Transcript
TROPICALCYCLONES
The mean tracks oftropical storms and hurri-canes are shown in red.These tracks represent av-erages, and movements ofindividual systems mayvary widely.
AIRTEMPERATURE
The mean air tempera-ture (C) in red lines isshown for every 2 de-grees. All weather narra-tives refer to air tempera
GALES The red numerals inthe center of each 5-de-gree square on this insetchart show the averagepercentage of ship re-ports in which winds of atleast force 8 have beenrecorded for the month.In cases where the obser-vation count is low thegale frequency may benonrepresentative andtherefore different fromthe values used in the text.Where 0 is given, galesmay have been recorded,but too infrequently togive a percentage value.
SEA SURFACETEMPERATURE
The mean sea surfacetemperature (C), in bluelines, is shown for everydegrees .
VISIBILITY Blue l ines show per-centages of observationsreporting visibil i t ies lessthan 2 miles.
SURFACEPRESSURE
This chart shows theaverage barometric pres-sure reduced to sea level.Isobars are solid bluelines for every 2.5 mil l i-bars difference in pres-s u r e .
LOCAL WEATHER.For extended remarks on the marineclimate along foreign coasts, see the appropriate SailingDirections and Planning Guides prepared and published bythe National Imagery and Mapping Agency ; for the coastsof the United States and its possessions, see the appropri-ate Coast Pilot prepared and published by the NationalOcean Service. The trimester publication MarinersWeather Log prepared and published by the National Oce-anic and Atmospheric Administration, National WeatherService, carries informative articles on marine climateconditions and tropical cyclone information.
MAYPRESSURE.In May the permanent anticyclone off theSouth American coast is centered near 30S, 85W with amean central pressure of 1021 millibars. Located oversoutheastern Australia is a second high pressure centerwith a mean central pressure of 1018 millibars. Pres-sures diminish slightly to the north and considerably tothe south of the subtropical high; mean pressures are aslow as 1008 millibars over the western half of the equato-rial trough and 990 millibars at 60S.
VISIBILITY.As in previous months, shipping routes northof 40S have little problem with poor visibilities (less than2 miles). At 130E longitude, poor visibilities increase from10% at 43S to over 40% at 60S; at 90W they increasefrom 10% at 48S to over 20% at 60S.
TEMPERATURE.Along the equator mean air tempera-tures range from 23C off South America to 29C west ofthe international date line. At this latitude approximately98% of the observations fall between 19C and 29C overthe Peruvian current and between 25C and 33C overMalanesia. Means vary little at 60Smost are found tobe near 2C to 3C. Of the observations at 60S, 98% fallbetween 4C and 5C.
WINDS.East to southeasterly winds prevail north of 30Sfrom South America to the Philippines; scalar winds av-erage force 3 to 4. South of 30S, prevailing winds arewesterly except for southerly winds between Australia andNew Zealand. Scalar winds average force 4 to 6 south of30S.GALES.With the decline of tropical storm activity overthe western half of the South Pacific, the occurrence offorce 8 winds is rarely reported north of 30S. Frequen-cies begin to increase south of 30S, reaching 10% southof 40S over the eastern half and south of 45S over thewestern half. Most regions east of 160W between 45Sand 60S report frequencies of 20% or more; a high of30% is reported off the southwest coast of Chile.
TROPICAL CYCLONES.With Mays cooler temperatures,the occurences of tropical storms (>_ 34 knots) have de-creased significantly. Historical records show that onlythree storms can be expected to reach 34 knots or greaterwithin an average 10-year period and that less than onestorm will reach 64 knots or greater (hurricane strength)within an average 20-year period. Such storms have beenobserved only in the northwest quadrant of the South Pa-c i f i c .
WAVE HEIGHTS.Wave heights of at least 12 feet aregenerally observed 10% or more of the time south of 20Sexcept for some coastal areas of Australia, New Zealand,and Chile. Frequencies continue to increase southerly,reaching over 40% in a large portion of the region southof 45S and west of 90W. Along the Chile coast frequen-cies reach 10% near 45S and increase to over 30% southof Cape Horn.
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EXPLANATION OF WIND ROSESPREVAILING WINDS AND CALMS.The wind rose inblue color is located in the center of each 5 squarewhere there was sufficient data. The rose shows thedistribution of the winds that have prevailed in thearea over a considerable period. The wind percentagesare summarized for the eight points and calm. The ar-rows fly with the wind indicating the direction fromwhich the wind blew. The length of the shaft, measuredfrom the outside of the circle using the scale below,gives the percent of the total number of observationsin which the wind has blown from that direction. Thenumber of feathers shows the average force of thewind on the Beaufort scale. The figure in the centerof the circle gives the percentage of calms. When thearrow is too long to fit conveniently in the 5 square,anything over 29 percent, the shaft is broken and thepercentage is indicated by numerals.
FOR EXAMPLE.The sample wind roseshould read thus: In the reported observationsthe wind has averaged as follows:From N. 3 percent, force 3; N.E. 16 percent,force 4; E. 61 percent, force 4; S.E. 17 percent,force 5; S. 1 percent, force 4; S.W. less than 1percent, force 3; W. 1 percent force 2; N.W. 1percent, force 4; calms 0 percent.
MAY: MAY