METEOROLOGY

The climate of the area surrounding Santiaguito is a highland modification of the tropical winter dry season type, but with a strong monsoonal influence. McBryde (1945) classifies the climate of the volcanic chain in southwest Guatemala as Cwbgn (Koeppen Classification), a tropical highland winter dry regime, with the hottest month having a temperature below 22°C and coming before the summer solstice. The climate is also characterized by frequent fog. Individual climate station points for points near Santiaguito are in Table 1. The mean annual temperature is about 15°C and the mean annual rainfall is in excess of 3.75 m, nearly all of it coming between June and October. The individual stations shown in Table 1 are nearby, but do not reflect precisely the conditions at the volcano, which probably has rainfall like Patzulin, and temperature like Quezaltenango.

Prevailing wind patterns are studied because they directly influence the areas affected by ashfall. Mercado et al. (1988) used an INSIVUMEH table of monthly averages (1982-1986) for upper air wind directions over Guatemala City to assess ashfall hazards. As this is only five years of data, it is only a preliminary assessment. Coupled with the fact that wind velocity was not considered, no definite ashfall model can be constructed for these four volcanoes at this time.

An arrow diagram of the average yearly upper level (10,000-15,000 ft) wind directions in Guatemala (see figure) shows a prevalence of easterly and westerly winds. Northerly and southerly winds are relatively uncommon. This indicates population centers due north and south of the volcanoes will rarely be affected by ashfall hazards, but those to the east and west have a higher potential to be affected.

Arrow diagrams for the monthly averages define 4 specific patterns of high altitude wind directions in Guatemala. Westerly winds predominate from January to March. April and May are transition months, where westerly winds still predominate, but other directions become significant. Easterly winds are prevalent from June to October. November and December are transition months, with westerly winds starting to predominate again.

Average wind direction from June to October.

Average wind direction from January to March.

The heavier rainfall in the country is caused by the cyclones which come from the two tropical cyclone zones: namely, the North Atlantic Ocean Zone and the Northeast Pacific Ocean Zone. Tropical storms and hurricanes from the North Atlantic Ocean zone annually rose 8.3 times. Tropical cyclones that hit the country one or two times a year occur from May to mid-December, and the occurrence frequency is at its highest level in mid-September. (Flood Control Project, 1985).

Tracks of the tropical cyclones of the northeastern Pacific Ocean (Flood Control Project No. 69, 1985).