| Smoke Management |
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| Meteorological Conditions |
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Meteorological Conditions
Open field burning, propane flaming, and stack burning are dependent upon weather conditions. To view the latest Willamette Valley weather forecast and burn advisory, visit the Oregon Agricultural Weather Center.
Wind Direction
The direction of smoke transport is the key element in determining areas in which burning may occur and the determination of northerly or southerly flow is required for certain burn decisions. Wind direction data is obtained from upper air and surface National Weather Service (NWS) observing stations at Salem and Medford. Surface wind measurements are received from NWS stations as well as DEQ meteorological stations. Pilot balloon tests performed at Salem and Harrisburg are used to help build a comprehensive picture of the transport wind pattern in the Willamette Valley.
Wind information from the NWS is transmitted to the ODA by modem communications. Surface winds from DEQ stations are available on a near real-time basis through use of their Data Acquisition System.
Wind Speed
Wind speed data is available from all of the sources mentioned above. Wind speeds that are light at the surface but increasing substantially through the mixed layer are optimal for good plume rise and smoke transport. Light winds aloft do not provide adequate transport, thus downward mixing may reintroduce smoke to the surface layer. As with wind direction, wind speed is substantially affected by surface heating patterns and synoptic scale pressure changes.
Experience indicates surface winds below 10 mph do not inhibit plume rise appreciably. Wind speeds between 10 mph and 15 mph sometimes hinder plume rise by entrainment of colder ambient air. Very little burning may be accomplished when higher wind speeds occur due to difficulties in fire control and turbulent transport of smoke back to the surface.
Mixing Height, Effective Mixing Height, and Ventilation Index .
The depth of the mixed layer normally grows throughout the day and collapses rather quickly in the evening in response to surface heating effects. The mixing height is calculated by the ODA meteorologist from an accurate maximum temperature forecast applied to the morning temperature sounding taken by the NWS.
The mixing height as calculated from the pseudo-adiabatic chart represents a minimum estimate of atmospheric mixing. Buoyant or high velocity plumes can often, through their own energy, overcome atmospheric limitations to vertical dispersion and effectively increase the mixing height.
To better ascertain actual atmospheric mixing capabilities, the Smoke Management Program personnel observe and measure plum rise from test fires or general burning as part of the routine observations. The maximum plume rise attained by these test burns is more representative of actual dispersion capabilities.
Rainfall
Rainfall is measured at NWS observation stations. This information is used to identify likely high fuel moisture content areas and the institution of restrictions on burning.
Weather Forecasts
Forecast briefings are received from the National Weather Service office in Portland at least twice each day at 0730 and 1230. Synoptic and meso-scale meteorological patterns and forecasts are developed by the ODA meteorologist and discussed in determining burn releases. During these briefings the NWS provides forecasts of maximum temperature, minimum relative humidity, and surface and upper level winds. Variations in regional wind flow fields (development of marine air intrusions, thermal troughs, etc.) due to daily heating patterns are discussed. These briefings provide the basis for early morning grower briefing and selection of test fire sites.
Additional forecast updates are received by the ODA meteorologist form the NWS and the Oregon Department of Forestry on an as needed basis.
Monitoring Visibility and Smoke Impacts
A nephelometer is used as a reliable indicator of smoke in the air. This instrument indicates smoke levels by measuring the amount of light scattering that occurs when a beam of light is sent through an enclosed volume of air. Since nephelometers are insensitive to ambient humidity, sun angle, and darkness, they have prevailed over visibility observations as the standard by which smoke intrusions are analyzed.
Nephelometers have been set up by the DEQ in major population centers throughout the Willamette Valley. The amount of light scattering that is caused by the tiny smoke particles is given a numerical value. The numerical values or "b scat" can indicate smoke intensity and overall ventilation capabilities. Currently, nephelometers are located in Portland, Salem, Corvallis, Lyons, Sweet Home, Eugene, and Springfield.
In general, the nephelometer b-scat readings correspond to the following visibility observations:
- 1.0 b-scat clear, no noticeable smoke, visibility 35 miles
- 2.0 b-scat noticeable smokey haze, visibility 15 miles
- 3.0 b-scat moderate smoke haze, visibility about 10 miles
- 4.0 b-scat haze becoming heavy, visibility about 7 miles
- 5.0 b-scat heavy haze, visibility about 5 miles
- 10.0 b-scat heavy smoke, visibility about 2 miles
- 20.0 b-scat heavy smoke, visibility less than a mile
Whenever b-scat readings exceed 1.8 above the background level (the smoke levels prior to any intrusion) for a period of one hour, an hour of official smoke impact is recorded for that station. A b-scat reading of 5.0 above background for one hour counts double, or for two hours of smoke impact.
High background levels during the day (above 3.0 b-scat) indicate that ventilation conditions are poor at the surface, and burning would not be advisable. On a good burn day, background levels are normally less than a 1.5 b-scat.
Between June 16 and September 14 of each year, smoke impact hours are defined as follows:
"Heavy" hours are 5.0 X 10-4 B-scat or more above background; equivalent to visual range of 5 miles or less. (One hour of heavy smoke impact is equal to two hours of moderate smoke impact.)
"Moderate" hours of smoke impact are defined as resulting in hourly nephelometer measurements exceeding 1.8 X 10-4 B-scat above the prior 3-hour background; equivalent to visual range of 12 miles of less.
"Light" hours of smoke impact are defined as resulting in hourly nephelometer measurements exceeding 1.0 X 10-4 B scat above the prior 3-hour background.
"Light" hours of smoke impact were not recorded prior to the 1999 season.
"Significant" hours of smoke impact are defined as resulting in hourly nephelometer measurements exceeding 1.8 X 10-4 B scat above the prior 3-hour background.
Note: Between September 15 and June 15 of each year, "heavy" hours are defined as 4.0 X 10-4 B-scat or more above background, equivalent to visual range of 5 miles or less. (One hour of heavy smoke impact is equal to two hours of moderate smoke impact.)
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