Weather Estimate (extract)
a. Weather and Climate: October falls during the transition period between the two Philippine monsoon seasons. The southwest monsoon typically runs from May through September; the northeast monsoon begins in October or November and ends in April. The normal disappearance of the southwest monsoon during October is followed by the appearance of northeasterly flow in the northern Philippines (north of 13ºN) during November. The Intertropical Convergence Zone (ITCZ), also known as the monsoon trough or the Near Equatorial Trough (NET), passes over the Philippine Islands moving southward during this period, bringing cloudy, hot, humid, and wet weather. Thunderstorm activity decreases after the ITCZ passes, from as many as 15 days a month in September to 9 days or less in November. For most of Luzon, the southwest monsoon is the rainy season and the northeast monsoon is the dry season. However, locations with an eastern exposure normally receive maximum rainfall shortly before and during the northeast monsoon. Prevailing winds and topography largely control rainfall on Luzon.. Topography plays an important role in the distribution of rainfall over Luzon. For example, Manila and Infanta are about 35 miles apart, but separated by a mountain range of about 4,000 feet (1,200 m) elevation. During the southwest monsoon season the mountain range acts as a barrier to the amount of precipitation received on its leeward side, so Infanta normally records 28 inches (71 cm) of rain, whereas Manila receives about 47 inches (190 cm). Local topography also has a strong effect on surface winds. Along coastlines, land and sea breezes predominate during the transition periods and also at times of weak monsoonal flow.
(1) Precipitation: Luzon's mean precipitation for October covers a wide range, depending on the terrain surrounding the observing site. Daet, on southeastern Luzon's Bicol Peninsula, has the Philippines' highest October average, 22.8 inches (57.9 cm); San Fernando, on Luzon's northwest coast, has one of the Philippines' lowest averages for this month, 5.6 inches (14.2 cm). Half the October values for Luzon are 9.0 inches (23 cm) or less, and more than 2/3 are less than 12.0 inches (31 cm). During the passage of a typhoon in October 1967, Baguio recorded a 24-hour rainfall amount of 47.8 inches (121.4 cm).
(2) Visibility: Rain and clouds shrouding the mountains are the most frequent restrictions to visibility near the ground. The simultaneous occurrence of ceilings below 1,000 feet and visibility less than 3 miles are infrequent at most locations on Luzon for which there are records; however, many mountain locations probably have a much higher frequency than those reported. Most locations report less than 10 annual days with ceiling < 700 feet and/or visibility <1-1/4 mile at 2000 LST, and several report 1 day or less. Exceptions are the north coast site of Aparri (20 days) and the mountain site of Baguio, elevation 4,962 feet (1,500 m), (54 days). October values for these two locations are about 2.5 for Aparri and 5.5 for Baguio.
(3) Wind: Because of the mountainous character of the Philippines, surface winds are quite variable in both speed and direction over the interior. Near the coasts, directions are more persistent, and on average, speeds are somewhat stronger than over the interior. In October, the mean wind direction over Luzon at 3,000 feet (915 m) above mean sea level is from the northeast. At the surface, however, only the northernmost sites reflect predominantly northeasterly flow. Dagupan, in the west, receives generally southeasterly flow, and Manila, in south-central Luzon, shows no dominant wind direction. On the east coast, Casiguran shows primarily northerly flow, but has calm winds 38% of the time. In the northern Philippines, the diurnal variation in wind direction associated with land and sea breezes is normally prevented during the northeast monsoon. Winds in excess of 35 knots during the northeast monsoon and its attendant transition periods are normally thunderstorm associated. During October, Aparri is the only station having wind speeds greater than 27 knots. These are reported on only 1 to 5% of the observations.
(4) Temperature and Humidity:
(a) Temperature: Mean temperatures remain moderate throughout the year. Temperatures in the Philippine Islands are remarkably homogeneous, and the only large regional temperature variations are those due to elevation. Aside from Baguio (elevation 4,962 feet [1,512 m]) with an October mean daily maximum temperature of 73º F (23º C), October maximum temperatures on Luzon average between 85º F (29º C) at Basco, a small island in the northern Philippines, and 91º F (33º C) at Dagupan. Similarly for the absolute maximums, Baguio's October figure is 81º F (27º C), whereas all other stations except Basco (90º F [32º C]), Tuguegarao (101º F [38º C]), and Olongapo (104º F [40º C]) are between 94 and 97º F (34-36º C). Baguio's October mean daily minimum is 60º F (16º C), and all others are from 71 to 77º F (22-25º C). Olongapo holds Luzon's absolute minimum for October, 47º F (8º C), whereas all but three of the other stations have reported minimums between 64 and 68º F (18-20º C).
(b) Humidity: As in most tropical island locations, the Philippines have high relative humidity year-round. Diurnal variations are more pronounced than seasonal or aerial variations. The diurnal temperature variation is generally so small that even the very early morning is almost as uncomfortable as the afternoon. At 0500 LST, October mean relative humidity on Luzon is generally between 88 and 95%, but Clark Air Base averages 80%. At 1400 LST, humidities range from 64 to 80%, with 5 of 13 stations averaging from 73 to 75%.
(5) Tropical Cyclones. The Philippines are located in a region recognized as having the highest frequency of tropical cyclones in the world. Over central Luzon they occur most frequently from October to December, but they can occur during any month of the year. In October and November 1974, the Philippines, and Luzon in particular, were struck by an unprecedented onslaught of tropical cyclones. Starting with typhoon Bess on 11 October and culminating with typhoon Irma on 28 November (6 typhoons, 1 tropical storm, and 1 depression struck the Philippines). The passage of 5 typhoons between Bess and Gloria over Luzon is unparalleled in climatological records available since WWII.
(6) Light Data. See Tab A (Light Data Tables).
(7) Effects of Weather on Friendly and Enemy Courses of Action.
(a) Precipitation. Negative effect on threat and friendly ground forces. Rain will degrade both mounted and dismounted cross-country movement both on and off road, making it hard to commit a mechanized reserve or quickly reposition troops in the defense. It may result in flooding, mudslides and landslides. Rivers will run faster and deeper, making it difficult to conduct fording and bridging operations. Cross-country movement on foot will also be degraded in steep terrain. Storms will degrade both rotary and fixed wing operations.
(b) Visibility: Negative effect on threat and friendly air recon and attack forces, positive effect on enemy and friendly dismounted forces. Low ceilings will degrade fixed wing air operations and may effect rotary wing as well. Ground fog and low clouds, especially in the morning, will degrade aerial photo reconnaissance ability in the valleys and on the coastal plains, allowing movement with less chance of being detected and successfully engaged. The dismounted infantry will be protected against observation and attack from the air. The ground fog will provide concealment for infiltration of dismounted infantry forces.
(c) Wind: Negative effect on enemy chemical weapons. Surface wind from the northeast will decrease the enemy’s use of chemical munitions. Turbulence associated with thunderstorms is generally moderate to severe and turbulence in mountainous areas, especially on leeward sides, is also common. On hot days, clear air turbulence extending several thousand feet can be expected. This turbulence, along with unpredictable wind direction in the mountains, will slightly degrade fixed wing and rotary wing operations.
(d) Temperature and Humidity: Negative effect on enemy and friendly ground forces. Unusually high temperatures during October may degrade the ability of troops to dig in a defense, and will reduce the load that attack and lift helicopters can carry. Temperatures at altitude combined with cloud formations may cause icing problems for fixed wing aircraft and force them to operate at lower altitudes.
(e) Light: Positive effect on friendly and enemy ground and air forces. Relatively low illumination during October will enhance movement of forces during hours of darkness as well as helo operations.