Weather And Other Phenomenon
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.
