WatchExplaining wildfire weather
Read the air, not just the fire
Weather kills more firefighters than flame does. This page is for firefighters anywhere in the world who do not have US forecasts and smartphones on every ridge: how to read humidity, wind, and the sky itself, and predict what the fire will do next.
Section 1
Relative Humidity (RH)
Relative humidity is how close the air is to being full of water vapor, written as a percentage. RH matters to firefighters because it controls your fine fuels: grass, needles and twigs absorb moisture when RH is high and dry out when it falls. The same grass that barely burns at 60 percent RH can carry a running head fire at 15 percent, and the change can happen in a single afternoon. RH drops as the day warms and rises overnight, so a fire that was quiet at dawn can be aggressive by mid-afternoon without the weather actually changing.
Measuring it: the standard field tool is the sling psychrometer, two thermometers, one with a wet wick, spun through the air. The difference between the wet and dry readings gives you RH from a simple table. Belt weather kits have been standard US issue for decades, but any pair of matching thermometers and a psychrometric table works the same way. If you have no instrument at all, you can still estimate RH by watching your environment: smoke that keeps rising in a tall column means the air is dry and unstable; smoke that flattens out and drifts means moisture is increasing. Watch how fast morning dew burns off, whether your lips and eyes feel dry, how far you can see, and whether burning operation smoke lies down at night. Small changes in those signs are the field RH forecast.
Section 2
Winds: Diurnal and Environmental Shading
Diurnal winds are the daily breathing of the air on slopes and valleys, driven by the sun. In the morning, the sun heats the slopes and air flows uphill (upslope wind) and up the valley. After peak heating it weakens, and in the evening the slopes cool and the flow reverses: cold air drains downhill (downslope wind) and down the valley. These winds are gentle but steady, and they matter because fire moves with them: an upslope fire in the afternoon can reverse direction at dusk. Every firefighter should be able to feel which phase the day is in before the fire tells them the hard way.
Environmental shading is the other daily rhythm. When terrain or cloud shades your fuels, the sun cannot heat them, fuel temperatures stay low, RH stays higher, and fire behavior stays quiet. When the shade leaves, whether because the sun moved off that slope or the clouds cleared, fine fuels can dry in minutes and fire behavior jumps. Always ask: when does the sun come onto this slope, and when does that shade end? That question often predicts the fire’s afternoon better than any forecast.
Section 3
Foehn Winds: The Firefighter's Worst Weather
A foehn wind is created when moist air is forced over a mountain range. It loses its moisture as rain or snow on the windward side, then sinks and compresses on the other side, arriving hot, dry and strong. Foehn winds can raise temperatures by 10 or 20 degrees and smash the RH to single digits in an hour, while blowing hard for days. For firefighters they are the nightmare scenario: the fire blows up, spot fires fly ahead of the front, and there is no overnight relief because the wind never stops. The same wind has different names around the world, and every firefighter should know the local version:
United States
The Santa Ana winds of Southern California: high pressure over the desert pushes air down through the mountain passes to the coast, arriving hot, dry and gusting hard. They drive the region’s most destructive wind-driven fires, often in autumn.
Canada
The Chinook winds of Alberta: warm, dry west winds off the Rockies that melt snow on the prairies and, in fire season, strip the moisture out of fuels along the foothills within hours of arriving.
Australia
Foehn-type winds were documented in the Canberra region, where air crossing the Brindabella Range gully-winds into the capital hot and dry; researchers have linked them to elevated fire danger days. The lesson is universal: know which ranges produce foehn winds where you fight fire.
Section 4
Predicting Weather Changes Around You
Most firefighters in the world do not get a point forecast on every ridge. But weather never hides: it always announces itself if you watch the air, the fuels and the sky. Why it matters: nearly every tragedy fire was preceded by a weather change that someone could have seen coming. The wind that shifts, the RH that crashes, the thunderhead that builds: each one is a trigger point to reassess your tactics, your escape routes, and whether you should be on that piece of line at all.
Recognizing hotter and drier changes
- • RH falling through the afternoon, especially below 20 percent, and fuels that crackle underfoot when you walk.
- • Wind that starts gusting from a new direction, especially from the high country toward the low ground: a foehn-type change may be starting.
- • A smoke column that goes from leaning steadily to standing tall and boiling, then shearing over: the fire has outrun the surface wind.
- • Lenticular clouds building over peaks, and a sudden temperature rise with a dry wind: treat it as a wind change warning and move.
- • Dust devils, swirling leaves, and smoke that changes direction two or three times in an hour: the wind is unstable and undecided.
Reading the clouds
Clouds are the sky’s forecast, posted hours ahead. Learn these six and you can predict most fire-weather changes without a forecast at all.
High cirrus (thin, wispy)
Often the first sign that a weather system is 24 to 48 hours away. Watch for it, then expect thickening cloud, wind shifts and RH changes in a day or two.
Photo: Pexels
Altocumulus castellanus (turrets on mid-level clouds)
Instability building above you. These turret clouds often grow into thunderstorms later in the day: lightning, gusty outflow winds and wind reversals are coming.
Photo: Pexels
Growing cumulus (cauliflower tops by midday)
Strong convection. If the tops keep climbing past mid-afternoon they can become thunderclouds. Flat-based clouds with dark bottoms carry water; watch for sudden gust fronts.
Photo: Pexels
Cumulonimbus (thunderhead with an anvil)
Thunderstorms are here or forming. Expect lightning, rain that may not reach the ground, and above all the outflow wind: a sudden strong wind blast that can turn a fire 180 degrees in minutes.
Photo: Pexels
Lenticular (smooth lens-shaped clouds over peaks)
Strong winds aloft flowing over mountains. A warning of possible foehn-type downslope wind events on the other side of the range.
Photo: Pexels
Low stratus or fog burning off
Shading is ending. RH will drop fast, fine fuels will dry quickly, and fire activity will step up through the morning.
Photo: Pexels