Evaluation of Exceptional Events Appendix Y

Y-1

Appendix Y

Evaluation of Exceptional Events 

Based on Rules of Thumb

Daniel M. Alrick

Clinton P. MacDonald
Evaluation of Exceptional Events Appendix Y

Y-3

1. Overview

In relation to regional air quality, evaluation of the historic influence of various 

meteorological and chemical processes upon air quality concentrations allows meteorologists to 

identify the weather conditions normally associated with high ozone concentrations. As part of

the SMAQMD’s air quality forecasting program, STI evaluated high ozone concentrations in the 

Sacramento region and developed several rules of thumb for meteorological conditions to 

predict when 8-hr ozone concentrations will be greater than 0.095 ppm in Sacramento County. 

This appendix describes (1) the conditions associated with high ozone concentrations in 

Sacramento County and (2) an evaluation of whether 1-hr ozone concentrations exceeding the 

federal 1-hr ozone standard of 0.120 ppm would have been expected in the absence of wildfires

on June 23, June 27, July 7, and July 10, 2008. The rules of thumb described in this appendix 

were developed independently to the regression equation described in Appendix X.

On these four dates, fires were burning in northern California and ozone levels in 

Sacramento were high. The 8-hr rules of thumb were used to analyze the smoke impact days

and evaluate whether 1-hr ozone concentrations on those dates would most likely not have 

exceeded the 1-hr standard in the absence of ozone precursor pollutants (hereafter referred to 

as smoke) from the wildfires. If high ozone levels occurred even though meteorological 

conditions did not meet the rules of thumb for high ozone concentrations, an exceedance

without smoke from wildfires would have been unlikely. Since the rules of thumb were designed 

to predict when 8-hr concentrations exceed 0.095 ppm, we determined what the 1-hr 

concentrations were when 8-hr concentrations exceeded 0.095. During the 2007 through 2010 

period, on days when any monitor in Sacramento County recorded an 8-hr ozone concentration 

greater than 0.095 ppm, the average 1-hr maximum concentration was 0.118 ppm, with a 

minimum of 0.107 ppm and a maximum of 0.137 ppm. Again, the rules of thumb for high ozone 

concentrations in Sacramento were developed specifically for 8-hr ozone and have not been 

fully vetted for 1-hr ozone. Thus, they can offer, at best, only a rough correlation.

The rules of thumb for high ozone levels in the Sacramento region were also used to

identify smoke-free days similar in meteorology to the four smoke-impact days. The peak 1-hr 

ozone concentrations in Sacramento County on these similar days were reviewed. If the 1-hr 

ozone concentration did not exceed the federal 1-hr ozone standard for a given smoke-free day, 

it is unlikely that an exceedance would have occurred on the smoke-impact day in the absence 

of smoke. There can be considerable difficulty in finding good matches between smoke and 

non-smoke days; thus, these comparisons do not provide conclusive evidence that an 

exceedance would or would not have occurred, but merely suggest what may have been likely.

In summary, the rules of thumb for high ozone levels in Sacramento suggest that, in the 

absence of smoke from the wildfires, 1-hr ozone concentrations exceeding the federal standard

Would likely not have occurred June 23, 2008. 

Would likely not have occurred June 27, 2008. 

Would likely have occurred July 7, 2008.

Would likely not have occurred July 10, 2008.
Evaluation of Exceptional Events Appendix Y

Y-4

2. Meteorological Indicators Overview

The rules of thumb to predict 8-hr peak ozone concentrations in Sacramento County use 

a combination of local- to synoptic-scale surface data and upper-level meteorological data. The 

meteorological processes represented include transport, recirculation, and horizontal dispersion 

of pollution by wind; variations in sunlight due to clouds; the vertical mixing and dilution of 

pollution within the atmospheric boundary layer; and air temperature. The variability of these 

processes, which affects the variability in pollution, is strongly governed by the diurnal heating 

and cooling cycle, the movement of large-scale high- and low-pressure systems, and local and 

regional topography. 

