Ever wondered why the grey cloud did not rain as expected though there enough and more indications? Or why unthreatening clouds can result in a deluge?
Some facets of rain are little understood and two Ph.D scholars at the Indian Institute of Technology — Madras is trying to develop a formula that would help the India Meteorology Department predict rain with more precision.
A rain-bearing cumulus cloud is typically one km in length. “In the cloud, the average size of a water droplet would be 20 microns at the upper layer. Only droplets that eventually increase to 60 microns in size fall to the ground as raindrops,” explains M. Shyam Kumar, one of the two Ph.D students in Aerospace Engineering Department of the Institute who is working on the ‘warm rain initiation’ programme.
For the droplets to increase in size, there must be nucleation. The process of nucleation occurs only when there are enough dust particles. One reason why cloud seeding operations failed is that no study has quantified turbulence. “We are trying to quantify what amount of turbulence could rapidly increase the growth of water droplets and cause them to initiate rain,” Mr. Kumar says.
Hi-tech equipment
The institute had installed equipment that can cause turbulence artificially and tiny droplets of water are passed through a nozzle from a height to form a fine mist. Laser is passed through the mist to study the turbulence and its effect on the mist. “Once we quantify turbulence we will compare it with the IMD data on wind velocity and droplet size,” the research scholar explains.
His professor and Ph.D guide S.R. Chakravarthy says: “We will have to look at the cloud physics, the wind velocity and turbulence. Rainfall prediction is done tracking clouds and connecting cloud cover to rainfall. It is more than just precipitation. We are studying the gap in understanding between turbulence and wind. What causes a water droplet to increase in size from 30 micron to 1 mm? Since the IMD is tracking cloud cover it may be able to predict turbulence. But by developing a model to measure wind turbulence we will help the IMD in tightening the requirement for rainfall prediction.”
