Boundary Layer Height

Unit: Metres (m)

The planetary boundary layer (PBL) is the lowest part of the atmosphere, directly influenced by the Earth’s surface. Its height varies from about 100–300 m at night to 1000–3000 m (or more) during a sunny afternoon.

Why it matters for fire weather

The boundary layer is where all surface weather happens — and where fires burn. Its depth controls several fire-relevant processes:

Wind mixing. A deeper boundary layer connects surface winds to faster winds aloft. When the boundary layer grows during the morning, stronger winds from higher altitudes are “mixed down” to the surface, causing sudden increases in surface wind speed. This morning transition is a common cause of fires becoming more active mid-morning.

Smoke dispersion. A shallow boundary layer (common at night and early morning) traps smoke near the ground, reducing visibility. A deep afternoon boundary layer disperses smoke vertically, improving visibility but also indicating vigorous atmospheric mixing.

Atmospheric stability. A deep, well-mixed boundary layer indicates an unstable atmosphere — favouring convective development, gusty surface winds, and (for large fires) the potential for pyroconvection. The Haines Index and CAPE capture related aspects of atmospheric instability.

How it works

The boundary layer grows and collapses on a daily cycle driven by solar heating:

1. Night — the surface cools, creating a stable, shallow layer (100–300 m). Winds are calm, smoke is trapped.
2. Morning transition (08:00–11:00) — solar heating warms the surface, the boundary layer begins to grow, and upper-level winds start reaching the surface.
3. Afternoon peak — the boundary layer reaches maximum depth (1000–3000+ m). Surface winds are gustiest, turbulence is strongest.
4. Evening collapse — heating stops, the boundary layer contracts rapidly, winds calm, and a temperature inversion often forms.

Key thresholds

PBL height	Physical significance
< 300 m	Shallow, stable atmosphere. Smoke is trapped near the surface. Winds are light. Typical of night-time and early morning.
300–1000 m	The boundary layer is growing. The morning transition is underway — surface winds are increasing as upper-level air mixes down.
1000–2000 m	Well-mixed afternoon boundary layer. Gusty surface winds. Active turbulence. Typical fire weather conditions during the afternoon.
> 2000 m	Deep mixing layer. Indicates strong surface heating and an unstable atmosphere. Vigorous turbulence. On hot days, this signals conditions where the atmosphere can amplify fire behaviour.

How to read it in Wildflyer

Boundary layer height appears in the expert view. The most important feature is the morning transition — the period when the boundary layer grows from its overnight minimum to its daytime depth. During this transition, surface wind speeds can increase substantially and fire activity often picks up.

Sources

• Stull, R.B. (1988). An Introduction to Boundary Layer Meteorology. Kluwer Academic Publishers.
• Srock, A.F., Charney, J.J., Potter, B.E., & Goodrick, S.L. (2018). The Hot-Dry-Windy Index: A New Fire Weather Index. Atmosphere, 9(7): 279.