What is Torrential Rain?

Torrential rain is defined as rain that accumulates at a rate of three tenths of an inch or more per hour.

Rain refers to water droplets forced to fall down to earth by gravitational push. The droplets form when atmospheric water vapour condenses to water. Rain is essential for human, plant, and animal life. For instance, rainwater provides water for plant irrigation and hydroelectric power. Torrential rain refers to the heavy downpour of rain. There is no definite definition of it other than the definition provided by the National Weather Service (NWS). The NWS defines torrential rain as rain that accumulates at a rate of three tenths of an inch or more per hour. There are several idioms that also bring out the meaning of heavy rainfall. They include "raining cats and dogs" and "raining pitchforks."

What Causes Torrential Rain?

Moisture that moves along the weather fronts is the major cause of torrential rain. The convective clouds cause precipitation to occur when enough moisture rises up due to an upward motion. Narrow torrential rainbands come as a result of cumulonimbus clouds. In mountainous regions, torrential rain falls on one side of the mountain since heavy precipitation occurs on one side of the mountain. The side of the mountain where much precipitation occurs is the windward side. Most of the moist air condenses and then falls as torrential rain on the windward side of the mountain. Dry air blows on the other side of the mountain due to the down slope. The urban heat experienced on islands results in torrential rain. Scientific research shows that torrential rain which pours on other planets contains volumes of iron, water, methane, sulphuric acid, and even neon gas.

Formation of Torrential Rain

The atmospheric air always contains varied amounts of water vapour. Relative humidity is the term used to describe the amount of moisture in the air. Furthermore, relative humidity is the amount of water vapour that the air can hold at a certain temperature. When the air saturates with the water vapour clouds are formed. The clouds suspend in the air and are visible from the surface of the earth. Cool air has more saturation of vapour than warm air. The main mechanisms through which air cools to dew point are radiational cooling, adiabatic cooling, evaporative cooling, and conductive cooling. Convection or physical barriers such as a hill cause the cool air to rise up. Condensation then forces the water vapour to form clouds. The type of cloud formed depends on the amount of condensation that occurs. In the case of torrential rain, dark nimbus clouds form in the clouds.

Coalescence and Fragmentation

During coalescence individual water droplets fuse to form larger water droplets. Once formed, they remain stationary in the cloud due to air resistance. The production of larger droplets occurs when air turbulence causes water droplets to collide. Coalescence continues as larger water droplets fall. These water drops are quite heavy, therefore overcoming air resistance and make the rain continuous. The process of coalescence is temperature dependent. The temperature difference between the earth’s surface and the clouds make crystallized air to melt as it falls as torrential rain.

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