In Mumbai (formerly Bombay), capital of the state of Maharashtra, the climate is tropical, hot all year round, with a long dry and sunny season from early or mid-October to early June, and a rainy season, due to the south-west monsoon, from June to September or early October, characterized by a stuffy weather, cloud cover and frequent rains, which occur almost every day and are sometimes torrential.
In the pre-monsoon period, from March to May, the heat becomes intense, so that sometimes the temperature can reach 38/40 °C (100/104 °F); this partly happens also in the period following the monsoon, in October and November, when it can reach 36/38 °C (97/100 °F).
In winter, from December to February, the weather is normally hot and sunny. During the day sometimes it gets very hot, in fact the temperature can reach 35/36 °C (95/97 °F), while at night it normally returns below 20 °C (68 °F), and from time to time it can drop below 15 °C (59 °F): it is better to bring a sweatshirt for safety.
Here are the average temperatures.
Average temperatures - Mumbai
MumbaiJanFebMarAprMayJunJulAugSepOctNovDec
MumbaiJFMAMJJASOND
Min (°C)171821242626252524232118
Max (°C)313133333332303030333432
Min (°F)636470757979777775737064
Max (°F)888891919190868686919390

As mentioned, the monsoon in Mumbai lasts from June to early October, and is definitely intense, since the wet currents come directly from the sea: in July, even 800 millimetres (31.5 inches) of rain fall. In July and August, the sky is almost always cloudy, and relative humidity is very high, although the sea breeze gives a bit of relief. In Mumbai, rainfall amounts to about 2,250 mm (89 in) per year.
Here is the average precipitation.
Average precipitation - Mumbai
MumbaiJanFebMarAprMayJunJulAugSepOctNovDecYear
MumbaiJFMAMJJASONDY
Prec. (mm)110115525800530310551552260
Prec. (in)00000.620.731.520.912.22.20.60.289
Days000011523211431078

The sun in Mumbai regularly shines in the dry season, while in the rainy season the sky is often cloudy, especially in July and August.
Sunshine - Mumbai
MumbaiJanFebMarAprMayJunJulAugSepOctNovDec
MumbaiJFMAMJJASOND
Sun (hours)9101010106336899

In theory, from April to December Mumbai may be affected by tropical cyclones, although they generally affect the areas to the northwest and to the south of the city (or at least this is what happened over the last decades); the fact remains that the city can be brushed by a cyclone passing at some distance, and bringing heavy rains, as happened with cyclone Phyan in November 2009, and with Ockhi at the beginning of December 2017.

Mumbai

The Arabian Sea is warm enough for swimming throughout the year, as you can see in the following table.
Sea temperature - Mumbai
MumbaiJanFebMarAprMayJunJulAugSepOctNovDec
MumbaiJFMAMJJASOND
Sea (°C)262526272929292828292826
Sea (°F)797779818484848282848279

When to go


The best months to visit Mumbai are January and February; in November and December the rains have already ceased, but sometimes cyclones can pass; in March it starts to get very hot.