How to Read a Fire Weather Forecast

A regular weather forecast is built for people planning their day. A fire weather forecast is built for people planning to put fire on the ground. The two overlap, but the fire weather forecast adds several parameters that determine whether smoke will lift and disperse, how fast a fire will spread, and how easily it can be kept inside your control lines.

The core parameters

Relative humidity

Relative humidity (RH) is one of the single best predictors of fire behavior. Low humidity dries out fine fuels and makes fire spread faster and hotter; very high humidity can make ignition difficult and keep a burn from carrying. For most broadcast burns, a mid-range RH — roughly 30–50% — offers a workable balance between a fire that carries and one you can still control.

Wind speed and direction

Wind drives both fire spread and smoke. Two different wind measurements matter:

• Eye-level / 20-foot wind: the wind that actually pushes the flame front. Steady, moderate winds give predictable spread; gusty or shifting winds are dangerous because they change fire direction without warning.
• Transport wind: the average wind through the mixing layer that carries your smoke column downwind. It determines where the smoke goes once it lifts.

Direction is as important as speed. A steady wind blowing smoke away from roads, homes, and sensitive areas is worth waiting for.

Mixing height

Mixing height is how high into the atmosphere smoke can rise before it stops mixing upward. A low mixing height traps smoke near the ground, creating visibility and health hazards; a higher mixing height lets the column lift and disperse. This is one of the most important — and most overlooked — safety parameters in a burn plan.

Dispersion index

The dispersion index combines mixing height and transport wind into a single measure of how well the atmosphere will carry smoke away. Higher values mean better dispersion. Very low values are a signal to postpone: even a well-behaved fire can create a smoke problem when the air is stagnant.

Temperature and fine fuel moisture

Temperature interacts with humidity to set the moisture content of fine fuels like grass and leaf litter. Hotter, drier conditions lower fuel moisture and increase fire intensity. Moderate temperatures make for more manageable burns.

Rule of thumb: no single number makes or breaks a burn day. It is the combination — humidity, wind, and dispersion together — that tells you whether conditions are within your prescription.

Putting it together

A good burn window is one where every parameter falls inside the range in your burn plan (your "prescription") at the same time. That is rarer than it sounds, which is why experienced burners track the forecast for days ahead and stay ready to move when the window opens.

BurnAtlas pulls these fire weather parameters directly from the National Weather Service and combines them into a single, easy-to-read Burn Score so you can spot a good window at a glance — then dig into the underlying numbers before you commit.

Always follow your written burn plan and local regulations, and never rely on a forecast alone — verify conditions on site before you ignite.