Wind scales and what they say about trees: Beaufort on land, Saffir-Simpson categories and the NWS wind page

This page reports what the official source says. It is not legal advice: ask the office named here. It reports published wind scales and sets no wind speed at which work stops, because none of the scales below is a work limit. What the safety codes say about wind and tree work is on the storm hazards wind page and the climbing in bad weather page.

Beaufort scale on land

The National Weather Service office in Portland, Oregon, prints a table for estimating wind speed from what can be seen. It has thirteen rows, numbered 0 to 12, each with a name, a speed range in miles per hour and a line of visual clues and damage effects. The rows that mention trees are below, with the clue worded in our own words.

Force	Speed	What the page says about trees
1, Light Air	1 to 3 mph	Wind barely moves the leaves.
2, Light Breeze	4 to 7 mph	Leaves rustle and small twigs move.
3, Gentle Breeze	8 to 12 mph	Leaves and small twigs are in constant motion.
4, Moderate Breeze	13 to 18 mph	Small branches move.
5, Fresh Breeze	19 to 24 mph	Large branches and small trees in leaf begin to sway.
6, Strong Breeze	25 to 31 mph	Large branches are in continuous motion.
7, Near Gale	32 to 38 mph	Whole trees are in motion.
8, Gale	39 to 46 mph	Wind breaks twigs and small branches.
9, Strong Gale	47 to 54 mph	The ground is littered with many small twigs and broken branches, and the page lists structural damage such as roofing tiles blown off.
10, Whole Gale	55 to 63 mph	Small trees may be blown over and uprooted. The page also lists considerable structural damage, especially on roofs.
11, Storm Force	64 to 75 mph	Larger trees are blown over and uprooted, with widespread damage.
12, Hurricane Force	over 75 mph	Trees are uprooted, with severe and extensive damage.

Force 0 is calm, with smoke rising straight up and no tree line. The same office prints a second Beaufort table for the sea. It uses knots, adds wave heights and describes clues on the water and on a ship, so its speed bands differ from the land table: it puts force 8 at 34 to 40 knots, for example, where the land table puts it at 39 to 46 mph. The two tables are not interchangeable.

Saffir-Simpson hurricane categories

The National Hurricane Center describes the Saffir-Simpson Hurricane Wind Scale as a 1 to 5 rating based only on a hurricane's maximum sustained wind speed. It says the scale does not take into account storm surge, rainfall flooding or tornadoes, and it calls categories 3 and above major hurricanes. In the western North Pacific it says the term super typhoon is used for tropical cyclones with sustained winds above 150 mph.

Category	Sustained wind	Trees, as the Hurricane Center words it
1	74 to 95 mph, 64 to 82 knots, 119 to 153 km/h	Large branches will snap and shallowly rooted trees may be toppled.
2	96 to 110 mph, 83 to 95 knots, 154 to 177 km/h	Many shallowly rooted trees will be snapped or uprooted, blocking numerous roads.
3, major	111 to 129 mph, 96 to 112 knots, 178 to 208 km/h	Many trees will be snapped or uprooted, blocking numerous roads.
4, major	130 to 156 mph, 113 to 136 knots, 209 to 251 km/h	Most trees will be snapped or uprooted. Fallen trees and power poles will isolate residential areas.
5, major	157 mph or higher, 137 knots or higher, 252 km/h or higher	Fallen trees and power poles will isolate residential areas. The extended table adds that nearly all trees will be snapped or uprooted.

The Hurricane Center also publishes an extended table with a separate Trees column for each category, plus columns for homes, signs, power and water. The wording in that column matches the short table in substance for categories 1 to 4. For category 5 it reads that nearly all trees will be snapped or uprooted and power poles downed.

The Weather Service wind safety page

The National Weather Service safety page on wind says high winds can come from a severe thunderstorm, a strong weather system or air flowing down a mountain. It says that when winds are sustained at 40 to 50 mph, isolated wind damage is possible, and that widespread significant damage can occur at higher speeds. It adds that straight-line wind speeds in strong thunderstorms can exceed 100 mph.

Reading the three together

The Beaufort table gives a speed to an observed effect on land and stops at force 12, over 75 mph. The hurricane scale starts at 74 mph and rates sustained wind. Neither table says how long a tree must be loaded or what kind of tree fails first: both describe what an observer would see.