Shimla, the hilly state capital of Himachal Pradesh usually remains colder than Delhi most of the time. However, during the end of December, which is between December 29 and January 1, it was the national capital which was bearing the brunt of winters, leaving Shimla warmer than itself.
Take a look at the minimum temperatures in degrees Celsius over Delhi and Shimla between December 29 and January 1:
Due to approach of Western Disturbance, there was cloud cover as well as light winds which increased the temperatures due to greenhouse effect. Whenever there is clouding, the Earth emits its energy in the form of long wave radiation which get trapped between surface of Earth and cloud, due to which minimums increase.
On the other hand, talking about Delhi, during the same time frame, the national capital was observing clear sky conditions and northerly cold winds were blowing over Delhi and the NCR region.
Therefore, temperatures were quite low as compared to the hilly region of Shimla which mostly remains colder than the plains. This upturn does take place at times during the winter season mostly due to cloud cover. In fact, whenever a Western Disturbance approaches, it is preceded by a warm front wherein the air is warmer, which further causes an increase in temperatures over the hills. Another factor contributing to this upturn is the Greenhouse Effect.
Currently, while Delhi and NCR are enjoying fog free days, and better weather conditions as compared to the first week of January, Shimla is all set to be battered by some rain and snow due to the presence of a Western Disturbance.
Image Credit: wikipedia
Please Note: Any information picked from here must be attributed to skymetweather.com