The severe threshold, the NWS estimation chart in full, and the well-documented reason a number you guessed is worth less than a sentence describing what moved.

In this lesson you will

  • State the severe wind threshold and the reporting trigger.
  • Explain why spotter wind estimates are unreliable and biased high.
  • Apply the measured versus estimated distinction as the record encodes it.
  • State the anemometer siting standard and the limits of home and vehicle instruments.

The numbers

Report this way, always
Report gusts of 40 mph or higher, or any wind damage. Severe is 58 mph, which is 50 knots, measured or estimated. And say which. Those are the same threshold in two units, not two thresholds, and a report that does not say whether the number was measured is missing the field that decides how much it is worth.

The chart, verbatim

Wind speed estimationDescription
less than 1 mphCalm; smoke rises vertically
1–3 mphDirection of wind shown by smoke drift, but not by wind vanes
4–7 mphWind felt on face; leaves rustle; ordinary vane moved by wind
8–12 mphLeaves and small twigs in constant motion; wind extends light flag
13–18 mphRaises dust and loose paper; small branches are moved
19–24 mphSmall trees in leaf begin to sway; crested wavelets form on inland waters
25–31 mphLarge branches in motion; whistling heard in telephone wires
32–38 mphWhole trees in motion; inconvenience felt walking against the wind
39–46 mphBreaks twigs off trees; wind generally impedes progress
47–54 mphSlight structural damage occurs
Severe begins at 58 mph
55–63 mphDamage to chimneys and TV antennas; pushes over shallow rooted trees
64–74 mphRarely experienced; structural damage becomes possible
above 75 mphVery rarely experienced

NWS New Orleans and Baton Rouge, derived from the Beaufort scale, reproduced exactly including the severe line placed between the 54 and 63 mph rows.

Notice what the chart is actually made of: things you can see happening to objects. Twigs breaking. Whole trees moving. Walking made difficult. It is not asking you to know the wind speed. It is giving you a way to bound it from something observable, which is the right way round.

Why your number is probably too high

Two studies, and they are worth knowing by name because this is a claim about you.

Trapp and colleagues compared Storm Data wind reports against proper damage surveys and found the reports "served as a poor characterization of the actual scope and magnitude of the surveyed damage", sometimes overstating minor events and sometimes understating major ones. The paper is titled Buyer Beware.

Edwards, Allen and Carbin then looked at the shape of the numbers themselves: 150,423 convective gusts over a decade, of which 135,240 were estimated and only 15,183 measured. The estimates spike, unphysically, on values ending in 0 or 5.

A schematic bar chart of how often each gust speed from 40 to 80 miles an hour is reported. Bars at 40, 45, 50, 55, 60, 65, 70, 75 and 80 tower over their neighbors, while values like 47, 53 or 62 are close to nothing. A dashed line marks 58 miles an hour as the severe threshold, just below the tall bar at 60.
Wind has no preference for multiples of five, so every spike is a person rounding. Note where 60 sits relative to the severe line. WxAlerts, schematic of the effect reported by Edwards et al. 2018
The operational adage, and why it is in a journal
Edwards and colleagues quote it: "human observers typically overestimate the wind speed, owing to a lack of experience with extreme winds." You have probably never stood in a 70 mph gust, so you have nothing to calibrate against, and the thing you are calibrating is frightening. A described object beats a guessed number, every time.

Measured, and what that word means

Storm Data has separated MG, measured gust, from EG, estimated gust, since 2006. NWS Instruction 10-1605 puts it: "If known, maximum gusts will be encoded as 'measured;' otherwise, they will be an estimate". The same instruction rules out one tempting shortcut: "Doppler-derived wind speeds will not be used to determine the character of the storm."

Neither the instrument behind an MG nor the person behind an EG is usually recorded, which is worth knowing before you treat a measured value as authoritative. This platform carries the same flag, and weights measured gusts higher without discarding estimates.

What counts as measured

The WMO standard is unambiguous: wind instruments over level, open terrain are exposed ten meters above the ground, with open terrain meaning obstructions are at least ten times their own height away.

  • A backyard station on a roof is not at 10 m over open terrain. Roof-mounted units are usually under-exposed and read low, or sit in accelerated flow over the ridgeline and read gusty. Report the number and say what it came from.
  • A vehicle-mounted anemometer is invalid while moving, and needs correcting for vehicle motion even when stopped. Parked, into the wind, on a known instrument, it is a real measurement.
  • Sustained is not gust. Sustained wind is an average, typically over two minutes at an automated station; a gust is a brief peak. Reporting one as the other changes the meaning entirely.
  • The units are interchangeable, the threshold is one number. 1 knot is 1.151 mph. 58 mph is about 50.4 knots, which is where the 50 kt criterion comes from.

Check yourself

You watch whole trees moving and find it hard to walk against the wind, then estimate the gust at 60 mph. What is the most useful thing to report?

  1. The description of what you saw, with the estimate clearly flagged as estimated
  2. 60 mph, since that is above the severe threshold
  3. Nothing, since an estimate is not usable
  4. 58 mph, since that is the threshold your observation supports
Hint

One option invents a number to fit a threshold, and one throws away a real observation.

Answer

The description of what you saw, with the estimate clearly flagged as estimated

The description is the observation; the number is a guess on top of it, and estimated gusts spike unphysically at multiples of five, with 60 sitting just above the severe line. Report both, flag the estimate as an estimate, and let the office weigh it. Whole trees in motion and difficulty walking is the chart's 32 to 38 mph row, which is worth noticing about your own 60.

Sources

  1. NWS New Orleans and Baton Rouge, "Estimating Hail and Wind"
  2. NWS Instruction 10-1605, Storm Data preparation
  3. Trapp et al. 2006, Wea. Forecasting 21(3), 408–415: buyer beware, some words of caution on the use of severe wind reports
  4. Edwards, Allen & Carbin 2018, J. Appl. Meteor. Climatol. 57, 1825–1845: reliability and climatological impacts of convective wind estimations
  5. WMO-No. 8, Guide to Instruments and Methods of Observation
  6. NWS Weather Spotter's Field Guide (YPA-201154, June 2011)