to
impact locally from severe to catastrophic, the Northern Mariana Islands
have been classified as category "C"  with the possibility of moderate to
severe impact with major changes in crop production and demographic
patterns as a result of a rise in sea level which in turn could effect
coastal settlements.
Earthquakes - Volcanic Activity
       While earthquakes can occur at any time in the Commonwealth, no
serious damage has yet occurred. Earthquakes that can be felt, or those
with a magnitude of 4 or 5 on the Richter Scale, occur once or twice a year
and there are weekly tremors which cannot be observed without instruments.
The epicenters of most earthquakes are located on the Pacific Ocean floor
and intensities generally diminish before reaching the Marianas. The
earthquake history of Saipan since 1800 records two major events, one in
1849 and the other in 1902, ( actual magnitudes are not known). In April
1990 an underwater earthquake measuring 7.5 on the Richter Scale was
recorded as occurring 225 miles northeast of Guam and 100 to 150 miles
southeast of the island of Anatahan. All the islands within the archipelago
experienced the tremor but no damage or injuries were reported. Tinian
experienced a series of 10 foot waves. This seismic event took place at a
time of increased volcanic activity on the island of Anatahan. The Marianas
Trench is a breeding area for earthquakes, 23 registering 7 or more on the
Richter scale have been recorded since 1900. In 1993 an earthquake occurred
on Guam, 120 miles south of Saipan which caused considerable damage to a
hotel which later had to be demolished.
       Government Design Manual 5-809-10 dated February 1982, shows Saipan
located in seismic probability zone 3 with a design maximum acceleration of
0.33g and a corresponding approximate magnitude of 7 on the Richter Scale.
The Richter Scale is a method of measuring earthquakes by which every
increase of just one number means a ten-fold increase in magnitude. An
earthquake of 3.5 on the Richter Scale can cause slight damage; 5 -
considerable damage; 6 - severe damage; a  reading of 7  is a major
earthquake capable of widespread heavy damage and 8 is a "great" quake,
capable of tremendous damage.
       The islands are located within what has been called the "Pacific Rim
of Fire", an area of active volcanic and seismic activity which extends
around the entire rim of the Pacific Basin along the coastal fringe of the
continental land masses and extending south from Japan through the South
Honshu Ridge and the northern islands in the Marianas Arc.
       According to U. S. Geological Survey scientists all islands north of
Farallon de Medinilla are vulnerable to future devastating volcanic
eruptions.Saipan, Tinian and Rota are not presently within an active
volcanic area.
      Esmeralda Bank, 24 miles west of Tinian, is the southern most active
volcano in the Izu - Volcano - Mariana Arc and is one of  the most active
vents in the western Pacific. It rises to within 100 feet of sea level and
is considered to be  an area of potential eruption or a  "hot spot." In the
early part of the 20th century the banks were reported to be above sea
level but disappeared below water as a result of an earthquake.

Northern Mariana Islands
Earthquake Magnitude Frequency
(January 1990 - February 1992)
   Magnitude Number Of
   (Mb, Ms) Earthquakes
   3.6 - 3.9  12
                          4.0 - 4.9                   284
   5.0 - 5.9  93
   6.5     1
   7.5     1
Source: U. S. Geological Survey & CNMI Disaster Control Office
Note: Most of the above activity occurs within the area of the Northern
Islands.

Soils And Land Slides
      Construction on slopes over 50 percent usually require extensive
engineering studies as mud and rock slides pose a threat particularly
during heavy rains where construction has occurred  on clay soil or loose
rock. All of the soils in the Marianas are lateritic in character having
evolved under high temperature and abundant rainfall. In the elevated
portions of Saipan the soil is surface clay while the lowland has rich top
soil. Limestone underlies some 83 percent of the island.  Generally, the
soil is less than 30 inches deep. Because of Saipan's complex geology and
the mountain range which extends along  three fourths of the island on a
north-south axis, there are many short and rugged valleys, but only two
perennial streams near the central sector, Sadog Talofofo and Sadog
Haangot.  These streams drain from west to east.
