**2. Rainfall potential**

In the tropics, the amount of solar energy is the most radiated here since it is perpendicular, to near perpendicular to the sun's rays. This results to more energy converting the liquid water into water vapor, resulting to an increase in humidity in the area. The amount of precipitable water that could potentially become rainfall is a function of specific humidity which is proportional to the amount of vapor pressure which is a function of the temperature in the area [2]. This makes the wet tropics vulnerable to effects of climate change. Moreover, in this area in the tropics, since the sun's rays is concentrated, differential heating of the oceans and the lands will cause the air masses of different energy to move relative each other. Aside from the relative movement of the air masses, their differential densities will tend warm air mass to be lifted. Lifting of an air mass cools down the temperature adiabatically and results to condensation in the form of precipitation, or in the tropics, rainfall.

The effect of the changing sun's available energy which is caused by the varying tilt of the Earth's axis as it revolves around the sun results to the changes in seasons that the planet experiences yearly. Near the equator, it creates two distinct seasons while beyond the Tropic of Cancer in the northern hemisphere and Tropic of Capricorn in the southern hemisphere, it results to four different seasons in a year. In the Philippines, which is near the equator, the seasons is composed of the wet and the dry seasons that happen around May and November, respectively. The seasons is dictated by the presence of the monsoon winds – the Southwest Monsoon (SW Monsoon), locally known as Habagat season and the Northeast Monsoon (NE Monsoon), locally known as the Amihan season. During the SW Monsoon season, the wind comes from the southwest of the Philippines which is a moisture-laden air mass originating from the Indian Ocean which when it weakens, will be replaced by the NE Monsoon coming from the northwest in the cold northern regions such as Siberia.

At the time that SW Monsoon is strong, the same solar energy received at the region in the Pacific Ocean brews low pressure area (LPA) that usually develop into tropical cyclones (TC) or typhoon as we call it in the Philippines and other countries in the Pacific. Forming over oceans where sea surface temperature as well as air temperatures are greater than 26°C, TC accumulates large amounts of sensible and latent heat as it spirals toward the center and further get strength [3]. The counter-clockwise rotation of the Earth that produces the trade winds or easterlies tends the low pressure area that forms at low latitudes but greater than 5degrees, to move westward toward the Philippines. Depending on the strength of the monsoons, the incoming TC moving as the trade winds will have its characteristic bend that determines if it will make landfall or not to the country. If the SW Monsoon is prevailing, it could interact with the TC and intensify the rainfall that the TC could bring to the area. **Figure 1** shows a record compiled by the Philippine Atmospheric, Geophysical, and Astronomical Services Administration or PAGASA showing the TC tracks per month from 1948 to 2016 that at times deflect away the country depending on the relative strength of the monsoons.

On average, 20 Tropical Cyclones enter the Philippine Area of Responsibility (PAR) every year, 8 to 9 of which make landfall, as tallied by PAGASA. Coupled with the SW Monsoon, the problem of flooding in the country is exacerbated by TCs which attracts the monsoon's moisture-laden airmass. Even during the cool months, some parts of the country experience excessive rainfall due to the tail-end of the cold front or TECF which happens when the cold air from the prevailing NE Monsoon meets with the warm Easterlies. The lifting of the warm air mass as it moves past the advancing cold air mass (a cold front) causes much rain that floods the eastern and south eastern sea board of the country.

*Flooding and Flood Modeling in a Typhoon Belt Environment: The Case of the Philippines DOI: http://dx.doi.org/10.5772/intechopen.98738*

#### **Figure 1.**

*Tropical cyclone tracks in the Philippines averaged from 1948 to 2016. Source: PAGASA, available at http:// bagong.pagasa.dost.gov.ph*

The same scenario happens in other countries in the tropics that are near a large body of water and are influenced by prevailing wind patterns that come together with different air mass of different thermal properties. Countries in the Indian Ocean experience cyclones while hurricanes are in the Atlantic, and typhoon in the Pacific – different naming that pertain to Tropical Cyclone that forms in the warm waters of the oceans and affects many countries in the tropics and dissipates as it reaches the colder latitudes.
