Delhi is currently experiencing an intense cold spell, with temperatures plunging to unusually low levels, leaving residents struggling to cope with a combination of extreme cold and persistent pollution.
The severity of the chill has raised questions about why the national capital and surrounding plains are colder than hill stations such as Shimla and Dehradun.
Record-Breaking Cold In Delhi And NCR
The cold wave has been severe enough to break long-standing records. In Palam, the mercury dropped to 2.3 degrees Celsius, the lowest recorded there in 20 years. The situation has been equally harsh across the National Capital Region (NCR), with Gurugram recording temperatures as low as 0 degrees Celsius.
Typically, such biting cold is associated with hill stations and mountainous regions. However, this winter, conditions in Delhi and NCR have been colder than in several northern hill cities, prompting widespread concern and curiosity.
How Plains Turn Colder Than Hills
According to the India Meteorological Department (IMD), on January 13, Shimla recorded a minimum temperature of 6.6 degrees Celsius, while Dehradun registered 6.5 degrees Celsius, both significantly warmer than Delhi and its adjoining regions.
IMD scientist Naresh Kumar explained that this is not the first instance when plains have experienced colder conditions than the hills. During winter, cold and dry winds descend from the Himalayan region into the plains of north India, leading to a sharp drop in temperatures.
This year, however, western disturbances brought cloud cover to the hill regions. The presence of clouds prevented heat from escaping into the atmosphere at night, keeping minimum temperatures relatively higher in mountainous areas. In contrast, the plains experienced clear skies and steady cold winds, allowing temperatures to fall rapidly after sunset.
In simpler terms, the ground in the plains cooled faster, making the cold more intense compared to hilly regions.
The Science Behind Extreme Morning Chill
Meteorologists attribute the intense early morning cold in Delhi to a phenomenon known as radiational cooling. Over the past several days, Delhi has experienced clear weather during daytime hours. As a result, the heat accumulated during the day escapes directly into the atmosphere at night.
This rapid loss of heat cools the surrounding air significantly, leading to sharply lower temperatures by early morning. The absence of cloud cover accelerates this process.
In contrast, cities like Shimla and Dehradun remain under cloud cover, which acts like an insulating layer. These clouds trap heat and prevent it from escaping easily, thereby limiting the drop in nighttime temperatures despite cold conditions.
When Is a Cold Wave Declared?
Several Indian states, including Delhi, Uttar Pradesh, Haryana, Rajasthan, Punjab, Gujarat, Madhya Pradesh, Chhattisgarh, Bihar, Jharkhand, West Bengal, Odisha, Telangana, Himachal Pradesh, Uttarakhand, Jammu and Kashmir, and Ladakh, are prone to cold wave conditions.
According to the IMD, a cold wave is declared when temperatures in hilly regions fall to zero degrees Celsius or below, and when minimum temperatures in the plains drop to 10 degrees Celsius or lower.
A cold day is declared when the daytime maximum temperature falls below 10 degrees Celsius and remains 4.5 to 6.5 degrees below normal. If the maximum temperature drops by more than 6.5 degrees below normal, the condition is classified as a severe cold day.
