Sustained Wind Versus Gusts and Which One a Limit Means

Why this matters

"Twelve miles an hour, gusting to twenty-eight" gets read on the morning call as a twelve-mile-an-hour day. It is not. For anything with a sail area, a person on a platform, a sheet of panel on a roof, a tarp, a spray pattern, it is a twenty-eight-mile-an-hour day that is quiet most of the time. The gust is what tips the platform, peels the tarp, rips the panel out of a hand, and it arrives without warning in the middle of an otherwise calm stretch.

Manufacturer wind limits are written against the peak, not the average, and a shop that compares the average against them is working past the limit with a clean conscience. This card is how to read the two numbers and which one a rating means. What wind does to an aerial lift itself, stability and the platform, is owned by 'What Wind Does to a Lift and When It Stops It'.

The two numbers

Sustained wind is an average. Surface weather stations in the US report it as the mean speed over a short averaging period, a couple of minutes, and forecasts give the expected sustained speed for each hour. It describes the steady push of the wind.

Gusts are short bursts above that average, measured over a few seconds. Forecasts give the expected peak gust for the period when gusts are expected to run meaningfully above the sustained speed. A gust figure well above the sustained figure is normal, not a warning sign by itself, and the spread between them grows over rough ground (trees, buildings) and on sunny afternoons when the air is mixing.

Both are measured at the standard height of 10 meters, about 33 feet, in open exposure. Wind generally gets stronger with height above the ground, and the obstructions on a real site change it locally: a house sheltered on one side can see channeled wind around its corners and over its eaves that is stronger than the open-ground figure.

 One 10-minute stretch on a day forecast 12 G 28 mph

 28 |                     G                 G
    |        G            |                 |
 20 |        |            |     G           |
    |  .     |   .        |     |    .      |
 12 |..:.....|...:........|.....|....:......|.. average
    |  : ..  | . :  . .   | .   | .  :  .   |
  6 | :    ..  .   ..   ..  . .. ..    ..  ..
    |
      G = a gust peak lasting a few seconds
      the average sits at 12; the peaks load the work

Most of the trace sits near or below 12. Four peaks rise above it, two of them reaching 28. A tech watching the trees for a minute sees a calm day. A platform at 30 feet takes the peaks.

Force goes with the square of speed

The reason the gust governs is that wind force on a flat object grows with the square of the wind speed, not in proportion to it. Double the speed and the push is four times as large.

Run that on the forecast above. The ratio of gust to sustained is 28 divided by 12, about 2.33. Square it: 2.33 times 2.33 is about 5.44. The gust pushes on a sheet, a tarp or a platform with about five and a half times the force of the sustained wind. Check it the other way: 12 squared is 144 and 28 squared is 784, and 784 divided by 144 is 5.44.

So the twelve-mile-an-hour average describes a load that is about a fifth of the load the gust delivers. Planning against the average is planning for a fifth of the peak load. How that turns into pounds on a sheet a person is holding is worked out in Wind on a Ladder and the Sheet You Carry Up It.

What a manufacturer wind rating means

Equipment that carries a wind rating, aerial lifts above all, is rated against the wind it can safely see at its worst moment, and the manual says what to compare.

• Many outdoor-rated aerial lifts carry a 28 mph (12.5 meters per second) maximum. That figure comes from the lift's design and the industry standards it is built to (ANSI/SAIA A92), and it appears in the manual and on the platform decal. Indoor-only lifts are rated for no wind at all and are not used outdoors. Read the rating on the machine you have rather than assuming 28.
• Manuals commonly state the limit includes gusts. A lift rated 28 mph does not operate when gusts reach 28, whatever the average is. Where the manual says "wind speed," read it as the highest speed the platform will see, because that is the load the stability calculation is designed around.
• The rating is at the platform, not on the ground. A lift at full height is up where the wind is stronger than at the truck. An anemometer reading taken at the base understates what the platform sees.

Products carry wind limits too, usually for application: many sprayed products, some roofing materials during installation, tarps and temporary covers. Where the sheet gives a figure, read it against the gust unless the sheet explicitly says sustained. See related: Spraying in Wind: Drift, Overspray and the Calm-Day Trap.