       Of the 29,811 acres of land on Saipan, only 1300 acres or about 5
percent of the land could be considered fair in overall productivity
ratings for agricultural use.
       Lands considered as poor  in overall productivity account for some
10,500 acres or 35 percent of Saipan. These lands can be developed into
productive grazing lands. The poorest lands are those with steep slopes,
very shallow soil, rocky surface or marshes, etc.  This land type accounts
for 56 percent or 16,600 acres on Saipan and is best adapted for watershed,
grazing or conservation uses.  Agricultural land uses are summarized as
follows:
-Vegetable Crops - are grown on moderately deep to deep soils under
unirrigated situations.  Generally the lands are fallow during the period
of low rainfall, January to April.  The existing farms are rather small in
size.
-Orchards - coconut, banana, breadfruit and citrus are most common.
-Row crops - taro, cassava, sweet potato, yam and corn are the most widely
grown crops.
-Grazing lands - the principal forage species is tangan-tangan (Leucaena
glauca), Guineagrass, Napiergrass and Johnson grass.
-Forest lands - there are scattered stands of Formosan koa (Acacia
confusa).
Tides
       In the spring and fall syzygies, semi-diurnal tides prevail, but at
other  times the daily inequality in the time and height of successive high
and low tides is considerable.  The daily inequality is greatest when the
moon is farthest from the equator; at this time there is practically only
one low and one high water per day.
      Tidal Information
      (Height above datum soundings)
Mean  High Water                      Mean Low Water
Higher     Lower                             Higher     Lower
2.1 ft.      2.1 ft.                                 0.3 ft.      1.6 ft.

Tidal Currents
       Currents in the vicinity of the Northern Mariana Islands are usually
westerly and strongest near and southward of Saipan. They gradually become
weaker to the north. In the vicinity of the islands, the flood current
usually sets westward and the ebb current eastward. The tidal currents turn
at the approximate times of high and low water. These currents are usually
weak, except in narrow passages, and their direction and rates are variable
at times. The tidal currents are often irregular off the eastern side of
the islands. This is a result of the islands' configuration.
        In the Garapan dock area the tidal currents set northward at rates
of 0.5 to 1 knot during rising tide and southward at rates of 0.5 to 0.75
knot during falling tide. At Tanapag Harbor the tidal currents set
northward on the rising tide and southward on the falling tide neither
exceeding a rate of 0.75 knot according to U. S. Army Corps Of Engineers
investigations.
Wave Conditions
       Both the Garapan Dock and Tanapag Harbor area are well protected
from waves generated by the prevailing easterly trade winds and are
primarily affected by waves from the southwest clockwise to the northeast.
The majority of waves affecting Saipan are easterly trade-wind generated
waves. Deepwater wind wave heights are generally 2 to 6 feet with periods
of 6 to 12 seconds.
Water Supply(1)
       Saipan has no rivers and only a few perennial streams. Other than
several small desalination plants, rainfall is the only source of fresh
water. This is recovered either through catchment systems, pumped ground
water or from the Ghyben Hertzberg  lens.
       Saipan's only  lake at Susupe, with an area of 2 square miles, is
brackish (1 p.p.t. to 2.5 p.p.t.) (2).  Most of the island's streams dry up
during the dry season.  Saipan does experience a shortage of potable water
during some months of the year.  Rota has two fresh water springs known as
Matanhanom and Asonan with a combined average flow of approximately 2
million gallons of water per day.  As the island’s local demand is far less
than that amount the potential for exporting bottled water as well as
producing  soft drinks for commercial sale should be evaluated.
