How Convectional Rain Forms
People living in Taiwan are surely familiar with the scene: in a summer afternoon, the thick black clouds move fast, the ground disperses the slight scent of dampness while the air gains more and more stifling heat and humidity. Passers-by take out colorful umbrellas; riders stop their motorcycles one after another to put on the rain coats along the roadside; street vendors hurriedly tidy up the commodities and move under the roof; however, most people wisely choose not to go out then. It is an uncomfortable weather condition, and Taiwanese people know that is the very sign of
convectional rain. Like other countries located in the tropical area, Taiwan
has proper hydrological condition to generate convectional rain in summer
afternoon, especially during July and August. Convectional rain has three main
traits: short raining time, intense rainfall, and little raining range. This climatic
characteristic is so common in Taiwan that people even creatively coin many apt
nicknames for it; for example, Shr-Bau Rain, which means the intensity is raging like lions and leopards. But, what is the mechanism behind it? How do the hydrology and heat energy work in the generating process to create such a representative scene of Taiwan’s summer days? We can simplify the whole process into three clear stages to explain the climate phenomenon to understand easily.
As mentioned previously that convectional rain occurs only in summer, it is because the basic first stage of generating this kind of rainfalls needs enough sun heat. In the first stage, the energy of sun heats the surface of Earth, prompting the water, such as river and sea, to evaporate. Meanwhile, the air starts expanding and rising due to being heated by the hot land. Therefore, the primary stage is making the two
main natural factors of rainfall, the water and the air, to rise above.
Afterward, in the second stage, also the most crucial stage, important
contributor to convectional rain, cumulonimbus clouds, is generated. During
the heavy air’s exciting journey to the clear and boundless sky, the air’s
temperature, however, lowers down with height. On the other hand, the water
vapor which has transformed from the liquid form in the previous stage
encounters the cool air and gradually condenses to form clouds. The
condensing process continues with the clouds keeping rising until reaching the limited height and enough volume to form large cumulonimbus clouds in the sky. The sky then starts to turn dark.
Eventually, the dark clouds can no longer bear the heavy volume, and fierce convectional rain thus falls down. It usually occurs with scaring thunder and lightning due to the electrical change created by unstable conditions between the electrons in cumulonimbus clouds and the air. With dark heavy clouds, deafening thunder and shocking lightning, the classical scene of Taiwan’s summer afternoon is presented.
Now, we know the mechanism is not as complicated as other natural phenomenon as expected. It is divided into three phases, which are stimulation from the sun heat causing the evaporation of water and expansion of air, and the formation of clouds facilitated by the high cool air and water vapor; at the end, convectional rain hit the ground. Though being irritating and muggy, convectional rain is actually a main contributor to irrigation of farmlands and resource to reservoirs of Taiwan. Next time before complaining about the inconvenience caused by convectional rain, we could try to think about its positive dedication to us.
References
Internet geography. What is convectional rainfall?
Retrieved from https://reurl.cc/1QvERW
文化部臺灣大百科全書 (2009)。西北雨 (王原賢)。
Retrieved from https://reurl.cc/24Yedv