Development

To develop the rules of thumb, STI conducted detailed analyses of the meteorological 

conditions and ozone concentrations in Sacramento County for May through October in 2006 

through 2009. In particular, STI obtained and reviewed surface and upper-air weather maps, 

daily ozone data for the Sacramento region, and surface and upper-air data from the National 

Oceanic and Atmospheric Administration for the Sacramento region. From past STI analyses 

concerning ozone concentrations in the Sacramento region, explicit meteorological parameters 

were identified and calculated (e.g., average morning wind speeds at Sacramento Executive 

Airport). STI meteorologists analyzed the relationships between the meteorological parameters 

and daily maximum 8-hr average ozone levels in the Sacramento region. From these 

examinations, threshold values for the meteorological parameters were identified as favorable 

for daily maximum 8-hr average ozone concentrations above 0.095 ppm. Surface and 

upper-level weather maps were also examined to identify weather patterns conducive to high 

ozone levels in the Sacramento region.

Rules of Thumb

In general, 8-hr ozone concentrations in excess of 0.095 ppm occur in the Sacramento

region when an aloft ridge of high pressure is located over California (see Figure 1). Aloft 

ridges produce subsidence in the mid-to-low levels of the atmosphere. As the air sinks, it 

warms due to compression, which forms a temperature inversion that stabilizes and dries the 

atmosphere. This process limits the vertical mixing of boundary layer air (which traps pollutants 

near the ground) and limits cloud production (which increases ozone photochemistry). 

On the other hand, aloft low-pressure systems are associated with rising air, weaker or 

no temperature inversion, strong vertical mixing, and possibly clouds or precipitation. Subtle 

variations in the location and strength of the high- and low-pressure systems have a strong 

influence on the local stability of the atmosphere; local instability can have a dramatic influence 

on cloud cover and the vertical dispersion of pollution and, thus, a dramatic influence on ozone 

concentrations. 
Evaluation of Exceptional Events Appendix Y

Y-5

Figure 1. Plot of 500-mb heights for 4:00 a.m. PST on August 15, 2008, indicating a 

ridge of high pressure over California, a favorable pattern for high ozone levels in the 

Sacramento region. Source: National Weather Service.

Besides influencing tropospheric stability and cloud cover, aloft pressure systems also 

influence the strength and location of surface high- and low-pressure systems. In turn, the 

large-scale pressure patterns interact with local mechanisms to produce the observed local 

flows in the Sacramento area. The local mechanisms are strongly driven by the diurnally 

changing temperature contrast between the land surface of the Sacramento Valley and the 

water surface of the Pacific Ocean. In the Sacramento area, the large-scale summertime 

boundary layer flow is typically a sea breeze from the southwest, commonly referred to as the 

Delta breeze. On clear summer days, the Central Valley typically warms much faster than 

areas nearer the coast; the end result of this differential heating is lower surface pressure inland 

(thermal trough) with higher surface pressure nearer the Pacific Ocean and, subsequently, a 

flow of maritime air through the Sacramento River Delta. High ozone concentrations normally 

occur in the Sacramento region when the thermal trough is over or just west of Sacramento (see 

Figure 2). 
Evaluation of Exceptional Events Appendix Y

Y-6

Figure 2. Surface weather map for 4:00 a.m. PST on August 15, 2008, indicating a 

thermal trough of low pressure extending from the Sacramento region northward into 

western Oregon (brown dashed line). Source: National Weather Service.

The timing and strength of the Delta breeze can strongly influence ozone concentrations 

in the Sacramento region. A strong Delta breeze will effectively disperse pollutants in the 

Sacramento area, resulting in low ozone concentrations. A weak Delta breeze that does not 

occur until late in the day, however, can limit pollutant dispersion, leading to high ozone 

concentrations (see Figure 3). Owing to its position near the Sacramento River Delta, onshore 

breezes are enhanced through Fairfield as maritime air surges inland through the Sacramento 

Valley; thus, Fairfield morning wind speed is a good indicator of Delta breeze strength. At night,

in the foothill region just east of Sacramento, winds can reverse direction and come from the 

east, fostering stagnation, reduced transport distances, and, under some circumstances, 

recirculation—all of which lead to pollutant buildup. 
Evaluation of Exceptional Events Appendix Y

Y-7

Figure 3. Map of the surface wind barbs for 21Z on August 15, 2008, indicating weak 

southwest onshore flow in the Delta and Sacramento area. 

