Trigger depth: when a snow service shows up

What does a trigger depth actually mean?

On a seasonal arrangement, your property sits on a route and the service comes when accumulation reaches an agreed depth. Below that depth, nothing happens unless something else in the agreement says so. Above it, you are on the list. The point is that nobody has to phone during a storm, which is exactly when everyone else is phoning too.

The number is a threshold, not a promise of a time. A trigger tells the service when your property needs a visit; where you sit on the route, how fast snow is falling and what else the storm is doing decide when the truck actually arrives. Response times are their own topic, covered in snow service response times, and both belong in the written agreement. Our guide to snow removal contract terms explains how they are usually worded.

Is it snowfall or snow depth that counts?

They are different measurements, and the difference matters. The National Weather Service defines snowfall as the maximum amount of new snow that has fallen since the previous observation, and snow depth as the total depth of snow, including any ice, on the ground at observation time, which includes new snow plus snow already there. A trigger is almost always about new snow on a cleared surface, which is closer to snowfall than to depth.

The NWS guidelines also explain why one ruler reading can mislead. Observers are told to measure where blowing and drifting are minimized, to take several measurements and average them in windy open areas, to avoid the largest drifts, and to measure as soon as possible after snow ends because snow settles and melts. Their own example: 4.2 inches that settled to 2.6 inches by the next morning is still a 4.2 inch snowfall. A drive that drifts, or a lot with shaded and sunny halves, can show very different depths a few yards apart, so it helps to agree where depth is judged.

How much snow does western Montana actually get?

Less than many newcomers expect on the valley floor, and more as you climb. The Western Regional Climate Center's period of record summary for the Missoula International Airport station (1948 to 2012) shows mean annual snowfall of 45.3 inches, with the largest monthly means in January (12.3 inches) and December (10.7 inches), then February (7.4), March (6.1) and November (5.7). The Hamilton station's summary (1895 to 2012) shows a mean of 26.3 inches a year, with January the snowiest month at 7.4 inches.

Those are averages for two valley stations, not a forecast for your property. Benches, side drainages and higher ground get more, and a single winter can be far above or below the mean. What the numbers do show is that much of the valley's snow arrives as a series of modest storms rather than a few huge ones, which is why the trigger setting has such an effect on how many visits a season involves. Our guide to what happens to snow contracts in a light winter looks at the other side of that.

Why not just clear every dusting?

Because the cost and the benefit both scale with visits. A very low trigger means more visits, a drive that is nearly always open, and less chance for snow to be driven on and packed down. A higher trigger means fewer visits, and some mornings where you drive through an inch or two before the service arrives. On a seasonal or per-visit arrangement, the number of visits is a large part of what you pay, which is one of the factors in our snow removal cost factors guide.

Light snow is also the snow that melts on its own on a sunny pavement, especially early and late in the season. Clearing it can be worthwhile on a shaded, north-facing drive and nearly pointless on a sunny one. That is a decision about your property, not a rule.

What happens to snow that sits below the trigger?

It gets driven on and walked on, and packed snow behaves differently from fresh snow. Western valleys here cycle above and below freezing, and packed snow that softens in the afternoon refreezes at dusk into a hard, slick layer that a blade will not lift cleanly. That is the main argument for a lower trigger on surfaces with steady traffic, steps, and grades.

OSHA's winter guidance for workplaces is to clear walking surfaces of snow and ice, and spread deicer, as quickly as possible after a storm. For a home, the same logic says the surfaces people walk on deserve a lower threshold than the drive. Many arrangements set one trigger for the drive and treat walks, steps and ice as a separate line, which our ice melt guide and the sand, salt or both comparison help you think through.

How do you choose a trigger depth for a home?

Start with how you leave in the morning. A household that has to be out at a set hour on weekdays, with a low car or a long drive, usually wants a lower trigger. A household that can wait, has a high-clearance vehicle and a short flat drive can tolerate a higher one. Then think about who walks on the property: an older resident, a delivery route, visitors who do not know the steps.

Next, look at the property. Grades, shaded sections and gravel all favor clearing before snow gets packed, because packed snow on a slope or on gravel is hard to remove without damage. Drifting changes everything: on an exposed lot, a modest snowfall with wind can close a drive that a calm storm of twice the depth would not. If your property drifts, say so when the arrangement is set up, because a trigger alone does not describe that problem.

How is it different for a business or HOA?

Commercial and association properties usually work back from a time: the lot and walks have to be open before staff or residents need them. That pushes triggers down, adds separate rules for walks and entrances, and often adds a plan for snow that keeps falling through the opening hour. Our commercial snow and ice management and HOA snow removal pages explain how we set those up, and the commercial snow and ice contract checklist lists what to put in writing.

Rules outside your property also matter. The City of Missoula requires sidewalks to be cleared within 24 hours following a snowfall, which a trigger set for the drive does not automatically cover. Our guide to Missoula sidewalk snow removal rules explains what the City asks of owners.

What should the agreement say besides the number?

A trigger works best when the agreement also says how depth is judged (on the cleared surface, not in a drift), whether a long storm gets repeat visits, what happens with the county plow berm at the entrance, and whether ice treatment follows its own rule. It should also say what happens when snow falls just under the trigger several days in a row and gets packed, because that is the situation people argue about.

None of that needs legal language. It needs the same picture in your head and the contractor's. If you are deciding between seasonal and per-visit pricing at the same time, our comparison of seasonal contracts, per-push and hourly explains how the trigger interacts with each.

Should the trigger change through the season?

It can make sense. Early and late season storms in the valleys tend to bring heavier, wetter snow, and FEMA's Snow Load Safety Guide puts a foot of wet, heavy snow at roughly seven times the weight of a foot of light, dry snow. Two inches of slush on a drive is more work to move, and more likely to freeze into ruts overnight, than two inches of powder in January. Some owners prefer a slightly lower trigger for those wet storms and a standard one in mid-winter.

Gravel drives are the other seasonal case. Before the ground freezes hard, a gravel surface is soft and easily cut by a blade, so clearing early with the blade held up, rather than waiting for deeper snow to be packed into it, protects the drive. Our guide to plowing a gravel driveway explains why frozen and unfrozen gravel are treated differently. Whatever you decide, write it down, because a trigger that changes by month only works if both sides remember it did.

How do we set it up?

We agree the trigger depth, the surfaces, the storage and the ice plan before the season, as part of our residential snow removal and commercial arrangements, so the route is set on the first storm rather than negotiated during it. You can see the full range of snow and 24-hour storm services we provide across Missoula and Ravalli counties. This guide is part of our Montana Winter Property Guide.

If you would like help picking a trigger that fits your mornings and your property, call (406) 215-7319 or ask for a free estimate.

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