Summer in Delhi is rather unpleasant. Temperatures shoot to about 47oC once in a while, thought the average maximum temperature is about 42oC. In the nights, the temperature falls to a minimum of about 28oC. The days are extremely hot and often accompanied by dust storms, thus making Delhi very dusty. If you do visit Delhi during the summer months which stretch from April to about mid July, you should use a cap or a hat during the day. Use very light cotton clothes. Do not expose skin, you might get sun burnt. So a sun screen lotion comes in very handy during these months. Do not forget to hydrate yourself often unless you want to know what a sun stroke in Delhi is like. Unfortunately, due to the suspended particulate matter in the air and the exhaust from vehicles, the level of pollution does shoot up during these months. Summer in Delhi is from early April to mid July.
Rains always come to Delhi a little later than predicted. There are bouts of rain during early July or maybe very late June. The monsoon finally catches on in the end of July. Depends from year to year, but rainfall is sufficient. Torrential? Well, yes sometimes. The monsoon are around till about middle August generally and till about early September for a year with good monsoon. Any ways, do remember to carry an umbrella.
The fall season starts just after the rains. Its not too pleasant in early autumn because of the humidity which is still there. This season remains till about the beginning of November. By end October nights become chilly and the days become quite bearable. The temperature fall to a low of about 15oC and might go as high as 30oC in the beginning of autumn. For most of the autumn, summer clothing will do. Maybe you could stop rolling the sleeves of your shirt. However, in the later parts of autumn, you would feel comfortable with a sweat shirt on during the nights.
Winter in Delhi is very pleasant. The mornings are very foggy, the nights cold (at least by Indian standards). But once the sun does shine, the days (inspite of cold winds from the Himalayas) are very pleasant. Very cricket like weather we must say. The municipal corporation does good work by planting lots of seasonal plants during autumn which start blooming now and make the traffic islands and roundabout look very colorful. The maximum temperature on a very cold Delhi afternoon would be about 12oC and the minimum would fall to about 2oC. These temperatures recorded by the Met guys belongs to the more populated areas of the city. Since Delhi is rather big in size, many parts of Delhi would be colder than this.
Spring in Delhi is very pleasant. Temperature ranges from a maximum of 25oC to a low of about 10oC. Delhi has spring during the period of mid February to about end of March. The nights and the mornings are rather nippy. Delhi does get a passing shower or two during this time and usually does get a bout of hailstorms somewhere in March. Light woollens will do very well.