       The annual rainfall on Saipan is equivalent to about 190 million
gallons per day. More important, however, than absolute rainfall is the
distribution of that rainfall throughout the year. Average monthly
precipitation is about 7 inches.  Much of this water is fugitive and its
escape through run-off is almost equally  large.  No stream has a large
sustained discharge and none has a site suitable for the construction of a
large reservoir for impounding flood flow. When rain falls on the islands a
part of the rain will percolate into the ground.  This water will move
downward to the level of the water table of the island (usually sea
level).  With continued accumulation, fresh water will displace salt water
downward and outward to form a lens slightly convex (upward). The fresh
water is lighter and floats upon salt water. The height to which the
surface of the lens rises above mean sea level depends upon flotation plus
the gradient needed to move any increment from rainfall down the slope of
the lens to the edges of the island. The excess water can escape in the
zone approximately at, and just below, sea level.  If no recharge occurred
from rain, the fresh water would gradually flow down the surface of its own
lens until it becomes a film and dissipate and disappear.  This effect
occurs during an extended dry period.  This entire process occurs because
sea water, having a specific gravity of 1.025, is one fortieth (1/40)
heavier than fresh water.
(1) Sources: Summary of Synoptic Meteorological Observations, Vol. 5, Area
14 - Saipan, Prepared by the National Climatic Center for the U.S.
Typhoons
       Tropical storms are closed pressure systems in which the air moves
counter- clockwise in the Northern Hemisphere. A tropical storm reaches
typhoon intensity at 64 knots or about 74 miles per hour.  While typhoons
have occurred during all months of the year, 68 percent have occurred
during the rainy season months of August- November.  The start of the
tropical cyclone season coincides with the eastern migration of the summer
monsoon trough towards the longitude of the Mariana Islands usually by the
end of August.  This trough provides a favorable  environment for the
generation of tropical cyclones.  The majority of typhoons approach from
the east south east.  Many tropical cyclones have made  major deviations in
their track making it difficult to predict  their approach. One interesting
aspect of some typhoons is the non-steady nature of their wind speed. A
screeching, howling wind is punctuated by moments of eerie calm only to
rise again to its former crescendo of shrieking violence. The gusts of the
storm will peak and then drop to a relative lull ; they have  been observed
to vary as much as 80 knots in a matter of minutes.  This pulsating and
gusty nature of the wind results in an uneven, intermittent pressure and
wrenching effect on some structures.  It should also be noted that the wind
causes not only pressure on the windward side but also a suction effect on
the leeward side of structures.  Pressure differences of 60 to 70 pounds
per square foot in less than a minute during the passage of the eye have
been recorded.
       Rainfall is difficult to record because of the steep angle of
incidence of driving rain, indeed, rain is usually blown horizontally
making accurate measurement  impossible. From the highest to the lowest
elevations the ground becomes saturated with water, like a sponge soaked to
the maximum, until the soil can hold no more. At that point the torrential
rainfall can no longer be absorbed by the land and the massive runoff
begins from the mountains to  low lying areas.
      While prospective investors and new residents should not be deterred
by  the seriousness of typhoons -- for they are infrequent -- it is well
to have some appreciation of their disruptive nature.
       The Commonwealth is in Tropical Storm Condition Four at all times,
(40 mile per hour winds possible within 72 hours).  The following table
lists the conditions of tropical storms and typhoons.

Condition Period Force Classification
 4 72 hours Winds up to 39 mph  Tropical  Disturbance
 3 48 hours Winds - 40  to 74  mph Tropical Storm
 2 24 hours Winds of 75 mph or higher Typhoon
 1 12 hours Winds from 75  to 150 mph  Super Typhoon

       Typhoon force winds can remove the roof from poorly  constructed
structures particularly those of corrugated tin.   Aside from the damage
caused by flying debris, water can be quite destructive as well. During the
height of a typhoon the atmosphere is saturated with water and all
distinction is lost between sea and air.  Low  lying areas are flooded and
trees and power poles are blown down sometimes blocking roads, sewer lines
overflow.  As power lines will most certainly be down, the restoration of
electricity  can take some time.  Without power, water pumps are
inoperative.  It is during such periods that swimming pools and water
catchment tanks take on added importance as an emergency water supply.
