Created: 6/14/1954

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This study and the accompanying tables have been prepared to determine the feasibility of carrying out low level air operations at the localities specified Xy the requesting agency. The Air Weather Service accepts no responsibility for any Interpretation of this study or the included data rade without ite concurrence.

Detailed lnforar>,tioD for all the localities specified were not available. However, this lack should not reduce the value of this study greatly since tbe observations taken at Guatemala City give the worst conditions oa tbe average which will be experienced lo this area. In this area low ceilings and visibilities, and poor flying conditions are closely osaocleted with the occurrence or rain. Therefore, since It is generally assumed that the duration of rainfall Increases with elevation while the amount of rainfalloxinaa at an elevation of0 feet, the Guatemala City observations should be representative of tbe poorest flying conditions. The only exception to thia rule of Increasing


duration of rainfall occurs at stations which are shielded by high mountains in both tbe northeast and south quadrants. Examples of this exception are Zacapa and Iuezaltenanga, Guatemala.

Flying conditions in this area, as in all of Central America, are quite good. During the months studied, which are in tha rainy season, contact weather (defined as ceilingeet and

visibility greater than tu. and one-half wiles) prevalla more thanercent or the time during eachat San Jose. Those conditions varyonthly low4 par cent of tba time in Septenbor to an average monthly high0 per cent of the time in November at Guatemala City. Of theae four monthseneral rule oontectprevail most frequently in this whole area during November, since the rainfall which la associated with poor flying weather ieinimum then. Tha only exception te this statement that waa found in this Investigation ia at Puerto Barrios and the lowlands io its general locality.

The greatest hasards to flying operations in Guatemala aro the tropical storms of sunrer and fall, winter storms fron the north, and the frequent thunderstorms which form in all tropical areas.

Of those three hasarda tha most widespread In its effoctespectacular by far la the tropical storm and hurricane. they do not pasatation if they come withinocf the coast, they will cause widespread and intenselasting three days or nore at all stations on theside of the mountains, and vary strong winds and turbulentthe

The season for the formation of hurricanes and tropical stoma is roughly Kay through November. In the Western Caribbean they occurfrequently In tho months of Septesshor and October. The stoma which frequently Conn in the southwestern Caribbean will affect, all of Guatemala north of the Mountains even though they do not crooa the country. Those that cross the country as they have on Infrequent occasions45 have had core widespread and Intense effects in northern Guatemala. _

On the Pacific side the effects of tropical sterns are not so great. Oftoms recorded0nlyad their origin far enough eastward to effect the coastal waters of Guatemala. Nono of these it attained hurricane foroe (to beurricane the winds near the centerropical storn must be greater thaniles) while in that locality. In September, tho month of greatest frequency in the Pacific, none has occurred far enough to the eastward to touch the Guatemalan coast.

The cold waves or northers that extend south past Guatemala during winter constitute the greatest hazard to flying conditions from November to May. They oauee high winds along the northern coaat and through mountain passes that open to the north. Otherphenomena are widespread cloudiness, general rains, low ceilings, and visibilitiea and Icinguch lower level than the average wintertime height, which laeet. However, these storms occur Infrequently averaging less than one occurrence per month In January and February this far south and are even lesa frequently observed in November.

Thunderstorms, although less hazardous than the two preceding phenomena, occur ouch mere frequently. In addition, they are the only storms dangerous to flying that can not be forecast with sufficient accuracy to allow for proper precautions in advance. While no attempt should be made to minimize the hazards associated with any thunderstorm, they are probably less severe and less turbulent than those of the Southeastern United States or of the Rocky Mountain Area. However, because of the mountainous terrain In this area, they are difficult to avoid, and often obscure the mountain peaks and even the peaees through the mountains. Sometimes

a steep slope ofeet suffices to cause turbulence and local showers in this*region. At Guatemala City, San Jose, aod Chimax thunderstorms occur ouch core frequently on the average in august and Septenber than In the next two months. At the inland stations they occur much mora frequently In the afternoon. Along the coaot they are most frequent In the evening and early morning hours since the more frequent night tiiw stones forming over the water often drift in over tho coast.

Generally during adverse weather conditions, one important consideration Is that torrlnalo on leeward sides of mountains likely hove better ceiling, visibility, and wind conditions than terminals on the windward side. Therefore, if the enrouta weather isuitable alternate can usually be found within the limits of Guatemala.

Fog Is not too frequently observed in this area. Those fogs which reduce visibility to less than ono irila occur mora frequently inlard than they do at the coastal stations. At all localities they are almost exclusively early morning phenomena and can usually be expected to start to dissipate0 hours.

Other weather factorsy restrict visibility at these localities are haze and heavy rainfall. Haze did not restrict visibility to less than one mile during the more than three years for which records were available at either San Jose or Guatemala City. Haze reducing visibility to less than six piles is observed more at coastal locations and will occur as often in the late morning as any other hour.

Visibility less than one mile due only to intense precipitation, was observed infrequently st both stations, with the greater frequency at San Jose. Visibility less than, three miles because of intense

hich lie on open slopes at elevations of0O feet. Since precipitation is usually most intnnsn in the afternoon and early evening, the low visibilities associated with it will occur most frequently during tbe same hours.

r the utationa will experience surface winds prevailing froa the northeast quadrant, econdary maximum will prevail frcia the south or southwest quadrant. The only exceptions will be in deep sheltered valleys where the winds are lighter and more variable than those of San Jcsc and Guatemala City or in mountain parses whose main axis lies in some other direction than northeast-southwest. In the parses vhich are oriented at an angle to the northeast, the average wind speed will be less than that of Guatesatla City in almost all casco. At the inlani stations wind speeds exceeding the naxinurc attained at Guateeala City will occur very infrequently. Along the coast the Guatemala City maximm will he exceeded onlysses cicseby orold outbreak crosses the north coast.

Along the coast or on elopes open to the sea, thereefinite diurnal pattern to the surface winds. 'During thereeze blowing onto tbe land fron Uia sea will ccjMAence in the lste morning and reach ita maximum development during tho afternoon, usually0 After sunset tbe kind speed will die down. Tills sea oreeze will be most prevalent along the north coast in Kovenber since then it is strengthened by tbe northeast trade wind during that month.

Sometime0 the opposing wind, the landill conuence blowing err the land. Itaxiszura in the early morning shortly before sunrise. The land breeze is seldom aa strong as the sea breeze. It is strongest on the middle or lower slopes on the south aide of Guatemala. There Is no month when it can be expected to reach its peak as is the case with the sea breeze.

lib hcuriy surface wind data were available to t'-ils office for the inland station. Since the winds do not follow any such well-developed diurnal pattern inland, it is iapcpsible tc make any statements about their variation during tbe day.

Low level air operations can be carried cut during all theae months at these localities. However, becauseie greater poss&uiXityi.urricenoa, whleli coji br'og air operationsa caffplete halt ever extendedand because of the greatercf hazardous tnunderstcrms during the ether three months, Kovenber is the best oonth for these operations. Since the following aonths areas good,if not better than I'oveuber, it is ly far tbe beat tenth tc VegJn any biurc Lions which may require woreo month for eotspietion. Rcveirler has the added advantage that there are usually iwny nore hours during the day which are suitable for air operations.

Diurnaily the best hours for operations during Kcvember are0O. However, tbe lastcura of this period arc not quite so good at all stations as tbe preceding hours. During ths remaining months the-loot hours are noroaily0 until UCO. If the planes are taking-eff iron coastal stations for

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