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Fog is localized phenomenon where horizontal visibility is reduced to less than 1000 m due to suspension of very small water droplets in the air. In fact Fog is cloud at surface level. This phenomenon matters to us in our daily activities due to its effect on life, public health, road safety, and economic prosperity.  Its knowledge is essential to meet critical societal needs. In India, radiation and advection fog are most common which occur mostly over north India in winter. This study presents results of spatial variability of average number of days having visibility less than 1000 m at 0300 UTC during winter season over Indian subcontinent (0° N-35° N and 60° E-100° E) using synoptic hour daily surface data. Analysis of Fog events based on half hourly METAR observations for winter months             (December 2010-February 2011) at selected ICAO stations of India is also presented.    The monthly spatial variability of average number of days with fog having visibility less than 1000 m indicates presence of fog over Indo-Gangetic plains on an average of 7 to 10 days during December and more than 8 days during January. South Gangetic West Bengal is susceptible to fog during February. Seasonal variation of fog shows that there is prominence of fog over Indo-Gangetic plains on an average of more than 6.5 days. Significant incidences of foggy days occur over South Interior Karnataka and Coastal Karnataka has incidence of foggy days on an average of 6.5 days. North India is susceptible to dense fog on average of 2.5 days during the season while Gangetic West Bengal, West Rajasthan and adjoining parts of east Rajasthan and East Uttar Pradesh have occurrence of fog on an average of 3.5 days. Thick fog occurs on an average of 3 days over northern India during the winter season. Analysis of time series of air temperature, dew point temperature, dew point depression, visibility, zonal and meridional components of wind and its magnitude at selected ICAO stations indicate that visibility reduces to below 1000 m while light winds are southeasterly at Ahmedabad, northerly to northwesterly at Amritsar, northerly to northeasterly at New Delhi, westerly to southwesterly at Kolkata, Guwahati and dew point depression is below 3 °C during such conditions. Long period 1971-2010 analysis of visibility conditions less than 1000 m over Indian subcontinent shows Indo-Gangetic plains region to have average wind speed to be 0.6 m/s, air temperature in the range 9 °C to 15 °C during December-February at 0300 UTC. Long period 1971- 2010 analysis of relative humidity shows Indo-Genetic plains region to have relative humidity in the range 72% to 84% at 0300 UTC during December-February.

In this study, an attempt has been made to bring out the observational aspects of vertical wind shear in thunderstorms over Minicoy. Case studies of thunderstorm events have been examined to find out the effect of vertical wind shear and instability on strength and longevity of thunderstorms. Role of vertical wind shear in thunderstorms and its mechanism has been explored in this study. Results reveal that for prolonged thunderstorms high and low instability along with moderate to high vertical wind shear (moderate: 0.003 S −1 ≤ vertical wind shear ≤ 0.005 S −1 and high: > 0.005 S −1 ) play a significant role in longevity and strength of thunderstorms. The mechanism of vertical wind shear in thunderstorms was investigated in a few cases of thunderstorm events where the duration of thunderstorm was covered by the radiosonde/rawin ascent observation taken at Minicoy. Empirical model has been developed to classify thunderstorm type and to determine the strength and longevity of thunderstorms. Model validation has been carried out for selected cases. Model could classify thunderstorm type for most of the cases of thunderstorm events over island and coastal stations.

The catastrophic floods over southeast peninsular India during 2015 northeast monsoon season (NEM, October-December) avowed 400 lives, affected more than 4 million people and reported a loss of property worth 3 billion US dollars. In the present study, an attempt has been made to understand the mechanism of occurrence of heavy rainfall events over southeast peninsular India during NEM, with a special emphasize on 2015 heavy rainfall events. In presence of strong El Niño, basin-wide warming and low-level easterly winds were observed over tropical Indian Ocean during NEM-2015. It is noted that dominant easterly wave (EW) activity aided the intensification of already developing synoptic systems over Bay of Bengal (BoB) and led to occurrence of heavy rainfall events. This finding encouraged us to explore the association of El Niño, EW activity and heavy rainfall occurrences over southeast peninsular India during NEM for the period 1951-2015. It is noticed that~43% of the heavy rainfall events during NEM are associated with El Niño,~31% with La Niña and~26% with neutral years, respectively. A 3-7-day band-pass filter is applied to the daily global reanalysis fields to elucidate the role of EW activity on heavy rainfall occurrences over the region during El Niño-Southern Oscillations (ENSO). Results indicate that EW activity over Indian Ocean plays a seminal role in occurrence of the heavy rainfall events during positive phase of the ENSO (El Niño), while it is found to be weak during negative (La Niña) and neutral phase. It is also revealed that significant above-normal sea surface temperature (SST) over BoB, strong west-east SST gradient between southwest BoB and tropical western Pacific Ocean and anomalous strong low-level easterly flow over tropical Indian Ocean during El Niño years offer favourable conditions for the initiation and westwards propagation of EWs.

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