Precipitation introduction

5.1 What is precipitation?

            The process of transportation of water from the atmosphere to the earth's surface is defined as precipitation. Precipitation can occur in liquid or solid forms. Precipitation is the result of water droplets around nuclei or the formation of ice crystals.

        Two conditions must be fulfilled for forming precipitation.

                          (1) The air must be saturated-Air masses can be saturated by three methods.

a)     Clashing of different air masses (Hot, humid masses and cool, dry air masses) 

b) The connection between moist air masses and cool objects.

c)  Adiabatic process (Without entering or leaving of heat, pressure and volume changes)

                                 (2) Presence of small airborne particles called aerosols.

The available condensation nucleus is two types.

a)     Hygroscopic particles (Salts)-Salts are made condensing before the air becomes saturated.

Ex: Crystals of Nitrate (NO-3), Phosphate (PO43-)

b) Non-Hygroscopic particles-According to their sizes, supersaturation is essential.

                             Ex: Smoke, dust, ash

5.2 Types of precipitation

Four types of precipitation have occurred on the earth.

               (1) Orographic precipitation- Warm, humid air masses move horizontally form coastal areas to nearby mountains. With clashing of air mass with mountains, air masses are started to rise by uplift forces. Warm air masses are getting cool with increasing height because the temperature is decreased with increasing height. After the saturation of air, additional water vapors are tended to condense because cool air cannot hold water vapors as much as hot air. Therefore clouds are formed after saturation. Precipitation has occurred.   

(2) Convectional precipitation-When the land surface heat, above the air, is heated by land. As air heat, the air began to move further. With the distance between molecules increases, they are less densely packed. The air becomes lighter and expands. Air masses get cooling. After saturation, additional water vapors are condensed and clouds are formed. Precipitation has occurred. 

(3) Cyclonic precipitation-Cyclonic precipitation has occurred in low-pressure areas. Air masses move form high pressure in nearby surface areas to the low-pressure areas above. Air getting cooled and clouds are formed. Precipitation has occurred.

(4) Frontal precipitation-When warm, humid air mass and cool, dry air mass are met, cool air tightly packed together so cool air is heavier than dry air so the warm air is forced to rise above cool air. When warm humid air rises, the air gets cool. After the saturation of air, clouds are formed. Precipitation has occurred.

5.4Forms of precipitations

         (1) Liquid precipitation

                                 (a) Rain (Diameter of water droplets is more than 0.5mm)

                                 Light rain – Intensity of rainfall is up to 2.5 mm/h

Moderate rain – Intensity of rainfall is 2.5 to 7.5 mm/h

Heavy rain – Intensity of rainfall is more than 7.5 mm/h

                                       (b) Drizzle (Diameter of water droplets is less than 0.5mm)

                 (2) Solid precipitation

Snow-Snow is precipitation of crystal ice. Solid water is consolidating together and become flakes.

                                              Sleet- Sleets are small snow particles. They are formed as snowflakes are melted and refreeze at a subfreezing temperature in the atmosphere before reach the earth's surface.

                                              Hail- Small pieces of ice are formed through up and down of the convectional currents by melting and cooling.

                                              Dew- Condensation of water vapors is at the temperature below the dew point. Water vapors are condensed by emission their energy to around the environment.