Source: http://mesowest.utah.edu

The specific rules of thumb for the meteorological parameters associated with 8-hr 

ozone concentrations in excess of 0.095 ppm in Sacramento County are shown in Table 1. In 

addition to these criteria, aloft ridges of high pressure over California and thermal surface lows 

over or just west of Sacramento are necessary for high ozone levels. In general, all or nearly all 

criteria must be met for high ozone concentrations to occur; however, experienced 

meteorologists use the rules of thumb subjectively because there are not a specific number of 

rules that absolutely must be met for high ozone levels to occur in the Sacramento region.

Note: KOAK=Oakland International Airport, KSAC = Sacramento International Airport, 

KSUU = Fairfield, KSFO= San Francisco International Airport, KRENO= Reno International 

Airport, and 12Z = 4:00 a.m. PST.
Evaluation of Exceptional Events Appendix Y

Y-8

Table 1. Rules of thumb for the daily maximum 8-hr average ozone concentrations 

above 0.095 ppm in the Sacramento region.

Meteorological Parameter Rule of Thumb

12Z KOAK 925-mb Temperature ≥25°C

12Z KSAC 500-mb Height ≥5850 m

KSAC High Temperature ≥93°F

9 to 11 a.m. PDT KSAC Wind Speed <4 knots

9 to 11 a.m. PDT KSAC Wind Dir. >150° and <270°

9 to 11 a.m. PDT KSUU Wind Speed <15 knots

9 to 11 a.m. PDT KSFO Wind Speed <10 knots

12 to 6 p.m. PDT KSAC Wind Speed <6 knots

12Z KSFO – KSAC Pressure Gradient <3.0 mb

00Z KSFO – KSAC Pressure Gradient <4.0 mb

12Z KSAC – KRNO Pressure Gradient <-2.0 mb

00Z KSAC – KRNO Pressure Gradient <0.0 mb

3. Meteorological Conditions on Smoke Days

This section describes the meteorological conditions observed on June 23, June 27, 

July 7, and July 10, 2008; provides an evaluation of whether the meteorological conditions were 

conducive to 8-hr ozone concentrations above 0.095 ppm (historically, 8-hr concentrations 

above 0.095 ppm have yielded 1-hr ozone concentrations ranging from 0.107 to 0.137 ppm, 

with an average of 0.118 ppm); and presents the meteorological details and ozone 

concentrations on the meteorologically similar days, or surrogate matching days, without smoke 

from wildfires. The ozone concentrations discussed and shown in tables below were observed 

at the Folsom-Natoma monitor.

Smoke Day: June 23, 2008

Summary: The maximum 1-hr ozone concentration was 0.161 ppm at Folsom-Natoma. 

However, as shown in Table 2, the meteorological conditions did not support these high ozone 

concentrations. In addition, the large-scale weather pattern was not conducive to high ozone

concentrations. Finally, a peak 1-hr ozone concentration of only 0.079 ppm occurred on the 

matching non-smoke impact day (June 16, 2008).
Evaluation of Exceptional Events Appendix Y

Y-9

Table 2. Meteorological conditions for the smoke impact day, June 23, 2008, and the 

matching day, June 16, 2008. Green indicates that a criterion for high ozone was not 

met. Note that, in general, all or nearly all criteria must be met for high ozone 

concentrations to occur.

