Cold wave, fog, smog: What these winter weather terms really mean

As winter grips north India, here is a guide to terms like cold wave, frost, fog, smog and low visibility, what they mean, and how they affect daily life

North India is experiencing bone-chilling winter conditions, with temperatures dropping below 5 degrees Celsius in several areas. On Sunday, Delhi recorded a minimum temperature of 4.2 degrees Celsius, below the seasonal average, while the Kashmir Valley continues to reel under sub-zero conditions.

 

During this period, terms such as cold wave, frost, fog, smog and low visibility frequently feature in weather bulletins. Here is what these commonly used winter terms mean.

 

Cold wave and severe cold wave

 

The India Meteorological Department (IMD) defines a cold wave as a situation where the minimum temperature falls below 10 degrees Celsius in the plains or below 0 degrees Celsius in hilly regions. A severe cold wave occurs when temperatures drop further or when the departure from normal exceeds minus 6.4 degrees Celsius.

 

 

Cold waves are common in winter due to the presence of high-pressure systems, the movement of cold air masses, and snowfall over the north-west Himalayas.

 

IMD warning system

 

Green: Temperatures near normal

Yellow: Cold wave in isolated places for two days

Orange: Severe cold wave for more than two days or prolonged cold wave

Red: Severe cold wave for more than two days or cold wave persisting over six days

 

Wind chill factor (feels-like temperature)

 

According to the IMD, wind chill reflects the temperature felt by the human body, factoring in wind speed and humidity. Strong winds increase heat loss from the body, making conditions feel colder than the actual air temperature. This effect is especially noticeable during windy winter days.

 

Frost and ground frost

 

Frost forms when moisture in the air freezes on surfaces. Ground frost occurs when the ground surface cools below 0 degrees Celsius even if the air temperature is slightly higher. In extreme cold, frostbite can occur, affecting exposed body parts such as fingers, toes, nose, ears and cheeks.

 

Fog

 

The IMD defines fog as tiny water droplets suspended in the air that reduce visibility to less than one kilometre.

 

Moderate fog: Visibility between 500 and 200 metres

Dense fog: Visibility between 50 and 200 metres

Very dense fog: Visibility below 50 metres

 

Smog

 

Smog is a mixture of fog and pollutants such as particulate matter, nitrogen oxides and sulphur dioxide, as classified by the Central Pollution Control Board. Cold and stable air conditions, along with temperature inversions, trap pollutants near the surface during winter, worsening air quality and visibility.

 

Low visibility conditions

 

In IMD terminology, visibility below 1,000 metres is considered poor for travel. More severe restrictions correspond with dense and very dense fog, increasing risks for road, rail and air transport.

 

Western disturbances

 

Western disturbances are low-pressure systems originating over the Mediterranean region. They bring rain or snow to northern India during winter and often intensify cold wave conditions and snowfall across the Himalayas.