Cloud Burst, Flash Floods and Thunderstorms

Cloudburst is a sudden, very heavy rainfall, usually local in nature and of brief duration. Most so-called cloudburst occurs in connection with thunderstorms. In these storms there are violent up rushes of air, which at times prevent the condensing raindrops from falling to the ground. A large amount of water may thus accumulate at high levels and if the upward currents are weakened the whole of this waterfalls at one time.
Cloudbursts are especially common in mountainous areas. This is probably because the warm air currents of a thunderstorm tend to follow the upward slope of a mountain. The effect of heavy rain is especially striking on mountain slopes because the falling water is concentrated in valleys and galleys. Mountain cloudbursts cause sudden and destructive floods. The intensity of rainfall in the most severe cloudbursts can only be conjecture. A rainfall 2.47 inches ( 63 mm) in 3 minutes was registered by automatic rain gauge at Porto Bello, Panama , on November  29, 1911 and one of 1.50 inches ( 38 mm) in 1 minute was registered at the Bardot rain gauge near Les Abymes , Guadeloupe on November  26, 1970. There have been cases, however, in which the excavation made in the ground by the falling water of cloudburst appears to indicate an even greater intensity of rainfall.
Flash Flood, an abrupt, unexpected torrent of water that rushes down a canyon gulch, valley or other narrow, low-lying area (including streets in urban areas). Flash floods often stem from heavy rains associated with thunderstorms or the rapid melting of snow and ice on mountains; however, they can also result from dam failures. They are relatively brief events, with waters rising and falling within hours or days. The sudden onset of a flash flood, which typically occurs less than six hours after a heavy rainfall event, can be extremely destructive. Some flash floods produce walls of water strong enough to wash out riverbeds, pull structures off their foundations, destroy roads and bridges and break or uproot large trees. They are among the deadliest weather-related phenomena, resulting in the death of more than 5,000 people worldwide each year, the majority of whom succumb to drowning.
Although extreme rainfall is the primary driver of flash flood, terrain factors can also influence how fast water levels rise and how widespread the flood becomes. A landscape characterized by steep slopes ( such as mountains or hills) and large areas of impervious surfaces ( including hardpan and rocks or roofs, roads and other paved areas) can cause water to run off more rapidly and fill up low-lying areas, compared with landscape made up of gentle slopes, thick vegetation and water permeable soils. A warming climate may be increasing the frequency and severity of flash floods. As average air temperature increase, the ability of Earth’s atmosphere to hold moisture ( water vapour) also increases; moisture concentration have been shown to increase by 7% for every 1 degree Centigrade  ( 1.8 degree F) of warming beyond preindustrial temperature levels. This excess moisture can yield more precipitation, which produces heavier rains that may make flash flood episode even more common in regions already prone to them, while also enhancing their intensity.
Thunderstorm, a violent short-lived weather disturbance that is almost always associated with lightening, thunder, dense clouds, heavy rains or hail and strong gusty winds. Thunderstorms arise when layers of warm, moist air rise in a large swift updraft to cooler regions of atmosphere, there the moisture contained in the updraft condense to form towering cumulonimbus clouds and eventually precipitation. Columns of cooled air then sink earthward, striking the ground with strong downdrafts and horizontal winds. At the same time electrical charges accumulate on cloud particles (water droplets and ice). Lightening discharges occur when the accumulated electric charge becomes sufficiently large. Lightening heats the air it passes through so intensely and quickly that shock waves are produced; these shock waves are heard as claps and rolls of thunder. On occasion, severe thunderstorms are accompanied by swirling vortices of air that become concentrated and powerful enough to form tornadoes.
Thunderstorms are known to occur in almost every region of the world, though they are rare in Polar Regions and infrequent at latitudes higher than 50 degree north and 50 degree south. The temperate and tropical regions of the world therefore are the most prone to thunderstorms. Most brief but violent disturbances in Earth’s wind systems involve large areas of ascending and descending air. Thunderstorms are no exception to this pattern. In technical terms, a thunderstorm is said to develop when the atmosphere becomes ‘’ unstable to vertical motion’’. Such an instability can arise wherever relatively warm, light air is overlain by cooler, heavier air. Under such conditions the cooler air rises, an updraft ( a strong current of rising air) will be produced. If the updraft is moist, the water will condense and form clouds; condensation in turn will release latent heat energy, further fuelling upward air motion and increasing the instability.
Once upward motions are initiated in an unstable atmosphere, rising parcels of warm air accelerate as they rise through their cooler surroundings because they have a lower density and are more buoyant. This motion can set up a pattern of convection wherein heat and moisture are transported upward and cooler and drier air is transported downward. Areas of atmosphere where vertical motion is relatively strong are called cells and  when they carry air to the upper troposphere ( the lowest layer of the atmosphere) they are called deep cells. Thunderstorms develop when deep cells of moist convection become organized and merge and then precipitated and ultimately lightning and thunder.
Flash floods due to cloud burst are a major natural hazard in India, causing sudden destruction of life and property so as thunderstorms. Understanding their causes – from cloudbursts and heavy monsoon rains to human-induced runoff – is essential for disaster management. Early warning systems and community preparedness can save lives, while mitigation measures (reforestation, zoning, and better infrastructure) reduce risk. In mountainous regions, special attention is needed due to steep slopes and intense storms.
(Writer can be reached at:sjugeshwor@gmail.com)