Fog or Smog? How to Tell, and What Winter Air Does Indoors

Fog is tiny droplets of water hanging in cold, still air; smog is pollution trapped close to the ground, often under the same cold, still conditions. In north Indian winters you frequently get both at once, which is why the grey looks the same. The quickest way to tell is to check the AQI: fog alone does not push it up, while smog does, and smog lingers long after the sun has burnt the fog away.

The Short Answer

From December to January, many mornings in Delhi, the NCR, Punjab, Haryana, Uttar Pradesh and Bihar start grey. Sometimes that grey is clean, damp fog. More often it is a mix: fog droplets sitting in air already loaded with smoke and dust. The difference matters because fog mostly affects visibility, while smog affects the air you breathe indoors and out. Check the reading for your city on the AQI by city page, or the Delhi AQI page if you are in the capital, before you decide whether to open the windows or step out.

What Fog Is

Fog is a cloud at ground level. Air always holds some water vapour, and cold air can hold less of it than warm air. On a clear, calm winter night, the ground loses heat quickly, the air just above it cools, and once it reaches its dew point the vapour condenses into droplets far too small to fall. Millions of them scatter light, which is why fog looks white and makes headlights glow.

Pure fog is mostly water. It feels damp on your skin, leaves windscreens wet, and has no smell of its own. As the sun climbs and warms the ground, the air warms, the droplets evaporate, and the fog thins out, often by late morning.

What Smog Is: Pollution Under a Lid

The word smog began as smoke plus fog, and in a north Indian winter that is still a fair description. The key is a weather pattern called a temperature inversion.

• Normally, air is warmest near the ground and cools with height. Warm air rises, carrying smoke and dust up and away to dilute.
• On cold, still winter nights, the ground chills the air right above it, while a layer higher up stays warmer. Cool, heavy air ends up trapped under warmer air, like a lid on a pot.
• Everything released below that lid stays there: traffic exhaust, smoke from burning waste and crop residue, construction dust, and smoke from wood, coal and dung used for cooking and heating. With little wind to carry it away, it builds up hour by hour.

Fog and smog then feed each other. Particles give water vapour something to condense on, and some pollutants dissolve into the droplets. That is why a foggy, polluted morning can look thicker and yellower than clean fog, and why the late-autumn burning season covered in our guide to stubble burning season and indoor air makes the following weeks so heavy.

How to Tell Fog From Smog

Sign	Mostly fog	Mostly smog
AQI reading	Normal for the season	Poor, Very Poor or Severe
Colour	White, bright around lights	Grey, brownish or yellowish haze
Smell	None, or damp	Smoky, burnt or acrid
Feel	Damp on skin and clothes	Scratchy throat, stinging eyes for many people
Time of day	Thickest around dawn, clears as the sun warms the ground	Lingers after sunrise, can hang through the day, thickens again after dark
Distant buildings	Vanish in a white wall, then reappear sharp	Look washed out and blurred even when visible

No single sign is proof, and on many days you have both. The AQI is the most reliable test, because your eyes cannot judge fine particles. If the sky clears by eleven but the air still smells of smoke and the haze stays brown, it was never just fog.

Why Winter Mornings Are Worst

The inversion is strongest after a long, cold night. The ground has been losing heat since sunset, the wind is usually at its weakest, and twelve hours of smoke and exhaust have collected under the lid. By dawn, the layer of trapped air is at its thinnest and dirtiest.

As the sun warms the ground, the lowest air warms too, starts to rise and mixes with the air above. The lid lifts, pollution dilutes, and the early afternoon is often the cleanest part of a winter day. After sunset the ground cools again and the cycle restarts, which is why evenings also worsen. On the worst days, authorities in the NCR step up restrictions under the Graded Response Action Plan; our explainer on GRAP stages covers what each stage means.

Morning Walks and School Runs

The habit of an early walk runs straight into the dirtiest hours of a winter day. Some practical adjustments when the AQI is Poor or worse:

• Move the walk later, to early afternoon when the air has usually mixed, or walk indoors for those weeks.
• Check before you go, not by looking out of the window. A white morning can be clean fog; a clear-looking one can still carry a high reading.
• On the school run, keep car windows closed and set the air conditioning to recirculate while you are in traffic.
• Follow school and local advisories on outdoor assembly and sport on high-AQI days.

