What fell, which way it fell, and why the same fallen tree means two different wind speeds depending on the ground it stood in.

In this lesson you will

  • Use tree damage indicators to bound a wind speed rather than name one.
  • Apply the soil-saturation and species caveats to tree damage.
  • Describe reportable wind damage precisely: limb diameter, count, structure type and what failed.

The scale rates damage, not wind

The Enhanced Fujita scale, operational since 1 February 2007, works through 28 damage indicators, each with degrees of damage and, for each degree, an expected wind speed with a lower and upper bound. It is a way of reasoning backwards from what broke to what the wind probably was, and it produces a range.

Two of the 28 matter most for straight-line wind: hardwood trees and softwood trees. Each runs the same ladder of degrees, from small limbs breaking, through large branches, through trees uprooted, through trunks snapped, to trees debarked with only stubs of the largest branches left standing.

The ladder is what to learn, not the numbers
This course prints the degrees and not the wind speeds attached to them, on purpose. A spotter does not assign a wind speed from a tree; a survey does, using the full indicator set, the construction quality, and the ground conditions below. What the ladder gives you is the vocabulary to describe damage precisely enough that somebody else can do that properly. Quote the degree, never the speed.

The same tree, two answers

This is the part that changes what a spotter in a wet climate should say, and it comes straight from the scale's own notes: when uprooting happens in saturated ground or very shallow soil, move toward the lower bound. The tree came out at a lower wind speed than the expected value implies.

Say it backwards until it sticks
Trees down in saturated soil implies less wind for the same damage, not more. After days of rain, wherever you are, that is the normal condition. A spotter who reports "trees down, must have been 70 mph" on soaked ground is reliably overstating it.
  • Species and rooting matter. Deep-rooted mature hardwoods push toward the upper bound. Shallow-rooted or badly planted urban trees fail early and push toward the lower.
  • Leaves matter. A bare tree in winter presents far less to the wind, so the same damage means a higher speed.
  • Health matters. A rotten trunk tells you about the trunk.

Which way did it fall

The single most useful thing a spotter can add, and it needs no instrument: the pattern.

Two overhead panels of fallen trees. On the left, marked straight-line, every trunk points outward from a single point in a spreading fan. On the right, marked tornadic, trunks on opposite sides of a track lean toward each other and lie across one another.
Divergent and fanned is a downburst. Convergent and crossing is rotation. Read the large trunks, and read a lot of them. WxAlerts

Surveyors take direction from large uprooted trunks, not from small snapped branches, which fall whichever way they were already leaning and are unreliable. One tree is an anecdote. A dozen is a pattern.

How to describe damage so it is usable

Precision here is worth more than a wind speed, because it lets somebody else do the estimating properly.

  • Trees and limbs: diameter in inches, and how many. "Three limbs, four to six inches" is a data point. "Big branches down" is not.
  • Trees down: whether snapped or uprooted, roughly what fraction of the trees present, and the ground condition.
  • Structures: the type, because it decides everything. A site-built house, a mobile home, a shed and a barn fail at very different speeds. Then what failed: shingles, roof decking, walls.
  • Power lines: down, and whether poles snapped or leaned.
  • The direction things fell, if you can see a pattern.

Check yourself

After three days of rain, several mature trees are uprooted in a fan pattern spreading outward from one area. How should the wind speed be characterised?

  1. Toward the lower bound for that damage, because saturated soil lets trees uproot at lower speeds
  2. Toward the upper bound, because uprooting mature trees takes exceptional wind
  3. At the expected value, since soil condition is not part of the scale
  4. It cannot be characterised without a measured gust
Hint

The scale's own notes say which way to move when the ground is saturated.

Answer

Toward the lower bound for that damage, because saturated soil lets trees uproot at lower speeds

Saturated or very shallow soil moves the estimate toward the lower bound: the tree came out more easily, so the same damage implies less wind. This is the caveat most likely to be inverted, and in a wet climate it applies most of the time. The fan pattern, separately, says straight-line rather than tornadic.

Sources

  1. NWS, the Enhanced Fujita scale
  2. WSEC / Texas Tech, the EF-scale damage indicator and degree of damage document
  3. Frelich et al., Electron. J. Severe Storms Meteor.: estimating wind speeds of convective storms from tree damage
  4. NWS Charleston spotter quick reference, for a worked example of damage bands