Tuesday’s weather, in fact, was “pleasant” — certainly not an adjective used early January. So, what's with the weather this time?

Why is it suddenly hot?
The answer lies in the fact that warm winds from Arabian Sea are replacing the colder winds blowing in from the Himalayas. The winds also bring with them a huge amount of moisture. This, in turn, traps more heat and keeps the city warm during the day.

What is causing the fog?
After a spell of dryness in early January, the warm winds coming from the Arabian Sea is bringing with it a lot of moisture. This moisture condenses and hovers near the surface of earth in the evening, leading to fog. So while humidity in the city from January 1-5 was 71%, it is above 100% now. The city had its tryst with densest fog so far this year on January 7 (Monday), with visibility reduced to 100 metres after 10 pm.

What determines Delhi’s weather?
A combination of western disturbances, northern winds, moisture, and winds blowing in from the Arabian Sea. While western disturbances affect the city in early- and mid-December, northern winds coming from the Himalayan region and Russia lower the temperature during the end of December and early January. At present, the winds from Arabian Sea are playing with the weather and heating up the city

Why did Delhi see its coldest New Year in five years?
Blame it on the dryness, and northern winds. According to the meteorological department, winds from Siberia and Himalayan region lowers the city's temperature around December-January every year. Last year, though, the Siberian region saw its warmest January in years; this year the temperature has reduced again. This is leading to colder winds. Add that to lesser humidity, and it means no fog. A sure-shot potion for bone-chilling cold.