The Boundary Layer Lid: Reading Winter Smog Through Potential Temperature

Smoke and gases live mostly in the lowest kilometre of air, the planetary boundary layer. Its potential temperature profile tells us how deep that layer is and how a chimney plume will behave.

By Saurabh Dhakad, PhD scholar, Atmospheric Chemistry and Climate Change Lab, School of Environmental Sciences, Jawaharlal Nehru University · 30 Oct 2026 · 4 min read

On a clear winter morning in Delhi, smoke from a chimney or a roadside fire often spreads out in a flat grey sheet instead of rising. By early afternoon the same smoke climbs and breaks up. To understand this we need to look at the lowest part of the atmosphere, the planetary boundary layer, and read it using potential temperature rather than plain temperature.

Two parts of the lower sky

The troposphere has two quite different parts. The planetary boundary layer (PBL) is the bottom layer that feels the ground: surface heating, friction from buildings and trees, and every exhaust pipe and chimney. Turbulence in it mixes heat, moisture and pollutants up and down. Above it is the free atmosphere, which hardly feels the surface.

Between them sits a stable layer called the capping inversion. It traps surface turbulence and pollutants below it. So whatever we emit near the ground, smoke, NO_x or water vapour, mostly stays in the PBL, and the depth of the PBL decides how much air is available to dilute it.

Why potential temperature?

Rising air expands and cools by about 9.8 °C for every kilometre, even if no heat is added or lost. So temperature falling with height does not tell us whether air will mix. Potential temperature removes this effect. It is the temperature a parcel of dry air would have if we brought it down, without adding heat, to a standard pressure of 1000 hPa.

• Potential temperature falls with height: unstable. Warm parcels keep rising and the air churns.
• Potential temperature stays the same with height: neutral. This is the well-mixed layer of a sunny afternoon.
• Potential temperature rises with height: stable. Parcels are pushed back, so vertical mixing stops. Meteorologists call any such layer an inversion, even when the temperature itself is not increasing.

A day in Delhi’s boundary layer

After sunrise the ground heats the air, and a mixed layer with nearly constant potential temperature grows upward, eating into the stable air above. After sunset the ground cools fast, a thin stable layer forms near the surface with potential temperature rising with height, and the old mixed air above becomes a residual layer that still holds the day’s pollution.

Winter keeps this layer shallow. Lidar measurements over New Delhi in 2008–09 found a mean daytime PBL height of about 695 m in winter, against about 1025 m in the monsoon. At night it can be far shallower. In a 2018 winter field study in Delhi, the PBL at times fell below 100 m. Ceilometer data from Lodi Road showed that in December and January, each 100 m rise in the afternoon mixing height went with about 13–14 µg/m³ less PM_2.5.

Reading a plume from the potential temperature profile

The classic plume shapes from textbooks are simply the potential temperature profile made visible.

Fumigation is the one to watch in winter. Smoke released into stable air at night stays aloft. When the morning mixed layer grows up to it, the plume is suddenly pulled down to the ground, so pollution near the surface can jump for an hour or two after sunrise.

What this means

The same emissions can give clean or choking air depending on how deep the boundary layer is. This is why Delhi’s worst air comes on calm, clear winter nights and mornings. Daily forecasts of mixing height and ventilation could help plan activities such as construction work or waste burning for hours of better mixing. Early morning walks are best moved to later in the day on smoggy days. More ceilometers and lidars across the Indo-Gangetic Plain would help us measure the lid and not just the pollution beneath it.

References and further readings

• Stull, R. B. (1988). An Introduction to Boundary Layer Meteorology. Kluwer Academic Publishers.
• American Meteorological Society. Glossary of Meteorology: ‘Capping inversion’ and ‘Potential temperature’.
• Nakoudi, K. et al. (2019). Planetary boundary layer height by means of lidar and numerical simulations over New Delhi, India. Atmospheric Measurement Techniques, 12, 2595–2610.
• Murthy, B. S. et al. (2020). Impact of mixing layer height on air quality in winter. Journal of Atmospheric and Solar-Terrestrial Physics, 197, 105157.
• Gunthe, S. S. et al. Planetary boundary layer height modulates aerosol–water vapour interactions during winter in the megacity of Delhi (ESS Open Archive preprint).
• Cimbala, J. M. Meteorology and plume behaviour, lecture slides, ME 433 Air Pollution Control, Penn State University.