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Lake Tahoe lies in a depression between the crests of
the Sierra Nevada and Carson mountain ranges. The bowl
shape of the Lake Tahoe Basin has significant implications
on air quality. There are two meteorological regimes that
affect air quality in the Basin. First, are thermal inversions
that occur when a warm layer of air traps a cold layer
of air at the surface of the land and lake. Locally-generated
air pollutants are often trapped in the “bowl”
by frequent inversions that limit the amount of air mixing,
which allows pollutants to accumulate. Inversions most
frequently occur during the winter in Tahoe but are common
throughout the year. Often, wintertime inversions result
in a layer of wood smoke, mostly from residential heating,
that can be seen over the Lake.
Thermal inversions develop under three conditions in
the Lake Tahoe Basin:
- A surface inversion can develop on a clear night,
when the earth's surface radiates heat rapidly.
Under these conditions, the air directly above the ground,
can be cooler than the air at higher altitudes.
- An advectional inversion, involves a horizontal inflow
of cold air from a source such as a coastal wind.
- Another type of surface inversion can develop at night
in valleys, when cold, dense air flows downslope, along
the mountains and rests at the bottom of the bowl.
The second most common meteorological event is atmospheric
transport of pollutants from the Sacramento Valley and
Bay Area. The location of Lake Tahoe, directly to the
east of the Sierra Nevada crest, allows the prevailing
westerly winds combined with local mountain upslope winds
to bring air from the populated regions, west of the Sierra,
into the Tahoe Basin. The strength of this pattern depends
on the amount of heat, usually strongest in summer, beginning
in April and ending in late October (Cahill et al., 1987;
Lake Tahoe Air Quality Research Scoping Document, TRPA,
2000).
Average
Meteorological Conditions for Lake Tahoe
| Month |
Avg.
High (*F) |
Avg.
Low (*F) |
Avg.
Precip. (in.) |
Avg.
Snowfall(in.) |
Avg.
Snow Depth (in) |
| Jan. |
38 |
19 |
6.2 |
44 |
21 |
| Feb. |
40 |
20 |
5.0 |
37 |
30 |
| Mar. |
44 |
23 |
4.2 |
36 |
28 |
| Apr. |
51 |
27 |
2.1 |
15 |
14 |
| May |
60 |
33 |
1.2 |
4 |
2 |
| Jun. |
69 |
39 |
0.7 |
Trace |
0 |
| Jul. |
78 |
44 |
0.3 |
0 |
0 |
| Aug. |
77 |
44 |
0.3 |
0 |
0 |
| Sept. |
70 |
39 |
0.6 |
Trace |
0 |
| Oct. |
59 |
32 |
1.9 |
3 |
0 |
| Nov. |
47 |
26 |
3.9 |
17 |
3 |
| Dec. |
40 |
21 |
5.7 |
34 |
11 |
Source:
Condo Vacation Concepts
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Meteorological ambient data such as temperature, wind
speed and direction, relative humidity, and precipitation
assist in measuring air quality indicators. Meteorological
conditions frequently dictate the amount of air pollution
blown into the Lake Tahoe Basin and how much of it is
deposited into the land and lake. It is important to measure
ambient temperature, wind speed and direction, relative
humidity, and precipitation to understand under what conditions
air quality samples are collected and to recognize how
each variable effects the behavior of individual pollutant
types such as, carbon monoxide, ozone, particulate matter,
and aerosols. For example, on a windy holiday weekend
in the summer, a spike in carbon monoxide can be expected,
rather than a surprise, due to the known meteorological
conditions and thousands of visitors driving to the Lake
and other destinations west of the Basin.
TRPA
Air Quality Indicators from Draft 2001 Threshold Evaluation
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Scientific research conducted by organizations, agencies,
and universities aims to detect, for example trends
in global warming, and discover ways to monitor and
ultimately eliminate environmental impacts. Ongoing
research efforts collecting baseline data will help
regulatory agencies establish regulations and develop
limits and indicators designed to improve environmental
health.
Meteorological conditions exert a strong influence
on the transport, dispersion and deposition of pollutants
in the Lake Tahoe Basin. In the past, limited meteorological
information has been available. Ambient information,
such as temperature, relative humidity, wind speed and
direction and precipitation has been collected for years
at various locations. However, additional information,
along with a larger network of monitoring sites, is
needed to refine estimates of pollutant source contribution,
transport pattern of both in- and out-of-Basin, and
deposition to the Lake. Research is currently underway
to gather a suite of data needed for the air quality
program, including enhanced meteorological data. The
data will aid in evaluating ambient pollutant concentrations
and will also provide input for a Tahoe specific air
quality model. This model will aid the Tahoe Regional
Planning Agency in planning for the future.
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