Meteorological Parameter Criteria for 8-hour Ozone 

Above 0.095 ppm

Event Date

(June 23, 2008)

Matching 

Surrogate Date 

(June 16, 2008)

12Z KOAK 925 mb Temp ≥25°C 21°C 19°C

12Z KSAC 500-mb Height ≥5850 m 5840 m 5830 m

KSAC High Temperature ≥93°F 86°F 84°F

9 to 11 a.m. PDT KSAC Wind Speed <4 knots 2 knots 6 knots

9 to 11 a.m. PDT KSAC Wind Dir. >150° and <270° 190° 211°

9 to 11 a.m. PDT KSUU Wind Speed <15 knots 17 knots 19 knots

9 to 11 a.m. PDT KSFO Wind Speed <10 knots 12 knots 10 knots

12 to 6 p.m. PDT KSAC Wind Speed <6 knots 7 knots 7 knots

12Z KSFO – KSAC Gradient <3.0 mb 3.3 mb 3.2 mb

00Z KSFO – KSAC Gradient <4.0 mb 4.1 mb 4.0 mb

12Z KSAC – KRNO Gradient <-2.0 mb -2.3 mb -1.9 mb

00Z KSAC – KRNO Gradient <0.0 mb 0.5 mb 0.9 mb

Max. 8-hr Avg. Ozone Conc. 0.123 ppm 0.068 ppm

Max. 1-hr Ozone Conc. 0.161 ppm 0.079 ppm

Meteorology Details

Large-Scale Pattern: An upper-level high-pressure system was centered offshore of Baja 

California and extended northeastward into the central United States (see Figure 4). An 

upper-level trough of low pressure was located off the Pacific Northwest coast. These two 

features were positioned on either side of Sacramento. At the surface, a thermal trough was 

positioned over Sacramento (see Figure 5). The upper-level and surface pressure patterns for 

the matching day of June 16, 2008, are shown in Figures 6 and 7, respectively. The

combination of these patterns is not conducive for high ozone concentrations.

Temperatures: Aloft and surface temperatures were mild. Warm temperatures are required for 

high ozone concentrations.

Local Winds: Winds were calm or light southwesterly at Sacramento during the morning hours 

and increased to over 7 knots from the southwest during the afternoon hours. The morning 

winds were conducive to high ozone concentrations, while the afternoon winds were not.
Evaluation of Exceptional Events Appendix Y

Y-10

Figure 4. Map of 500-mb heights on a smoke day at 4:00 a.m. PST on June 23, 2008. 

Source: National Weather Service.

Figure 5. Surface weather map for 4:00 a.m. PST on June 23, 2008, a smoke day. 

Source: National Weather Service.
Evaluation of Exceptional Events Appendix Y

Y-11

Figure 6. Map of 500-mb heights at 4:00 a.m. PST on June 16, 2008, a surrogate 

matching day to the smoke day of June 23, 2008. Source: National Weather Service.

Figure 7. Surface weather map for 4:00 a.m. PST on June 16, 2008, a surrogate 

matching day to the smoke day of June 23, 2008. Source: National Weather Service.
Evaluation of Exceptional Events Appendix Y

Y-12

Smoke Day: June 27, 2008

Summary: The maximum 1-hr average ozone concentration was 0.129 ppm. While some 

meteorological conditions satisfied the rules of thumb for high ozone on this day, other rules of 

thumb were not (See Table 3). In addition, peak 1-hr ozone concentrations of only 0.084 ppm, 

well below the 1-hr standard, occurred on the matching non-smoke impact day. This suggests 

that ozone concentrations in excess of the 1-hr standard would not have been expected on this 

day in the absence of smoke. 

Table 3. Meteorological conditions for the smoke impact day, June 27, 2008, and the 

matching day, July 9, 2007. Green indicates that a criterion for high ozone was not met. 

Note that, in general, all or nearly all criteria must be met for high ozone concentrations to 

occur.