Anyone with asthma, a heart or lung condition, or who is pregnant should follow their own doctor’s plan for high-AQI days. See a doctor if anyone has breathlessness, wheezing, chest tightness, or a cough that does not settle. Do not wait for the AQI to improve before seeking care.

What to Do Indoors

Smog does not stay outside. It leaks in around windows and under doors, so indoor air in winter follows the outdoor curve, just a little lower and a little later. A simple routine helps:

1. Keep windows shut during the worst hours, usually night and early morning.
2. Air the home briefly when the AQI dips, often early afternoon. Ten minutes with windows open across the flat changes the stale indoor air, then close up again.
3. Run a purifier in the bedrooms with the door closed, especially overnight, when you spend the longest stretch in one room.
4. Do not add smoke indoors. Skip agarbatti and dhoop on the worst days, and use the chimney while cooking.
5. Never use a coal or charcoal angithi in a closed room. It produces carbon monoxide, which no air purifier removes. Use an electric heater instead, and keep any combustion source well ventilated.

The full routine for the smog months, room by room, is in our guide to protecting your home from winter smog.

Fog Days, Humidity and Your Purifier

Foggy days are humid days, by definition: the air outside is close to saturated. With windows shut and people breathing, cooking and drying clothes indoors, indoor humidity can climb too. You may notice condensation on cold windows in the morning, damp patches on outside walls, or a musty smell in cupboards.

An air purifier does not change humidity. It does not dry the air and it does not add moisture. If indoor humidity stays high for days, use the dry mode on an AC, a dehumidifier, the bathroom and kitchen exhausts, and those short airing windows when the AQI allows. Wipe condensation off window frames so it does not feed mould.

It helps to see both numbers at once. Boon Airvana’s AtmoScan sensing reads PM1.0, PM2.5 and PM10 alongside temperature and humidity every second, and shows them on its full-colour display. In Auto mode the fan follows the real-time AQI, so it works harder on the smoggy nights and eases off as the room clears. It will not touch the humidity or remove carbon monoxide, but it tells you what is in the room, which is the first step. Airvana is ₹14,900 at the time of writing.

Frequently Asked Questions

What is the difference between fog and smog?

Fog is tiny droplets of water that form when cold, still air reaches its dew point. Smog is air pollution, such as smoke, exhaust and dust, trapped close to the ground, often by a temperature inversion on cold, calm nights. In north Indian winters the two often occur together, which makes them look alike.

How can I tell if it is fog or smog outside?

Check the AQI first, since your eyes cannot judge fine particles. Fog is white, damp and odourless and usually clears by late morning as the sun warms the ground. Smog looks grey, brown or yellowish, can smell smoky, and lingers after sunrise, sometimes all day. If the AQI is Poor or worse, treat it as smog.

Why is smog worse in the morning in winter?

On cold, still nights the ground chills the air just above it while a layer higher up stays warmer. This temperature inversion acts like a lid, trapping smoke and exhaust near the ground all night. By dawn the trapped layer is at its dirtiest. As the sun warms the ground, the air mixes and pollution usually dilutes by early afternoon.

Is it safe to go for a morning walk in smog?

When the AQI is Poor or worse, early morning is usually the dirtiest part of a winter day. Consider walking in the early afternoon, when air has mixed, or indoors. Anyone with asthma, a heart or lung condition, or who is pregnant should follow their doctor’s plan. See a doctor for breathlessness, wheezing or chest tightness.

Should I open windows on a foggy morning?

Not if the AQI is high. A foggy morning in north India often carries smog too, and opening windows lets it in. Keep them shut during the worst hours, usually night and early morning, and air the home for about ten minutes when the AQI dips, often in the early afternoon, then close up again.

Does an air purifier reduce humidity on foggy days?

No. An air purifier filters particles and some odours and gases, but it does not dry or add moisture to the air. For high indoor humidity, use an AC’s dry mode, a dehumidifier, kitchen and bathroom exhausts and short airing when the AQI allows. A purifier with a humidity sensor can show you the level.