Important Facts For Prelims
Atmospheric River
- 15 Apr 2025
- 5 min read
Why in News?
In April 2025, a large part of the US experienced heavy rain, strong winds, and severe thunderstorms due to a meteorological phenomenon known as an Atmospheric River (AR).
- This phenomenon, though not new, is gaining attention due to climate change and increasing extreme weather events.
What is the Atmospheric River?
- Definition: An atmospheric river is often described as a "river in the sky". It refers to a relatively long and narrow band in the atmosphere that transports enormous amounts of water vapor from the tropical oceans to continental regions. Unlike rivers on Earth, they are not visible.
- The "Pineapple Express" is a well-known example of AR storms that bring heavy rainfall to the US West Coast, particularly California.
- Characteristics:
- Shape & Size: ARs can be up to 1,600 km long and 400–600 km wide, according to the National Oceanic and Atmospheric Administration (NOAA).
- Moisture Content: Carries water vapor equivalent to the average flow of water at the mouth of the Mississippi River. The strongest ones can carry up to 15 times more.
- Categories:
- Category 1 (Weak): Primarily beneficial, helps in replenishing soil moisture and drought recovery.
- Category 2 (Moderate): Mostly beneficial for replenishing reservoirs after droughts, but prolonged rain may cause localized flooding and mudslides.
- Category 3 (Strong): Balance of beneficial and hazardous. It helps refill drought-hit reservoirs, but heavy rain raises river levels, posing flood risks in already saturated areas.
- Category 4 (Extreme): Mostly hazardous, but also beneficial for drought recovery. Extreme rainfall can cause severe flooding, with rivers often reaching flood stage.
- Category 5 (Exceptional): Primarily hazardous, as prolonged heavy rainfall causes catastrophic flooding, landslides, and widespread destruction, often resulting in billions in economic losses.
- Formation: An AR forms when warm ocean waters, usually in tropical (e.g., the central Pacific, and most of the Indian Ocean) or subtropical regions (Southern California), cause high levels of evaporation, loading the atmosphere with moisture.
- Low-level jet streams (fast-moving winds in the lower atmosphere) guide the moist air from the tropics toward mid-latitudes.
- As the moist air is forced upward by mountain ranges or frontal boundaries, it condenses into rain or snow.
- Significance: ARs help replenish water supplies by increasing snowpack in mountainous regions.
- ARs are responsible for over 90% of poleward water vapor transport in the mid-latitudes and can produce extreme precipitation when making landfall.
- AR in India: A study found that between 1951 and 2020, India experienced 596 major atmospheric river (AR) events, with over 95% occurring during the summer monsoon season (June to September).
- Notably, 70% of major floods from 1985 to 2020 were linked to ARs, including the devastating 2013 Uttarakhand floods, 2018 Kerala floods, and 2007 South Asian floods.
- The frequency and severity of ARs have increased in recent decades, particularly in the Indo-Gangetic plains and peninsular India, leading to significant destruction and loss of life.
- Impact of Climate Change on AR: Rising global temperatures are intensifying the impact of ARs. Warmer air can hold about 7% more moisture for each 1°C increase, making AR-fueled storms more powerful.
- Future ARs are expected to be longer, wider, and more intense, doubling extreme AR events and increasing flood risks in vulnerable areas.
UPSC Civil Services Examination, Previous Year Question (PYQ)
Prelims:
Q. The formation of ozone hole in the Antarctic region has been a cause of concern. What could be the reason for the formation of this hole? (2011)
(a) Presence of prominent tropospheric turbulence; and inflow of chlorofluorocarbons
(b) Presence of prominent polar front and stratospheric clouds; and inflow of chlorofluorocarbons
(c) Absence of polar front and stratospheric clouds; and inflow of methane and chlorofluorocarbons
(d) Increased temperature at polar region due to global warming
Ans: (b)
