Type

The Hernandez type of classification under PAGASA defines the climate of the province as generally arid (Type E) wherein there are more dry than wet months. At most, there are only 4 ½ wet months in a year with Q ranges from 1.00 to 1.67, where Q is the ratio of the number of dry months to the number of wet months. However, the southernmost portion of the province (part of the municipality of Cervantes) was observed to be humid (Type B), where rain is evenly distributed throughout the year with at most 3 dry months and the eastern part of Sugpon is dry (type D) with rain not sufficiently distributed with at most 6 dry months.

Rainfall

The total annual rainfall in Ilocos Sur is 2,552 millimeters, with a monthly average of 213 millimeters. Maximum rainfall is observed during the month of August with a mean rainfall of 710 millimeters, with rainfall means of 3 and 6 millimeters, respectively.

Temperature

The mean annual air temperature in the province is 27°C. April and May were observed to be the hottest months with temperatures of 28.3°C and 28.7°C, respectively, while the coldest month is January with a temperature of 25.4°C.

Physical Constraint

The province is located within a region of the country where either typhoon or prolonged drought prevails yearly. The low portions of the province are flooded during the rainy season. The rivers and the sea sometimes pose a threat of flooding and erosion in some coastal municipalities of the province like Caoayan and Santa.

Geophysical

Landslides are to be considered only in mountain areas. Erosions occur in coastal areas, along the rivers, creeks and also along the roads more particularly during the occurrence of typhoons.