How Slope, Aspect and Wind Change Wildfire Risk on Your Lot

Why does fire run uphill?

Because the flames and rising heat dry and preheat the fuel above them. The extension guide Reducing Fire Risk on Your Forest Property, from Oregon State, Idaho and Washington State universities, explains that fires tend to spread faster up a slope than down one: as heat rises in front of the fire, it more effectively preheats and dries upslope fuels, making for more rapid combustion. Missoula County's 2018 wildfire protection plan says the same thing in local terms: steeper slopes generally result in faster spread of wildfires and also make mechanical fuel reduction more difficult.

For a house, this means the slope below it matters more than the slope above it. A fire starting downhill has a run at the house. A fire starting uphill tends to back down slowly. Homes on the rim of a draw, at the top of a hillside, or on a bench above a steep face carry the most exposure.

How steep is the terrain around Missoula and the Bitterroot?

Much of it is steep. Missoula County's 2018 plan reports that about 28 percent of the county is flat to gently sloped, under 15 percent slope, mostly in valley bottoms; another 25 percent has moderate slopes of 15 to 30 percent, mostly on lower slopes next to the valleys; and the remaining 47 percent has slopes over 30 percent. The plan notes that the areas under 30 percent are where mechanical fuel reduction is usually considered.

Ravalli County's 2024 plan describes a clear contrast across the Bitterroot Valley. The Bitterroot Mountains on the west side are steep, with deep glacial valleys and few foothills, and the plan says those steep slopes are conducive to rapid fire growth and spread and limit fuel treatment because of rugged access. The Sapphire Mountains on the east are characterized by more moderate slopes and foothills. Homes tucked against the west-side range and homes in the east-side foothills sit in different fire settings.

How much more defensible space does a slope need?

More, and mostly on the downhill side. An older Colorado State University Extension fact sheet gives a worked example: for a home on a 20 percent slope, the minimum defensible space would be 90 feet uphill and to the sides, and 104 feet downhill. The Colorado State Forest Service's current guide says its spacing figures are minimums that can be increased, particularly on slopes, and that the outer zone may be larger for homes on steep slopes.

Crown spacing widens too. The PNW guide lists steep slopes and high winds among the factors that increase the crown spacing needed to reduce crown fire. NIST's tests on shed fires near homes were done on flat ground, and its lead researcher said homes on slopes may need more buffer than the minimum distances found. Our guide to thinning conifers for defensible space covers spacing on slopes.

What is aspect, and why does it matter?

Aspect is the direction a slope faces. The PNW guide's appendix explains that south slopes receive much higher solar radiation, are warmer, and their fuels dry out sooner and more thoroughly during the fire season. South and west-facing slopes tend to have less vegetation and lighter fuel loads, particularly in lower elevation forests. North slopes are cooler and shaded, carry more vegetation and heavier fuel, and dry later in the season, but because of that heavier fuel they can burn more severely when they do burn.

Apply that to your own hillside. A south-facing grass and pine slope below a house tends to be ready to burn earlier in summer and carries fast-moving surface fire; a north-facing timbered slope tends to carry more intense fire later in a dry season. The cheatgrass and dry grass guide covers the first case.

How does elevation change fire risk?

The PNW appendix says lower elevations, where most private land is, tend to dry out earlier in the year because of higher temperatures and lower precipitation, and that higher elevations see the opposite, along with more lightning strikes and ignitions. Missoula County's plan reports that 21 percent of the county lies below 4,000 feet, 61 percent between 4,000 and 6,000 feet, and 18 percent above 6,000 feet.

For a homeowner on the valley floor or the lower benches, that means your fuels are among the first to cure each summer, while lightning fires often start higher up on public land and burn down toward you. The 2016 Roaring Lion Fire is an example: Bitter Root RC&D's assessment says it started on the Bitterroot National Forest and burned east with high intensity into the wildland-urban interface on the west side of the valley.

What terrain features make fire worse?

Canyons, chimneys, saddles and ridges. The PNW appendix says narrow and wide canyons, ridges and saddles can dramatically affect fire behavior by funneling air, increasing wind speed and intensifying fire. It also notes that fires on lateral ridges can burn in any direction and be affected by wind moving up canyons and through saddles. A house at the top of a narrow draw, or in a saddle between two hills, sits where fire and wind are channeled.

The same appendix points out the opposite case: rock outcrops, streams, rivers, lakes and roads act as fire barriers and can anchor fuel breaks. On acreage, it makes sense to find those features first and build any fuel break from them. See planning a fuel break on acreage.

How does wind behave in western Montana valleys?

Wind follows a daily pattern in mountain country. The PNW appendix explains that in summer wind often moves upslope in the afternoon, when heating peaks, and downslope in the evening once the land cools, and that fires burn hotter and faster in the warm part of the day. Missoula County's plan reports that summer winds at the Missoula airport average about seven miles per hour from the northwest, and that typical July winds are often calm in the morning, pick up with daytime heating, and calm again around 9 pm.

Fire weather is a different thing from average weather. The same plan notes that during the 2003 Black Mountain Fire, sustained winds were measured at 20 to 25 miles per hour with gusts to 40 to 45. Ravalli County's plan says weather systems and prevailing winds there generally blow from the west. A wind-driven fire, the PNW appendix notes, often forms a long, narrow front that is hard to control when the wind shifts. When the National Weather Service issues a Red Flag Warning, critical fire weather is happening or about to.

Does a steep lot change how fuel reduction is done?

Yes. Both county plans make the point that slope limits equipment. Missoula County's plan says steeper slopes make mechanical fuel reduction more difficult, and Ravalli County's plan says the steep Bitterroot Range reduces opportunities for fuel treatment because rugged terrain is hard to reach. On a steep residential lot, that usually means more hand work: felling and limbing by saw, carrying or winching material to where a chipper can reach it, and leaving cut material where it will not roll onto a road or a neighbor's yard.

Slope also shapes what happens after a fire. The Northern Rockies Fire Science Network's summary of the 2000 Bitterroot fires reports that debris flows were most common on severely burned sites where much of the granitic soil was exposed. Keeping fire intensity down on the slope below a house protects the soil as well as the house; see erosion and flooding after a fire.

How do you use this on your own lot?

Start by finding three things: the direction your ground falls away from the house, the direction the prevailing wind usually comes from, and any draw, saddle or canyon mouth that points at you. The side of the house that is downhill and upwind is the side to work hardest, and it is often the side where the defensible space has to extend farthest. Ownership notes in the Roaring Lion assessment describe one house that survived partly because of a large open area on the side the fire came from.

Then adjust the plan from our guide to defensible space zones: wider crown spacing and a longer treated distance downslope, more attention to grass on south and west aspects, and an access route that does not run up a chimney. HJ Property Care plans and does that work through our fire mitigation service across Missoula and Ravalli counties, including steep-ground tree services. This guide is part of our Montana Wildfire Guide.

Call HJ Property Care & Tree Service at (406) 215-7319 or request a free estimate and we will walk the property with you.

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