This paper highlights the temporal and spatial variability of human bio-meteorological conditions based on temperature and relative humidity related index called Heat Index (HI) in India. Monthly mean maximum temperature and relative humidity records from 283 surface meteorological stations in India were used to analyze HI during summer (March to May) and monsoon (June to September) seasons for the last 60 years (1951 to 2010). The long-period climatology of HI suggests that all the regions of the country except the hilly regions are affected by human discomfort of varying levels during summer and monsoon seasons. The results showed increasing trend of HI in almost all the regions of India during the study period. Averaged over the country, HI is increasing during summer and monsoon seasons at the rate of +0.56 o C/decade and +0.32 o C/decade respectively which is statistically significant at 95% level. The increasing HI indicates high level of discomfort in both the seasons which is primarily due to increase in humidity in summer season and maximum temperature in monsoon season. Spatial distribution of HI indicates greater chances of heat related illness in India, more prominently in the southeast coastal regions (Andhra Pradesh, Orissa and Tamil Nadu) in summer and over northwest India (Rajasthan and Indo-Gangetic plains) in monsoon.
A study has been carried out to analyze the annual and seasonal spatial patterns of trends in mean maximum temperature (MMAX), mean minimum temperature (MMIN) and diurnal temperature range (DTR = MMAX-MMIN) in India during 1961-2010. Additionally, we analyzed the spatial dependence of DTR trends on the low cloud cover (LCC) and number of rainy days (NRD). The study has revealed uneven changes in DTR across regions and time of the year in India. All India averaged annual MMAX and MMIN are increasing at the rate of 0.05 o C/decade but there is no trend in annual DTR. Seasonal averaged MMAX and MMIN are increasing for all seasons except MMIN in monsoon. But country averaged seasonal DTR is decreasing for winter and post monsoon mainly due to higher increases in MMIN or lesser decreases in MMAX, increasing for monsoon mainly due to higher increases in MMAX or lesser decreases in MMIN and showing no trend for summer during the period of study.Spatially, both MMAX and MMIN have increased all over India with the exception of northeastern region. However, DTR trends show a distinct contrast between northern and southern half of India. The amplitude of increase of MMIN compared to MMAX is high over north causing significant decrease in DTR. However, the amplitude of increase in MMAX compared to MMIN is high enough to cause significant increase in DTR in southern half of India. Stations in Western Himalayas show significant increase in DTR in all seasons except monsoon. MMAX trends in the Indo-Gangetic plains are significantly decreasing. The correlation analysis reveals that LCC and NRD are responsible for explaining much of the variance in DTR over India during the study period. Furthermore, local factors like land-use change, deforestation, urbanization, anthropogenic aerosols may also have contributed to the overall trends in DTR.
Spatial and temporal variations in summer and winter extreme temperature indices are studied by using daily maximum and minimum temperatures data from 227 surface meteorological stations well distributed over India for the period 1969-2012. For this purpose, time series for six extreme temperature indices namely, hot days (HD), very hot days (VHD), extremely hot days (EHD), cold nights (CN), very cold nights (VCN), and extremely cold nights (ECN) are calculated for all the stations. In addition, time series for mean extreme temperature indices of summer and winter seasons are also analyzed. Study reveals high variability in spatial distribution of threshold temperatures of extreme temperature indices over the country. In general, increasing trends are observed in summer hot days indices and decreasing trends in winter cold night indices over most parts of the country. The results obtained in this study indicate warming in summer maximum and winter minimum temperatures over India. Averaged over India, trends in summer hot days indices HD, VHD, and EHD are significantly increasing (+1.0, +0.64, and +0.32 days/decade, respectively) and winter cold night indices CN, VCN, and ECN are significantly decreasing (−0.93, −0.47, and −0.15 days/decade, respectively). Also, it is observed that the impact of extreme temperature is higher along the west coast for summer and east coast for winter.
Premonsoon season in north-east India is known for thunderstorm activity with moderate to heavy rain, gusty wind, squall and sometimes with the occurrences of hail fall. In this paper an attempt has been made to find out the climatological and synoptical aspects of hailstorms as well as squall over Guwahati Airport (26.18° N, 91.75° E) during Premonsoon season. 20 years data from 1987 to 2006 have been analyzed. The frequency of hail and squall during premonsoon season is observed to be 1.35 and 3.65 respectively. The most favourable time of occurrence of hail is during afternoon to late evening and that for the occurrence of squall is during evening to early morning over Guwahati Airport. The significant synoptic situations associated with occurrence of hail are Sea level trough from East Uttar Pradesh/Bihar to North-East India and low level cycir over Bihar and neighbourhood.
Climatologically thunderstorm activity is quite common features over Guwahati Airport and its vicinity during premonsoon season. But a severe hailstorm with large size of hailstones and long duration is a rare event. Such a hailstorm with hailstone size of about 25mm occurred over Guwahati Airport (26.1°N, 91.6°E) on 2nd April, 2006 during midnight. In this paper an attempt has been made to investigate prerequisite synoptic and thermodynamic conditions of the atmosphere associated with this hailstorm. Under favourable synoptic and instability condition of thunderstorm occurrence, veering of winds over Guwahati between freezing level and 500 hPa level with vertical wind shear of horizontal winds exceeding 6.2 ms-1km-1 more appears to be conductive for the development of a hailstorm. Stability Indices viz., Showalter, K, Total-Total and Severe Weather Threat (SWEAT) index have shown predictability for this event.
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