Without power, ice becomes a rare commodity and frozen foods quickly
spoil.  With air conditioners not functioning in the tropical climate the
interiors of offices and homes become hot, damp and humid.  Electric ranges
are of no value for cooking meals.
       After  the initial thrust of high wind and rain which can last for
hours, a period of calm follows when the wind slackens and frequently,
during daylight, the sun shines -- this is the center of the storm when the
"eye" is passing.  The force of the wind and rain will quickly resume to
full fury  with the only difference being a change in the direction of the
wind -- it blows in the opposite direction of the first  phase of the
typhoon.
       Telephone cables on Saipan are underground with the result that
communication is not usually interrupted during the most ferocious storms.
Unfortunately, electric transmission lines are above ground and as a
consequence can break during high winds causing power outages.
       There are many measures one can take to minimize the temporary
discomfort when water and power service has been interrupted.  Stock up on
canned food, batteries and candles.  Have a battery powered radio and
lantern nearby.  Maintain a reserve supply of water in buckets and in the
bathtub for drinking and washing. Above all stay indoors to avoid downed
power lines and flying debris.
       While the Joint Typhoon Warning Center on Guam provides ample notice
of impending storms and tracks their movement in the western Pacific, it
should be kept in mind that the average margin of error for the Center's
predictions is 250-350 miles.  The wind velocity varies inversely with the
distance from a storm center.  The following is a typical example:  35
miles from the center, above 75 m. p. h. ; at 50 miles about 60 m.p.h. at
100 miles about 50 m.p.h., at 150 miles about 40 m.p.h., at 200 miles about
30 m.p.h.
       Observers have determined that the point of origin of typhoons is,
in general,between 5 degrees and 25 degrees north latitude and 125 degrees
and 155 degrees east longitude.In the winter storms tend to form in the
southern and eastern portions of this area, whereas in the summer they
originate in the northern and western area of these coordinates.
Investigations have also indicated that the rate of progress of a typhoon
center varies with its latitude. South of 13 degrees north latitude the
rate of progress is relatively constant at approximately 5 or 6 nautical
miles (knots). Further north it increases from 6.5 knots to as much as 36
knots in the higher latitudes. In the north east Pacific these storms are
known as hurricanes, in the south-west Pacific they are referred to as
cyclones and in the western Pacific they are called typhoons.
Potential Effect OF Storms
       Typhoon strength and intensity  are totally unpredictable. During a
tropical storm when maximum sustained winds are 35 knots  (gusting to 63
knots) one can expect damage to crops, trees, foliage and poorly
constructed signs. Storm surge may reach a height of 4 to 8 feet above
normal tide level. Residents in low lying areas along the western shore
line may be effected by flooding. Boats moored in exposed  surge direction
will either break and sink or break free of their mooring.
       During a typhoon when winds are 64 knots or greater, trees will be
blown down or uprooted, power poles will snap and major structural damage
can be expected to occur to all temporary or poorly constructed wooden
homes. Total damage may be anticipated for poorly constructed signs,
shacks,false fronts, etc., with extensive damage to roofing materials other
than concrete. The storm surge may reach  a height  of 12 to 20 feet above
the normal tide level.  On Saipan all areas  on the western shore are
highly vulnerable to flooding. If the wind direction shifts to the
southeast,westerly or north westerly, terrain that is 10 feet or less above
mean sea level will be flooded by wave surge action. Major beach erosion
can be expected and evacuation of the beach and low  lying residential
dwellings within 500 yards of the shore is required.
       During a super typhoon with winds of 151 knots or greater, total
evacuation of all low  lying areas is required. Major flood water damage
can be expected on the lower floors of all structures. Winds at this speed
are capable of  blowing down all trees and demolishing tin structures,
wooden buildings, semi-concrete and pre-fabricated structures that are not
well secured. Severe damage to windows, doors and glass fixtures can be
expected on concrete buildings. The storm surge may be greater than 20 feet
above normal tide level.
