It's due to the humidity of the region. Kochi is almost at the sea level and New Delhi is 239 meters above sea level. Even in rainy days the temperature is always around 28-30. The atmospheric pressure is always high in Kochi. Humidity is always above 95%. Its is a sea shore city and lots of back water make this place look like a pressure cooker. But its temperature always have a low dynamic range whereas in Delhi in summer you have very high humidity and winter low humidity. The temperature of Delhi during rainy seasons, its always around 28-35, with high humidity of 95%. Humidity plays a major role in fluctuating the temperatures.