The part of India that experiences the highest diurnal range of temperature is typically the northwestern region, particularly in desert areas like Rajasthan. This high diurnal temperature range is due to the following reasons:

• The arid and desert climate results in very low humidity, which allows temperatures to drop significantly at night after a hot day.
• Lack of cloud cover in these areas causes intensified solar heating during the day and rapid heat loss at night.
• Sandy and rocky surfaces heat up quickly during the day and cool down rapidly after sunset.

Thus, regions such as the Thar Desert in Rajasthan show the highest difference between day and night temperatures in India due to these climatic and geographical factors. This results in a very high diurnal temperature range in that area. If a more precise, cited source is needed, a focused search on the "highest diurnal temperature range in India" may be done next.