The average annual rainfall is about 125 cm, but it has great spatial and temporal variations.

Temporal variation:
The distribution of rainfall varies temporally as per an annual cycle of seasons. The meteorologists recognise four seasons.The rainfall in these seasons varies in the following manner:
1. Cold Weather Season:

• Little rainfall in some parts of India.
• Some weak temperate cyclones from the Mediterranean Sea cause rainfall in north-western India, which are called Western Disturbances.

2. Hot weather season

• A sudden contact between dry and moist air masses gives rise to local storms which are associated with torrential rains.

3. Southwest Monsoon season

• Over 80% of the annual rainfall is received in the four rainy months of June to September.
• The monsoon may burst in the first week of June in the coastal areas of Kerala, Karnataka, Goa and Maharashtra while in the interior parts of the country; it may be delayed to the first week of July.                                                                                                                                                        Monsoonal rainfall is largely governed by relief or topography and rainfall has a declining trend with increasing distance from the sea.

4. Retreating Monsoon

• By the end of September, the monsoon becomes weak in response to the southward march of the sun.
• The weather is dry in north India but is associated with rain in the eastern part of the Peninsula.

Spatial Distribution

Areas of High Rainfall (Over 200cm): Highest rainfall occurs along the mountain ranges obstructing the approaching moist winds, like the west coast, as well as in the sub-Himalayan areas in the northeast.

Areas of medium Rainfall (100-200 cm): In the southern parts of Gujarat, east Tamil Nadu, north- eastern Peninsula covering Orissa, Jharkhand, Bihar, eastern Madhya Pradesh, northern Ganga plain along the sub-Himalayas and the Cachar Valley.

Areas of low Rainfall (50-100 cm): Most of the regions having the effect of continentality like Western Uttar Pradesh, Delhi, Haryana, Punjab, Jammu and Kashmir, eastern Rajasthan, Gujarat and Deccan Plateau.

Areas of inadequate Rainfall (Less than 50 cm): These are arid regions lying in the interior parts of the Peninsula, especially in Andhra Pradesh, Karnataka and Maharashtra, Ladakh and most of western Rajasthan.

In the north India, rainfall decreases westwards and in Peninsular India, except Tamil Nadu, it decreases eastward.

Changing pattern of rainfall in India:

• There is a general consensus that monsoon pattern in India is changing in terms of intensity, duration, frequency and spatial distribution:
• Rainfall extremes have increased threefold over the last few years.
• The frequency of floods in northwest and the northeast while rainfall deficit in south has increased.
• The onset of the monsoon has been delayed due to a regime shift in climate i.e. from a weak to a strong El Niño period.
• Monsoons have also been ending sooner thereby reducing the length of the rainy season.
• Monsoon seasons are witnessing random ‘break periods’ when there is little to no rainfall.

Though it’s difficult to attribute exact reasons for changing pattern, the following factors have affected the Monsoon pattern:

• The ripple effects of global warming and climate change
• Frequent El-Nino and La-Nina, the Indian Ocean Dipole and the Atlantic Nino
• Break periods are associated with rainfall systems moving northwards from the equatorial region.
• The high rate of deforestation

Thus it becomes imperative for India to work towards restoring the balance of nature in collaboration with other countries, so that monsoon pattern doesn’t change permanently.