Meteorological Parameter Criteria for 8-hour Ozone 

Above 0.095 ppm

Event Date

(June 27, 2008)

Matching 

Surrogate Date

(July 9, 2007)

12Z KOAK 925 mb-Temp ≥25°C 24°C 22°C

12Z KSAC 500-mb Height ≥5850 m 5850 m 5870 m

KSAC High Temperature ≥93°F 90°F 90°F

9 to 11 a.m. PDT KSAC Wind Speed <4 knots 0 knots 8 knots

9 to 11 a.m. PDT KSAC Wind Dir. >150° and <270° -- 203°

9 to 11 a.m. PDT KSUU Wind Speed <15 knots 12 knots 17 knots

9 to 11 a.m. PDT KSFO Wind Speed <10 knots 4 knots 2 knots

12 to 6 p.m. PDT KSAC Wind Speed <6 knots 6 knots 10 knots

12Z KSFO – KSAC Gradient <3.0 mb 1.6 mb 2.4 mb

00Z KSFO – KSAC Gradient <4.0 mb 3.0 mb 2.7 mb

12Z KSAC – KRNO Gradient <-2.0 mb -4.9 mb -0.4 mb

00Z KSAC – KRNO Gradient <0.0 mb -3.0 mb 1.7 mb

Max. 8-hr Avg. Ozone Conc. 0.112 ppm 0.069 ppm

Max. 1-hr Ozone Conc. 0.129 ppm 0.084 ppm

Meteorology Details

Large-Scale Pattern: A broad upper-level ridge of high pressure was located along the west 

coast of the United States, with a cutoff upper-level low-pressure system located southwest of 

California over the Pacific (see Figure 8). These two systems were located on either side of 

Sacramento. A thermal trough at the surface extended from far southeastern California 

northwestward to San Francisco and continued north to the Oregon border (see Figure 9). 

These patterns are somewhat conducive to high ozone levels; however, the high-pressure ridge 

was only moderate in strength, as indicated by the modest height of the 500-mb pressure level 
Evaluation of Exceptional Events Appendix Y

Y-13

of 5,820 m above msl. The upper-level and surface pressure patterns for the matching day of 

July 9, 2008, are shown in Figures 10 and 11, respectively. 

Temperatures: 925 mb and surface temperatures were both below the thresholds for high 

ozone.

Local Winds: Winds were calm at KSAC and weakly onshore at KSUU and KSFO during the 

morning hours. Winds increased from the southwest during the afternoon hours at KSAC, 

indicating a developing Delta breeze. The morning winds were conducive to high ozone

concentrations, but the afternoon winds were not.

Figure 8. Map of 500-mb heights on a smoke day at 4:00 a.m. PST on June 27, 2008. 

Source: National Weather Service.
Evaluation of Exceptional Events Appendix Y

Y-14

Figure 9. Surface weather map for 12Z on June 27, 2008, a smoke day. Source: 

National Weather Service.

Figure 10. Map of 500-mb heights 4:00 a.m. PST on July 9, 2007, a surrogate matching 

day to the smoke day of June 27, 2008. Source: National Weather Service.
Evaluation of Exceptional Events Appendix Y

Y-15

Figure 11. Surface weather map for 4:00 a.m. PST on July 9, 2007, a surrogate 

matching day to the smoke day of June 27, 2008. Source: National Weather Service.

Smoke Day: July 7, 2008

Summary: The observed daily maximum 1-hr average ozone concentration was 0.166 ppm. 

As shown in Table 4, all of the meteorological conditions satisfied the rules of thumb for high 

ozone, meaning that, even without smoke, conditions were favorable for 8-hr ozone 

concentrations above 0.095 ppm; historically, an average maximum 1-hr concentration of 

0.118 ppm occurred on days with 8-hr concentrations above 0.095 ppm. A peak 1-hr ozone 

concentration of 0.111 ppm, which is below the national standard, occurred on the matching day 

(June 22, 2006). However, it should be noted that concentrations below the 1-hr standard on 

the matching day were likely due to moderate northerly winds in the morning; northerly winds 

did not occur on July 7. This suggests that 1-hr ozone concentrations above the standard were 

likely to occur on July 7, even in the absence of smoke.
Evaluation of Exceptional Events Appendix Y

Y-16

Table 4. Meteorological conditions for the smoke impact day, July 7, 2008, and the 

matching day, June 22, 2006. Green indicates that a criterion for high ozone was not 

met. Note that, in general, all or nearly all criteria must be met for high ozone 

concentrations to occur.

