Due to the moderating influence of the sea, the temperature in Bombay undergoes little seasonal fluctuation. May is the warmest month of the year, when the temperature averages 32o C (90o F), although temperatures as high as 40o C (105o F) can be reached occassionally. January is the coldest month, and averages 24o C (75o F), although a temperature as low as 18o C (64o F) is not unknown. The humidity is often very high in the\ months when the temperature is at its peak.
The Indian Ocean monsoon usually occurs between June and September, and deposits more than 90% of the annual 1800 mm (70 in) of rainfall. The first monsoon showers are usually expected to reach Mumbai on the 7th of June, but there are large fluctuations in the date of onset. The monsoon is not a period of unremitting rain. More than half the rainfall is usually deposited in approximately seven days of rain during these three months. There may also be extended periods without any rain. The temperate remains in the high 20s (ie, around 75o F). Monsoon is characterized by heavy westerly winds. The sea is very rough, with extremely high waves on the western coast of the city. The harbour is calmer, but too choppy for boats and sailing.
The maximum daily rainfall ever recorded in the city was 944 mm on July 26, 2005. This is the highest ever rainfall recorded in one day in any city. The previous highest in Mumbai was 575.6 mm on July 5, 1974. The previous record highest anywhere in India was 838.2 mm in one day in Cherrapunji in the north-eastern state of Meghalaya. The world's highest ever rainfall in one day was 1825 mm, as recorded on January 6, 1966 in Fac Fac, on La Reunion island in the Indian ocean.
Immediately after the monsoon there are a few weeks of uncomfortable weather in which high temperatures are rendered harder to bear by high humidity. This post-monsoon return to conditions which prevail in early summer is the root of the English phrase "Indian summer". With the absence of the monsoon wind, the breezes take on the classic diurnal land and sea breeze pattern characteristic of coastal cities. Boats begin to ply in the harbour and sailing begins.
The four month period from November to February are the most pleasant months of the year. The temperature and humidity fall together, reaching their low points late in December and early in January. The temperature remains pleasant till February in most years. The calm sea turns a particularly pleasant shade of blue in these months. Boats ply to nearby beaches, and competitive sailing takes place in these months.
The hot season sets in from March. The temperature usually rises quickly through March. In April the humidity reaches discomfort levels. Towards the end of May a few scattered pre-monsoon showers may provide relief. By the end of May, the onset of the monsoon wind muddies the coastal waters. Boat services to the islands and beaches are usually suspended at the end of May, and sailboats have to be secured for the monsoons.