The climate of Irosin belongs to the Type II of the modified Corona System, the classification of Philippine Climate. The Type II climate is characterized of having no dry season or not a single dry month, a very pronounced maximum rain period from December to February and minimum monthly rainfall occurring during the period from March to May.
It must be noted that the local climate is mainly influenced by the proximity of a great body of water and the effects of elevation and mountain barriers. In this regard, Irosin’s topographical make-up and its proximity to South China Sea on its west and the Pacific Ocean on the east makes the municipality’s climate fall under the Tropical Wet with Short Dry Season of the Tropical Rainy Climate as classified under the World Climate. Northeast Monsoon or “amihan” and Southwest Monsoon or “habagat” are the two main air streams which influences the climate pattern of the municipality. Cold northeast winds usually start in October and prevails until April. During the month of January the wind intensity reaches its peak and gradually weakens in March and April. Abnormal wind direction happens during the month of May on the onset of the southwest wind, which normalizes towards the months of June and July and intensifies in August. The southwest wind gradually weakens until the next main stream in October.
Irosin experiences rain almost all year round. Rainfall is observed at an annual average of 234 days a year. Maximum rainfall comes in November to January caused by the intensification of the Northeast Monsoon. In the period 2012 to 2013, the heaviest rainfall is in the month of January with a monthly mean of 381 millimeters and with 26 days of rain. On the other hand, May has the lowest rainfall with a monthly mean of 61 millimeters with only 9 days of rain. The mean annual rainfall is 2,632 millimeters. The heaviest recorded rainfall in a day in the municipality was 200 millimeters which occurred on February 22, 2008.
The annual average temperature registered is 27.06 ⁰C. The hottest month is May with a mean temperature of 28.23 ⁰C while January is the coldest month with a monthly mean of 25.76 ⁰C. The hottest recorded temperature was 36.36 ⁰C on May 19, 2013 while the coldest was 17.6 ⁰C on January 26, 2014.
Irosin’s annual relative humidity is recorded at 79.90%. January is the most humid month at 83.24% while April is the driest at 75.69%. The highest humidity recorded is 98% on January 2012 and the lowest humidity at 48% was measured on May 17, 2013. It must be noted that Irosin’s humidity is the result of the interaction of the municipality’s topography and its proximity to Philippine Sea, San Bernardino Strait, Sorsogon Bay and Ticao Pass. The annual average wind speed recorded was 1.01 meters per second or 3.636 kilometers per hour. November is the windiest month with a monthly average wind speed of 2.07 meters per second and the calmest month is May at 0.37 meters per second.