Climate of West Bengal
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The Climate of West Bengal varies with relief from tropical wet-and-dry in the deltaic south to subtropical highland in the Darjeeling Himalaya. The India Meteorological Department treats the state under two meteorological subdivisions: Gangetic West Bengal and Sub-Himalayan West Bengal and Sikkim.[1]
Climatic zones
Köppen-Geiger classifications follow the relief of the state.[1] The Darjeeling and Kalimpong hills are subtropical highland (Cwb). Most of the western and northern plains are humid subtropical (Cwa). Kolkata, Haldia and the deltaic south are tropical wet-and-dry (Aw).[1]
Seasons
Four seasons are recognised: winter (December to February), pre-monsoon (March to May, with Kalbaishakhi or Nor'wester thunderstorms in the south), the southwest monsoon (June to September) and the retreating or post-monsoon (October to November).[citation needed]
Winter
Winter is mild over the plains with mean minimum temperatures around 15 °C and harsher in the hills with occasional snowfall above 2,000 m at Sandakphu and Phalut.[1] A cold dry northerly wind lowers humidity through January.[citation needed]
Pre-monsoon
Pre-monsoon temperatures rise sharply across the Rarh and Gangetic plains, reaching 38-45 °C in the lateritic west during April and May.[citation needed] Kalbaishakhi thunderstorms, locally called Nor'westers, bring brief violent squalls and hail to southern Bengal during this period.[citation needed]
Southwest monsoon
The southwest monsoon arrives over Gangetic West Bengal in early to mid-June and over Sub-Himalayan West Bengal slightly earlier.[citation needed] The Bay of Bengal supplies moisture and the Himalayas force orographic uplift, so rainfall increases northward and into the foothills. Seventy-five to eighty per cent of annual rainfall falls during the southwest monsoon.[1]
Retreating monsoon
The monsoon withdraws from West Bengal through October.[citation needed] October and November also bring most of the state's cyclone activity.
Rainfall distribution
Rainfall ranges from about 1,200 mm in the southwestern Rarh to over 3,500 mm in parts of the Darjeeling Himalaya and the upper Dooars.[1] The deltaic south receives 1,500-1,800 mm, the central and northern plains 1,200-1,700 mm.
| Station | Köppen | Mean annual temperature (°C) | Mean annual rainfall (mm) |
|---|---|---|---|
| Darjeeling (2,042 m) | Cwb | 14.9 | 3,100 |
| Jalpaiguri (Dooars) | Cwa | 24.5 | 3,200 |
| Kolkata (Alipore) | Aw | 26.7 | 1,711.5 |
| Malda | Cwa | 25.4 | 1,349 |
| Asansol (Rarh) | Cwa | 25.3 | 1,294 |
| Haldia (coast) | Aw | 26.2 | 1,654 |
Cyclones
Tropical cyclones forming in the Bay of Bengal periodically affect the deltaic and coastal districts. Notable recent events include Aila (May 2009), Amphan (May 2020) and Yaas (May 2021). Cyclone activity concentrates in the pre-monsoon (April-May) and post-monsoon (October-November) windows.[citation needed]
Climate change
A 20-year study at the Darjeeling Tea Research and Development Centre experimental farm at Kurseong recorded a 0.51 °C rise in maximum temperature, a decline of about 56 mm in annual rainfall and a 16.07 per cent decline in relative humidity between 1993 and 2012.[2]
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 India Meteorological Department, Climatological Tables of Observatories in India 1981–2010, New Delhi: IMD, 2015.
- ↑ Choubey, M. et al., Darjeeling Tea Research and Development Centre, "Effects of climate change on Darjeeling tea production," presented at the Indian Himalayas Climate Adaptation Programme media workshop, Darjeeling, 2013, reported in India Science Wire and Down to Earth, 17 May 2018.