Acold wave swept Delhi on Wednesday as the minimum temperature dipped to 3.5 degrees Celsius, the India Meteorological Department said.

It is likely to get even colder on New Year's Eve, it predicted, as dry and icy winds barrelled through the plains.

The Safdarjung Observatory, which provides representative data for the city, recorded a minimum of 3.5 degrees Celsius as against 3.6 degrees Celsius on Tuesday.

The weather stations at Jafarpur and Lodhi Road recorded a low of 3.5 degrees Celsius and 3.7 degrees Celsius, respectively, it said.

During nighttime, ''dense'' fog reduced visibility to 50 metres in the Palam area. However, it improved to 400 metres by 9 am. The visibility at Safdarjung was 500 metres. According to the IMD, ''very dense'' fog is when visibility is between 0 and 50 metres, 51 and 200 is ''dense'', 201 and 500 ''moderate'', and 501 and 1,000 ''shallow''.

In the plains, the IMD declares a cold wave if the minimum temperature dips to 4 degrees Celsius. A severe cold wave is when the minimum is 2 degrees Celsius or less. Cold wave conditions are predicted in parts of the city over the next two days, it said.

Kuldeep Srivastava, the head of the regional forecasting centre of the IMD, said cold and dry northerly/northwesterly winds from the western Himalayas have been barrelling through the plains, bringing the minimum temperature in north India down. According to the IMD, a cold wave is also declared when the minimum temperature is 10 degrees Celsius or below and is 4.5 notches less than normal.

A ''severe'' cold wave is when the minimum temperature dips to two degrees Celsius or the departure is more than 6.4 degrees Celsius. On December 20, the Safdarjung Observatory recorded a minimum of 3.4 degrees Celsius, the lowest this season so far.

The mean minimum temperature (7.06 degrees Celsius) in December this year so far is less than last year when it was 7.6 degrees Celsius. Last year, the national capital broke the record of the longest cold spell, registering 18 consecutive cold days.

At 9.4 degrees Celsius, Delhi had also recorded the lowest maximum temperature in December in 119 years. This year, however, the city has witnessed only three ''cold'' days and 7 ''cold wave'' days so far.

A cold day is declared when the maximum temperature is less than or equal to 16 degrees Celsius.