Mumbai, by and large, observes humidity throughout the year. Since it rests in the tropical zone, moreover close to sea, it enjoys more or less same weather. Its climate can be broadly divided into three seasons, namely winters (November-February), summers (March-May) and monsoons (June-September). The months between March and October are characterized by high humidity, while dryness persists from November to February.
During summers, the temperature rises up to 35°C and the climatic conditions flit around mugginess. Monsoons afflict the residents of Bombay with heavy showers. The temperature comes down to some extent, but extreme clamminess makes the conditions worse. Water-logging is a major problem in the city during this season. However, good news is that the air-conditioners really work well around this time.
The Island City receives an annual rainfall of 2,200 metres. The months of October and November bring a respite from the heavy rains. However, winters appear at their apex in December and January. The weather remains pleasant during these months, where the temperature goes down to the minimum of 15°C. February too, offers an enjoyable climate. In fact, the months from October to February provide the best time to visit Mumbai.
The South-west monsoons can put many a plan into chaos. Bombay sees heavy and unpredictable rains between June and September. Watter logged roads, traffic jams, humidity and much confusion all round is often the result...
Bombay winters are almost non-existant. December to February is usually a very plesant period, though never really getting cold.
Now I won't say monsoons are bad but then you will only enjoy the rains there. Trust me don't go there during Monsoon season. There are so many things you can explore and see if there is no rain otherwise rain will spoil all your mood.....
Dehydration and Sun/Heat Stroke - The sun and the heat it generates can be a powerful force in India,even in the cooler winter months. You may not notice the process of dehydration until weakness and dizziness set in. Make it a point to drink a lot of liquids, especially water. Rehydration salts from a medical shop may help. You can eat them as powder or mix them in water.
Be wary of sunburn. Use a sunscreen. Wear a hat. Good sunglasses will protect your eyes. Avoid too much activity in the hotter parts of the day. Go native. Take a siesta if you can.
As such I can't think of any major warnings or dangers, Bombay is usually safe in the day as well as night.
But I would advise tourists to plan their trip to Bombay when it's least hot, because as such all year round Bombay has a hot climate of highs of 32-33 degrees C on avg, but could go as high as 39-40 (sometimes) during April May. The best time to visit Bombay is probably during November to March, that is also the time when Bombay has a dry climate, the rainfall is during June-October, with the rains measuring almost upto 1000 mm during July/Aug.
Adjoining pic : Nehru Science Centre at Worli