Worked example: one lift, one afternoon

A two-story gutter and fascia replacement is booked for a boom lift rated 28 mph. The crew plans to run the lift from 8 a.m. to 3 p.m., with an hour off the platform at lunch, and do ground-level cutting and assembly in between.

The hourly forecast for the address, sustained and gust in mph:

• 8 a.m.: 6, gusts not forecast
• 10 a.m.: 10 G 18
• noon: 12 G 22
• 1 p.m.: 14 G 28
• 3 p.m.: 16 G 31

Reading it on the average. The sustained speed never goes above 16 mph, which is 12 mph under the lift's rating. On that reading the whole day is a go.

Reading it on the gust. Gusts reach the 28 mph rating at 1 p.m. and exceed it at 3 p.m. The forecast is a forecast, and a figure at the rating is a figure that could be over it; the crew treats 28 as a stop, not as the last legal minute.

The plan. Lift work runs 8 a.m. to noon, four hours, and the gutter runs that need the platform go first, starting with the highest, most exposed elevation while the wind is lightest. Lunch moves to noon. The afternoon goes to the ground work that was going to fill the gaps anyway: cutting, mitering, assembling downspouts. Whatever platform work is left goes to a calmer day rather than into the 1 p.m. window.

The check on site. At 10:30 the lead raises a handheld anemometer at the platform's working height and watches it for several minutes, noting the highest reading rather than the typical one: 17 mph. That sits close to the forecast gust of 18 for the 10 a.m. hour. At 11:45 the peak reads 23, against a noon forecast of 22, and the trees around the house are swaying in their crowns. The crew finishes the run they are on and brings the platform down at noon as planned.

What it would have looked like otherwise. On the average, the crew stays up through 3 p.m., and from 1 p.m. on the platform is taking gusts at and above its rating, with a person and a 10-foot gutter section in the basket. The gutter section is itself a sail, which is the subject of a sibling card.

The lull is not the reading

Gusts come and go, and the trap they set is the lull. A crew that has come down for wind waits, sees a calm stretch of a minute or two, and goes back up. A calm minute on a gusty day says nothing about the next gust, which by definition arrives out of a calmer average. The same trap runs the other way at the start of the day: the crew arrives in a still hour, sets up, and is aloft when the afternoon mixing starts.

So a gust limit needs a time rule, not just a number. A workable default: watch the meter at working height for at least 5 minutes and take the highest reading, not the typical one; if any reading reaches the limit, the work comes down; resume only after 30 minutes in which no reading has come within a few mph of the limit, and only if the hourly forecast is not rising. That is a shop default, set it tighter if your equipment or your exposure calls for it. The full hold procedure, who calls it and what gets logged, is owned by The Wind Hold for Ladders, Roofs and Spraying SOP.

Field cues when there is no meter

An anemometer is cheap and belongs on any truck that runs a lift or works on roofs. Without one, the standard land descriptions from the Beaufort scale give a usable read on speed, and they work because they describe what the wind is doing to things around you:

• 13 to 18 mph: small branches move, dust and loose paper lift off the ground.
• 19 to 24 mph: small trees in leaf begin to sway.
• 25 to 31 mph: large branches move, overhead wires whistle, an umbrella is hard to hold.

Watch for the gusts specifically. If the large branches are calm for a minute and then all thrash at once, the thrash is the reading that counts.

What changes the answer

• Height and exposure. A rooftop on a building that stands above its neighbors, a ridge, an open field or a waterfront sees more than the forecast area figure. A courtyard may see less, until a gust comes down into it.
• The weather type. A frontal passage or thunderstorm outflow can bring gusts far above anything in the hourly forecast with little warning; see related: Lightning and the Outdoor Crew and The Lightning Stop Work SOP for the storm itself.
• A limit that really is sustained. A few product sheets state a sustained limit explicitly. When the sheet says so, use it, but remember the gusts are still loading whatever the person is holding.

References

• Aerial lift manufacturer operating manuals and platform decals (the rated wind speed and what it includes)
• ANSI/SAIA A92 standards for mobile elevating work platforms, which the lift's rating is designed to
• National Weather Service definitions of sustained wind and gusts; Beaufort scale land descriptions
• See related: What Wind Does to a Lift and When It Stops It
• See related: Wind on a Ladder and the Sheet You Carry Up It; The Wind Hold for Ladders, Roofs and Spraying SOP