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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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