Thunder-storms and hail. Generally before & after the monsoon season, thunder storms with associated showers of rain & hail are the predominant weather phenomena. When sufficient moisture is not present in the air, as over north-west India, only a dust-storm may result. Thunder-storms, with associated squals, are of short duration, but some are very violent and destructive like the Kalbaishakis of Bengal. Dust-storms or andhis occur over north-west India. Some violent thunder-storms are accompanied with hail, especially in northern and central India & occasionally in the interior of peninsular India. The frequency of hail-storms is small in winter,but increases generally as the season advances to summer.During the monsoon season, hail-storms are practically absent from the whole country.

The number of days with hail is about 6-7 per year over Himachal Pradesh and its neighbourhood, but it decreases to one in two years over the adjoining plains. Over Bengal,Bihar,Uttar Pradesh & Madhya Pradesh, hail-storms occur about once a year. Hail-storms are comparatively rare over the coastal tracts of the Peninsula.In central India,especially in February & March,hailstorms occur on one or two occasions in the year, and pose a potential hazard to rabi crops. Protection against violent hail-storms is possible by adopting artificial hail-suppression methods. It is difficult to forestall a hail-storm, since its occurence is sporadic & confined to very limited areas in a thunder-storm which in itself is a highly localised phenomenon.

Cyclonic storms. Cyclones from the Bay of Bengal & the Arabian Sea ravage the Indian coasts during the pre-monsoon period from April to June & during the post-monsoon period from October to December. On an average 2-3 storms may be expected in a year. On the coast,many storms cause widespread damage to the standing crops owing to high winds & the associated tidal waves,which are responsible for about 90 percent of the deaths in low-lying areas. With the aid of modern tools for detecting & tracking storms, such as the radar & meteorolgical satellites, the Indian Meteorological Department is now in a position to give advance warnings of impending cyclones with sufficient accuracy. A chain of eight powerful cyclone-warning radars is to be installed in the Fourth Plan ,of which two have already been installed at Vishakhapatnam and Madras, and two more are to be installed at Calcutta & Paradeep by the middle of 1973. weather-satellite-monitoring stations are operating at 5 centres of the IMD, including the three cyclone-warning centres.

Western disturbances During November to May, cloudy weather & light rainfall in the plains & snowfall in the hills in the north occur in association with a series of disturbances that enter India through Iran,Afghanistan & Pakistan & move eastwards across northern India & Assam. The average frequency of these western disturbances is two in November & May & four or five from December to April.These disturbances result in light rains which benefit the rabi crops. In the winter months, the disturbances are also followed by cold waves over northern & central India. These cold waves are sometimes injurious to vegetable & fruit crops.

The actual rainfall in any year in a sub-division may differ widely from the normal, as the quantum of precipitation depends upon the nature & the intensity of synoptic weather systems.Areas with low or scanty rainfall are generally the areas of high variability in rainfall. In parts of Saurashtra & Kutch, the coefficient of variation,(i.e.standard deviation/mean *100) is 40-50 percent,whereas in western Rajasthan, it can be as high as 60-80 percent.The years of deficient rainfall & the years of excessive rainfall occur with no recognizable periodicity . Further,it is not unusual that,in some years, whereas some parts of the country are in the grip of drought, some others are paradoxically ravaged by floods.

Drought. Food production in India being marginal, drought poses many problems.Irrigation facilities available in this country are also limited &,therefore, when droughts occur, they cause partial or complete crop failure.If failures occur in consecutive years, it becomes a national calamity,putting great strain on the economy of the country.

According to the practice folowed by the Indian Meteorological Department, drought is taken to have occured over an area where the annual rainfall is less than 75 percent of the normal.When the annual rain is less than 50 percent of the normal,it is called severe drought.Drought can occur anywhere in India,depending upon the distribution of rainfall.

Attempts to quantify drought in the form of an index have been made by using various techniques.Pilot studies have been conducted in India,with special reference to Bihar State, in order to arrive at a rational definition. The Palmer drought index,which takes into account rainfall,evapo-transpiration and soil moisture is considered a comprehensive approach to this problem.The computation of the Palmer drought index has ben carried out for the different sub-divisions of the country. The computations show that,on the average,drought is experienced on 20-25 percent of the days in each of the months of the kharif season over large areas of the country.

Another aspect of great interest in respect of drought is whether there is any periodicity in its occurence.The power-spectrum analysis of the rainfall series & that of the Palmer drought index series show some relation to the quasi-biennial & the eleven year sunspot cycles in some areas.The amplitude of the cycle is, however, too small to exert a significant influence.

Connected with the above points is also the problem over the rainfall series over an area exhibits any trend, cyclic variation or persistence.A number of studies have been carried out on this aspect over different regions of the country. By & large, over the areas studied, it has been found that the rainfall series is found to be of a random nature with no significant trend.Hence it would not be feasible statistically to predict the amounts of rainfall during any particular year from a knowledge of the past rainfall occurences alone.The probabilities of the rainfall occurences can, however,be used to estimate the risk of poor rainfall & the attendant drought.

Floods. Many parts of the country, particularly the areas drained by large river systems, suffer from devastating floods.Widespread floods occured in 1878,1892,& 1917 in the then many provinces of India,& again in many states of the country in 1954. Rainfall, exceeding 40-50 cm in 24 hours, occurs occasionally along the west coast, in the coastal districts of Tamil Nadu & in southern Andhra Pradesh,in Assam, West Bengal & in the foothills of the Himalayas.

Some of the long term measures to minimise the damage because of floods consist in the afforestation of the upper catchment areas of the rivers, the construction of river embankments,& the execution of multi-purpose river valley projects.Individual farmers can lessen the damage to their crops owing to heavy rains or floods by making the necessary drainage channels in their fields.Plans are in hand for forecasting floods & for arranging expeditious facilities to take timely action to save the standing crops from devastation by floods.A flood meteorological organisation has been drawn up for the country & is under implementation during the current & the subsequent plan periods.