NEED AND PURPOSE
Arborists and urban foresters frequently refer to “wind loads,” “sail areas,” and so forth when discussing the risk of tree failure. Whether the discussion is generally descriptive or specifically quantitative, wind speed is an essential consideration. Wind load is a function of wind speed. Acceptable levels of risk must be associated with wind speeds and their probability of recurrence (Cullen 2002a).
It is conventional in scholarly literature to cite wind speed in SI (Systeme International) units of m/s (NIST, no date). It is customary, however, for meteorological agencies to report wind speeds in miles per hour, knots, or kilometers per hour. The preferred units vary by both agency and country. In addition, various classification scales are used to describe weather events by wind speed ranges. Some of the tree-risk literature employs these scale numbers (e.g., Sinn and Wessolly 1989; Wessolly 1995; Peterson and Rebertus 1997; Wessolly and Erb 1998; Hayes 1999; Peterson 2000; Brudi 2002).
These various usages may be an obstacle to proper understanding of the tree–wind literature (Cullen 2002b), to practical tree-risk management, and to dissemination of knowledge and methods across national and cultural boundaries. The tables in this article should facilitate simple and accurate comparison.
COMPARING SCALES AND SPEEDS
A comparison of wind speed scales and wind speeds in various units of measure is presented in Table 1. The table is not intended to catalog all wind speed scales, but includes several common systems. The World Meteorological Organization provides a list of weather reporting agencies around the world (WMO, no date) that employ various systems.
Wind scales and speeds (copyright Scott Cullen 2002).
Some scales incorporate other elements of storm severity in addition to wind speed. For example, the Fujita Scale (Fujita 1987; NCDC 2001a; TTU 2002)— or Fujita-Pearson Scale—(Stormfax 2001) considers the width and length of tornado damage paths; and the Dolan-Davis Northeaster Storm Scale (Dolan and Davis 1992) considers the “storm surge” in coastal water bodies. A variety of scales are catalogued by the Natural Hazards Research Centre (2001). These scales may be useful in assessing risks of tree damage in particular locales.
TREES AS WIND SPEED INDICATORS
A number of scales have been developed to use tree movement or deformation as an indication of current or experienced wind speed.
Tree Movement and Damage
Historically, the Beaufort Scale was developed to scale wind speeds over water and describe wind effects that could be observed by mariners. The scale has been adjusted for use on land, including description of wind effects on trees. The Fujita Tornado Scale and Safffir-Simpson Hurricane Scale (NWS, no date–b; Simpson and Riehl 1981; NCDC 2001b) use tree damage as an index of wind speeds. These effects are presented in Table 2. Rating damage using the Fujita Scale has been characterized as “highly subjective and variable” (Edwards and Harmon, no date; Doswell and Burgess 1988).
Wind speeds and trees (compiled by Scott Cullen 2002).
Deformation of Trees
In chronically windy locations, wind can also affect tree growth resulting in permanent deformation. Robertson (1987) notes that a number of investigators have developed indices of deformation related to wind speeds. These indices include the Griggs-Putnam index (Putnam 1948) for North American conifers; the Barsch index (Barsch 1963) for European broadleaves; and Yoshino (1975) and Yoshino et al. (1976) for European and Japanese trees. Koeppl (1982) reports that the Griggs-Putnam index was further developed to scale deformation against wind characteristics including mean annual wind speed, mean growing season wind speed, mean nongrowing season wind speed, and percentage of winds from prevailing direction. The predictive reliability of these indices is not without critics (Hennessey 1980).
Other studies of tree deformation by chronic wind include those by Cordero (1999), Musselman et al. (1990), and Noguchi (1979, 1992).
- © 2002, International Society of Arboriculture. All rights reserved.
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