      When two typhoons develop in proximity of each other the possibility
of a Fujiwhara  effect bears close monitoring. During such an event a
binary interaction is possible where tropical cyclones within 750 nautical
miles of each other begin to rotate about one another. When the two are
within 400 nautical miles of each other they may also begin to be drawn
together.
       In the northwestern Pacific the following typhoon facts have been
recorded. The intensification period from a tropical depression to a
typhoon averages approximately 3 days. However, there are cases where the
intensification period was only 30 hours. The record for typhoon longevity
is 22 days (Rita, 1972). The largest typhoon diameter was 720 nautical
miles (Marge, 1951). Maximum wind speed recorded gusts of 166 knots (Cora,
1963- Ryukyu Islands). Maximum recorded rainfall in a 24 hour period was 66
inches (Gloria, 1963- Taiwan). Five typhoons are the most recorded at one
time with 26 occurring in a single year (1964). Over a twenty year period
(1970 - 1990) there has been an average of 2.5 storms per year with
February and March the only months not experiencing a single storm during
the period. Twenty five percent of the fifty-one recorded storms occurred
during the month of October with fourteen percent experienced during the
month of November. July, August and September averaged  about five storms
each over the two decades.
Flooding
       Severe tropical storms and typhoons usually bring about a rise in
the near-shore mean water level including that  of the Saipan lagoon.  Such
a storm surge can cause substantial damage to structures and facilities
along the lagoon shoreline. A  typical storm surge can rise  4 to 6 feet
above normal.  When, under severe conditions,  both storm surge and storm
waves coincide with a normal  high tide,  inundation and flooding can be
expected to occur in the low lying coastal areas below elevations of 10
feet.  Five areas along the Saipan lagoon shoreline which are prone to
flooding are:  San Roque village,the road at Tanapag, the lower base
industrial area, Garapan / Putan Muchot, and the Chalan Kanoa /Lake Susupe
area.
     The Commonwealth participates in the National Flood Insurance Program,
(NFIP). Developers should review the flood risk maps of Saipan, Tinian and
Rota and be aware that federal financial assistance can be provided as a
result of any disaster within the area depicted on these flood risk maps.
The NFIP requires new construction within these flood risk areas to be in
compliance with previously established building codes and regulations.
     The federal government underwrites the coverage sold by private
insurance companies to individual policy holders. For information contact:
Federal Emergency Management Agency, Presidio of San Francisco, Building
105, San Francisco, California, 94129, Tel. (415) 556-9840 or the
Commissioner of Insurance, CNMI Department of Commerce and Labor.
     Interested parties should determine if the CNMI has licensed insurance
companies to sell flood insurance. It is necessary that the local
government adopt adequate flood plain management regulations for its flood
prone areas since only through the enactment of certain laws can the
community regulate construction and establish construction priorities. This
is necessary to reduce flood loses.
                          Storms of 34 Knots or Greater Which Passed
Within 180 Nautical Miles of Guam
1945 - 1986 by Month of Incidence

 Jan  Feb Mar April
 2 1 1 3
 May June  July Aug
 6 4 9 13
 Sept Oct Nov Dec
 15 26 17 7
Source: Naval Oceanography Command
Tsunami
      There is no historical record of Tsunami (tidal wave) occurrences in
the Northern Mariana Islands.
Tsunamis In The North Pacific Ocean
                                         Maximum Run-up
Date                  From            ( Meters)                   Damage
                            (Observed at Guam)                  Sustained
1952  Japan  <0.1   None
1952  Kamchatka  0.1   "
1956   Kamchatka  <0.1   "
1957  Aleutians  0.3    "
1963  S. Kuril Is.  0.1   "
1964  Alaska  0.1   "
 Source:  Iida, Preliminary Catalog of Tsunamis Occurring in the Pacific
Ocean, Hawaii Institute of Geophysics, 1967.

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