What are clouds?

• Clouds blanket 70% of the Earth’s surface at any given time.
• They are like a thin layer of the floating ocean, with enough water to cover the entire surface of Earth with about one inch of rain. A modest-sized cloud (1 cubic km) may contain more than 5,00,000 litres of water.
• Clouds form from water in the sky. The water may evaporate from the ground or move from other areas. Clouds form when an area of air becomes cooler until the water vapor there condenses to liquid form. At that point, the air is said to be “saturated” with water vapor. The air where the cloud forms must be cool enough for the water vapor to condense. The water will condense around things like dust, ice or sea salt – all known as condensation nuclei. The temperature, wind and other conditions where a cloud forms determine what type of cloud it will be.
  • High clouds form several kilometers up in the sky, with the exact height depending on the temperatures where they form. Low clouds generally form within a kilometer or two of Earth’s surface. (Low clouds can even form touching the ground, when they are called fog). Middle-level clouds form between low and high clouds.
    • Cirrus clouds are thin and wispy and often curve with the wind.
    • Cumulus clouds tend to be big and fluffy. These clouds look kind of like giant cotton balls or other shapes in the sky. As a middle-level cloud, this type also can form parallel stripes of clouds.
    • Stratus clouds form sheets of clouds that cover the sky.

What is a cloudburst?

• Cloudbursts are short-duration, intense rainfall events over a small area. According to the India Meteorological Department (IMD), it is a weather phenomenon with unexpected precipitation exceeding 100mm/h over a geographical region of approximately 20-30 square km.
• Cloudburst events are often associated with cumulonimbus clouds that cause thunderstorms and occasionally due to monsoon wind surges and other weather phenomena.
  • Cumulonimbus clouds can grow up to 12-15 km in height through the entire troposphere (occasionally up to 21 km) and can hold huge amounts of water.

• A study published last year studied the meteorological factors behind the cloudburst over the Kedarnath region- They analysed atmospheric pressure, atmospheric temperature, rainfall, cloud water content, cloud fraction, cloud particle radius, cloud mixing ratio, total cloud cover, wind speed, wind direction, and relative humidity during the cloudburst, before as well as after the cloudburst.
• The results showed that during the cloudburst, the relative humidity and cloud cover was at the maximum level with low temperature and slow winds. 

Will we see more such cloudbursts?

• Climate change will increase the frequency and intensity of cloudbursts in many cities across the globe.
• The World Meteorological Organization noted that there is about a 40% chance of the annual average global temperature temporarily reaching 1.5°C above the pre-industrial level in at least one of the next five years.  It added that there is a 90% likelihood of at least one year between 2021 and 2025 becoming the warmest on record and dislodge 2016 from the top rank.
• Also, there is evidence suggesting that globally short duration rainfall extremes are going to become more intense and frequent. 
• Climate change is projected to increase the frequency and intensity of cloudbursts worldwide. As the air gets warmer, it can hold more moisture and for a longer time. We call this the Clausius Clapeyron relationship. And this moisture comes down as a short very intense rainfall for a short duration resulting in flash floods in the mountainous areas and urban floods in the cities.
• The change in monsoon extremes and cloudbursts we see now are in response to the 1-degree Celsius rise in global surface temperature. A 1-degree Celsius rise in temperature may correspond to a 7-10% increase in moisture and rainfall. This increase in rainfall amount does not spread moderately throughout the season. As the moisture holding capacity of air increases, it results in prolonged dry periods intermittent with short spells of extreme rains. More deeper cumulonimbus clouds form and the chances of cloudbursts also increase. As emissions continue to increase and global commitment to reduce emissions proves insufficient, these temperatures are set to hit 1.5°C during 2020-2040 and 2°C during 2040-2060.
• We do not have long-term (20 years or more) hourly data to. With IMD enhancing its automatic weather stations, we may have hourly data that can help map cloudburst-prone regions.

Characteristics

• Cloudbursts are not defined based on cloud characteristics and do not indicate clouds exploding. Cloudbursts are defined by the amount of rainfall. According to the India Meteorological Department (IMD), 100 mm of rain in an hour is called a cloudburst. Usually, cloudbursts occur over a small geographical region of 20 to 30 sq. km.
• In India, cloudbursts often occur during the monsoon season, when the southwesterly monsoon winds bring in copious amounts of moisture inland. The moist air that converges over land gets lifted as they encounter the hills. The moist air reaches an altitude and gets saturated, and the water starts condensing out of the air forming clouds. This is how clouds usually form, but such an orographic lifting together with a strong moisture convergence can lead to intense cumulonimbus clouds taking in huge volumes of moisture that is dumped during cloudbursts.
• Tall cumulonimbus clouds can develop in about half an hour as the moisture updraft happens rapidly, at a pace of 60 to 120 km/hr. A single-cell cloud may last for an hour and dump all the rain in the last 20 to 30 minutes, while some of these clouds merge to form multi-cell storms and last for several hours.

Cloudburst prone areas in India

• Cloudbursts, hence, occur mostly over the rugged terrains over the Himalayas, the Western Ghats, and northeastern hill States of India. The heavy spells of rain on the fragile steep slopes trigger landslides, debris flows, and flash floods, causing large-scale destruction and loss of people and property.
• Recent cloudbursts that caused significant devastation occurred over the Himalayan foothills in Himachal Pradesh (in the year 2003), Ladakh (2010), and Uttarakhand (2013). Cloudbursts were reported from the northeastern States and Western Ghats States during the 2022 monsoon season.
• Monitoring stations on the ground can hardly capture the cloudburst characteristics due to their highly localised and short occurrence. Hence, most of these events go unreported due to the lack of monitoring mechanisms in the region, weakening our ability to understand these events in complete perspective.
• Strong monsoon wind surges along the coast can also result in cloudbursts, as in the case of Mumbai (2005) and Chennai (2015).
• Coastal cities are particularly vulnerable to cloudbursts since the flash floods make the conventional stormwater and flood management policies in these cities dysfunctional.

Detecting cloudbursts

• Satellites are useful in detecting large-scale monsoon weather systems, but the resolution of the precipitation radars of these satellites can be much smaller than the area of individual cloudburst events, and hence they go undetected. Weather forecast models also face a similar challenge in simulating the clouds at a high resolution.
• The forecasting of rainfall in hilly regions remains challenging due to the uncertainties in the interaction between the moisture convergence and the hilly terrain, the cloud microphysics, and the heating-cooling mechanisms at different atmospheric levels.
• The IMD’s  weather prediction scenario has advanced such that widespread extreme rains can be predicted two-three days in advance. Cyclones can be predicted about one week in advance. However, cloudburst forecasts still remain elusive.
• Short term measure: Multiple doppler weather radars can be used to monitor moving cloud droplets and help to provide nowcasts (forecasts for the next three hours). This can be a quick measure for providing warnings, but radars are  expensive, and installing them across the country may not be practically feasible.
• Long term term measure: Mapping the cloudburst-prone regions using automatic rain gauges. If cloudburst-prone regions are co-located with landslide-prone regions, these locations can be designated as hazardous. The risk at these locations would be huge, and people should be moved, and construction and mining in nearby regions should be restricted as that can aggravate the landslides and flash flood impacts.

References:

1. https://www.thehindu.com/sci-tech/science/why-cloudburst-forecast-in-india-still-remains-elusive/article65872538.ece
2. https://www.nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-are-clouds-58.html