Meteorological Parameter

Criteria for 8-hour 

Ozone Above 0.095 

ppm

Event Date

(July 7, 2008)

Surrogate Date 

(June 22, 2006)

12Z KOAK 925 mb Temp ≥25°C 29°C 28°C

12Z KSAC 500-mb Height ≥5850 m 5880 m 5920 m

KSAC High Temperature ≥93°F 102°F 102°F

9 to 11 a.m. PDT KSAC Wind Speed <4 knots 1 knot 7 knots

9 to 11 a.m. PDT KSAC Wind Dir. >150° and <270° 190° 352°

9 to 11 a.m. PDT KSUU Wind Speed <15 knots 5 knots 6 knots

9 to 11 a.m. PDT KSFO Wind Speed <10 knots 4 knots 4 knots

12 to 6 p.m. PDT KSAC Wind Speed <6 knots 2 knots 4 knots

12Z KSFO – KSAC Gradient <3.0 mb 1.9 mb 1.5 mb

00Z KSFO – KSAC Gradient <4.0 mb 1.8 mb 1.2 mb

12Z KSAC – KRNO Gradient <-2.0 mb -6.2 mb -7.1 mb

00Z KSAC – KRNO Gradient <0.0 mb -4.9 mb -4.0 mb

Max. 8-hr Avg. Ozone Conc. 0.114 ppm 0.091 ppm

Max. 1-hr Ozone Conc. 0.166 ppm 0.111 ppm

Meteorology Details

Large-Scale Pattern: An upper-level high-pressure system was centered off the shore of 

California and extended eastward over the Sacramento area (see Figure 12). At the surface, a 

thermal trough extended from the northern Gulf of Mexico to a position west of Sacramento near 

Fairfield (see Figure 13). These conditions are favorable for high ozone levels. The 

upper-level and surface pressure patterns for the matching day of June 22, 2006, are shown in 

Figures 14 and 15, respectively.

Temperatures: With the upper-level high-pressure system over Sacramento, aloft and surface 

temperatures were well above the thresholds for high ozone levels.

Local Winds: Morning winds were light at KSAC, KSUU, and KSFO. Afternoon winds at 

KSAC remained light and generally variable before becoming southwesterly during the evening, 

indicating a late, weak Delta breeze. The light winds were conducive to high ozone levels.
Evaluation of Exceptional Events Appendix Y

Y-17

Figure 12. Map of 500-mb heights on a smoke day at 4:00 a.m. PST on July 7, 2008. 

Source: National Weather Service.

Figure 13. Surface weather map for 4:00 a.m. PST on July 7, 2008, a smoke day. 

Source: National Weather Service.
Evaluation of Exceptional Events Appendix Y

Y-18

Figure 14. Map of 500-mb heights at 4:00 a.m. PST on June 22, 2006, a surrogate 

matching day to the smoke day of July 7, 2008. Source: National Weather Service.

Figure 15. Surface weather map for 4:00 a.m. PST on June 22, 2006, a surrogate 

matching day to the smoke day on July 7, 2008. Source: National Weather Service.
Evaluation of Exceptional Events Appendix Y

Y-19

Smoke Day: July 10, 2008

Summary: Highly complex meteorological and air quality conditions present on July 10, 2008,

introduced considerable uncertainty regarding the impact of smoke on ozone levels in 

Sacramento. The observed daily maximum 1-hr average ozone concentration was 0.150 ppm. 

As shown in Table 5, most of the meteorological conditions satisfied the rules of thumb for high 

ozone levels. However, the synoptic pattern was not ideal for high ozone concentrations 

because there was a short-wave trough of low pressure passing through northern California 

(see Figure 16). In addition, only a peak 1-hr ozone concentration of 0.091 ppm, which is 

below the 1-hr standard, occurred on the matching day (June 28, 2009). We suspect that the 

relatively modest ozone concentrations on the matching day were due to strong vertical mixing 

generated by hot surface temperatures. Hot surface temperatures were not observed on July 

10, 2008, probably because of smoke blocking the sun (see ―Temperatures‖ below). This

suggests that, in the absence of smoke, ozone levels likely would not have exceeded the 1-hr 

standard on July 10, 2008.

