A ''kalbaisakhi'' (, ; , ), also known as ''bordoisila'' () and in some

English English usually refers to: * English language * English people English may also refer to: Culture, language and peoples * ''English'', an adjective for something of, from, or related to England * ''English'', an Amish ter ...

publications as a nor'wester, is a localised rainfall and thunderstorm event which occurs in

Bangladesh Bangladesh, officially the People's Republic of Bangladesh, is a country in South Asia. It is the List of countries and dependencies by population, eighth-most populous country in the world and among the List of countries and dependencies by ...

as well as the Indian states of

Bihar Bihar ( ) is a states and union territories of India, state in Eastern India. It is the list of states and union territories of India by population, second largest state by population, the List of states and union territories of India by are ...

Jharkhand Jharkhand (; ) is a States and union territories of India, state in East India, eastern India. The state shares its border with the states of West Bengal to the east, Chhattisgarh to the west, Uttar Pradesh to the northwest, Bihar to the north ...

Odisha Odisha (), formerly Orissa (List of renamed places in India, the official name until 2011), is a States and union territories of India, state located in East India, Eastern India. It is the List of states and union territories of India by ar ...

Tripura Tripura () is a States and union territories of India, state in northeastern India. The List of states and union territories of India by area, third-smallest state in the country, it covers ; and the seventh-least populous state with a populat ...

Assam Assam (, , ) is a state in Northeast India, northeastern India, south of the eastern Himalayas along the Brahmaputra Valley, Brahmaputra and Barak River valleys. Assam covers an area of . It is the second largest state in Northeast India, nor ...

, and

West Bengal West Bengal (; Bengali language, Bengali: , , abbr. WB) is a States and union territories of India, state in the East India, eastern portion of India. It is situated along the Bay of Bengal, along with a population of over 91 million inhabi ...

during summer. These storms generally occur in the afternoon or just before sunset, when thick dark black clouds start appearing over the sky and then bring gale-speed wind with torrential rain, often with hail, but spanning only a short period of time.

Etymology

In Bengali, the term ''kālbôiśākhī'' refers to a "fateful thing" which occurs in the Bengali month of Bôiśākh. The term ''nor'wester'' is more commonly used in English newspapers since these storms travel from northwest to southeast. In Assamese, the event is known as a ''bôrdôisilā'', which is derived from the

Bodo Bodo may refer to: Ethnicity * Boro people, also called ''Bodo'', an ethno-linguistic group mainly from Northwest Assam, India * Bodo-Kachari people, an umbrella group from Nepal, India and Bangladesh that includes the Boro people Culture an ...

word ''bôrdôisikhlā'', in which ''bôr'' means wind, ''dôi'' means water and ''sikhlā'' means girl, represents the goddess of nature, wind and rain and also marks the beginning of the Assamese month Bôhāg or Baishagu.

Genesis

The period of kalbaisakhi formation generally begins in April and lasts until the

monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in Atmosphere of Earth, atmospheric circulation and precipitation associated with annu ...

establishes itself over

Northeast India Northeast India, officially the North Eastern Region (NER), is the easternmost region of India representing both a geographic and political Administrative divisions of India, administrative division of the country. It comprises eight States and ...

, although March kalbaisakhi are not unknown. Based on event descriptions and the meteorological environments involved, these storms can be classified as progressive

derecho A ''derecho'' (, from , 'straight') is a widespread, long-lived, straight-line wind storm that is associated with a fast-moving group of Severe weather#Categories, severe thunderstorms known as a mesoscale convective system. Derechos cause Bea ...

s. Kalbaisakhi originate over the Chhotanagpur Plateau between the states of Bihar and Jharkhand. They gradually gain strength as they move southeastwards and impact the states of Odisha, West Bengal, and Assam with gale-speed winds (often exceeding ), torrential rains, and

hail Hail is a form of solid Precipitation (meteorology), precipitation. It is distinct from ice pellets (American English "sleet"), though the two are often confused. It consists of balls or irregular lumps of ice, each of which is called a hailsto ...

storms. On some occasions, incursion of moist air in these elevated places, when combined with high surface temperatures, causes violent thunderstorms.

Tornado A tornado is a violently rotating column of air that is in contact with the surface of Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. It is often referred to as a twister, whirlwind or cyclone, although the ...

es also rarely form if there is enough moisture, convection and heat in the Ganges-Brahmaputra Delta; one such tornado killed over a thousand people when it struck Bangladesh in 1989, making it the deadliest tornado in history.

Effects

Kalbaisakhi brings destruction by uprooting trees due to gale winds and waterlogging roads due to heavy rainfall. It often damages crop by hailstorms. However it is extremely helpful for

kharif crop Kharif crops, also known as monsoon crops or autumn crops, are domesticated plants that are cultivated and harvested in India, Pakistan and Bangladesh during the Indian subcontinent's monsoon season, which lasts from June to November dependin ...

s like

jute Jute ( ) is a long, rough, shiny bast fibre that can be Spinning (textiles), spun into coarse, strong threads. It is produced from flowering plants in the genus ''Corchorus'', of the mallow family Malvaceae. The primary source of the fiber is ...

paddy Paddy may refer to: People *Paddy (given name), a list of people with the given name or nickname *A nickname or slur for an Irish person Birds *Paddy (pigeon), a Second World War carrier pigeon *Snowy sheathbill or paddy, a bird species *Black ...

, etc. and give relief after the mid-day heat and give rainfall to the dry soils for the development of the crops. Due to

global warming Present-day climate change includes both global warming—the ongoing increase in global average temperature—and its wider effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes ...

, these storms are becoming infrequent and becoming stronger, causing more destruction.

In Bengali literature

Many Bengali poets and artists have been regarded "kalbaisakhi" as events of astounding beauty. Bengali poet Mohit Lal Majumder described the storm in his poem ''Kalbaisakhi'', and the storms also inspired

Rabindranath Tagore Rabindranath Thakur (; anglicised as Rabindranath Tagore ; 7 May 1861 – 7 August 1941) was a Bengalis, Bengali polymath who worked as a poet, writer, playwright, composer, philosopher, social reformer, and painter of the Bengal Renai ...

's poem ''Esho Hey Baisakh'' ().

Photography

File:বৈশাখ মাসে বৃষ্টি আসার আগ মূহুর্ত.jpg, At 8:28 am on the 8th of Baishakh, 1432, the moment before the month's rainfall in

. File:Before the rains arrive in the month of Baishakh in Bangladesh.jpg, Before the rains come in the month of Baishakh in

References