Table 5. Meteorological conditions for the smoke impact day, July 10, 2008, and the 

matching day, June 28, 2009. Green indicates that a criterion for high ozone was not 

met. Note that, in general, all or nearly all criteria must be met for high ozone 

concentrations to occur.

Meteorological Parameter Criteria for 8-hour 

Ozone Above 0.95 ppm

Event Date

(July 10, 2008)

Matching 

Surrogate Date 

(June 28, 2009)

12Z KOAK 925 mb-Temp ≥25°C 34°C 30°C

12Z KSAC 500-mb Height ≥5850 m 5890 m 5900 m

KSAC High Temperature ≥93°F 93°Fa108°F

9 to 11 a.m. PDT KSAC Wind Speed <4 knots 0 knots 0 knots

9 to 11 a.m. PDT KSAC Wind Dir. >150° and <270° -- --

9 to 11 a.m. PDT KSUU Wind Speed <15 knots 11 knots 2 knots

9 to 11 a.m. PDT KSFO Wind Speed <10 knots 4 knots 4 knots

12 to 6 p.m. PDT KSAC Wind Speed <6 knots 6 knots 4 knots

12Z KSFO – KSAC Gradient <3.0 mb 2.5 mb 1.4 mb

00Z KSFO – KSAC Gradient <4.0 mb 3.3 mb 2.1 mb

12Z KSAC – KRNO Gradient <-2.0 mb -6.2 mb -6.0 mb

00Z KSAC – KRNO Gradient <0.0 mb -3 mb -2.9 mb

Max. 8-hr Avg. Ozone Conc. 0.115 ppm 0.078 ppm

Max. 1-hr Ozone Conc. 0.150 ppm 0.091 ppm

a Reduced visibility due to reported smoke and haze may have contributed to the relatively cool high 

temperature.
Evaluation of Exceptional Events Appendix Y

Y-20

Figure 16. Map of 500-mb heights on a smoke day at 4:00 a.m. PST on July 10, 2008. 

Source: National Weather Service.

Meteorology Details

Large-Scale Pattern: An upper-level ridge of high pressure was located west of California, with 

a weak upper-level trough of low pressure passing through Northern California. At the surface, 

a thermal trough extended from the northern Gulf of California northward through the 

Sacramento region and into northern California (see Figure 17). The upper-level and surface 

pressure patterns for the matching day of June 28, 2009, are shown in Figures 18 and 19, 

respectively. 

Temperatures: The 925-mb temperature was well above the threshold for high ozone levels in 

Sacramento. The combination of warm aloft temperatures with relatively mild surface 

temperatures acts to reduce vertical mixing. Reduced mixing is conducive to high ozone 

concentrations. The relatively mild surface temperatures were likely due to smoke blocking the 

sunlight, as indicated by the smoke-induced haze and reduction in visibility that were reported 

for most of the day in Sacramento. It is not clear how the reduced visibility impacted the 

photochemical reactions that form ozone. 

Local Winds: During the morning hours, calm winds were reported at KSAC, with onshore 

winds reported at KSUU and light winds at KSFO. Afternoon winds gradually increased from 

the southwest at KSAC, indicating a light afternoon Delta breeze. These conditions are 

conducive for high ozone levels.
Evaluation of Exceptional Events Appendix Y

Y-21

Figure 17. Surface weather map for 4:00 a.m. PST on July 10, 2008, a smoke day. 

Source: National Weather Service.

Figure 18. Map of 500-mb heights at 4:00 a.m. PST on June 28, 2009, a surrogate 

matching day to the smoke day of July 10, 2008. Source: National Weather Service.
Evaluation of Exceptional Events Appendix Y

Y-22

Figure 19. Surface weather map for at 4:00 a.m. PST on June 28, 2009, a surrogate 

matching day to the smoke day of July 10, 2008. Source: National Weather Service.