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<br />OD1764
<br />
<br />CHAPTER 2
<br />
<br />Temperature (degrees Fahrenheit) varies mostly with
<br />ejevallon, and to a lesser extent, local microclimate. Summer
<br />temperatures usually range from lows in the 40s to highs
<br />lD the 70s (mountains) and 80s (valleys). In winter, cold
<br />air often sinks down the mountains, filling the San Luis
<br />Valley and making it as cold as, or colder than, the mountains.
<br />Winter temperatures typically range between zero degrees
<br />and the mid-30s. Extieme temperatures have btxn as iow
<br />as -42 degrees aod as high as 97 degrees, At higher elevations,
<br />freezlOg temperatures and snowfall are possible year around,
<br />WIth snow accumulation likely from September to May.
<br />At lower elevations, freezing temperatures are likely from
<br />October to May with snow accumulation from October
<br />to April.
<br />
<br />Annual precipitation (Map 2-1) is highly variable, primarily
<br />because of the orographic (mountain-related) effect of the
<br />San Juan Mountains. Within tbe resource area, annual
<br />precipitation is among the highest and lowest in Colorado;
<br />Wolf Creek Pass ( 40.8 inches) and Alamosa (7.1 inches),
<br />Except for areas with extreme snowpack, most precipitation
<br />comes from summer thunderstorms. Snowfall varies from
<br />around 20 inches in the lower elevations to over 360 inches
<br />on Wolf Creek Pass; mouotainaus accumulation may vary
<br />from 60 to 80 inches.
<br />
<br />Upper-level winds prevail from the southwest, and are not
<br />normally modified as they blow across the San Luis Valley.
<br />However, the diverse and rugged terrain of the surrounding
<br />mountains results in complex wind flows and surface winds.
<br />Pressure gradient winds may be channeled or forced around
<br />hills; however, without strong gradient flows, daily upslope!
<br />downslope winds are predominant. Upslope winds usually
<br />occur on sunny mornings when the air at higher elevations
<br />heats rapidly and rises. Downslope winds occur when the
<br />air oear the ground becomes cool and dense, sinking
<br />downward along drainages, Similar light daily winds occur
<br />ajong the Rio Grande drainages,
<br />
<br />The extent that vertical and horizontal mixing takes place
<br />is related to the atmospheric stability and mixing depth.
<br />Unstable conditions normally result from strong surface
<br />heating, typical of summer afternoons, and produce vertical
<br />winds. Neutral conditions reflect a breezy, well-mixed
<br />atmosphere. Stable conditions are enhanced by rapid
<br />radiative cooling and downslope drainage, producing the
<br />least amouot of dispersion,
<br />
<br />Because of the relatively level terrain throughout the San
<br />Luis Valley, dispersion is normally good in spring and
<br />summer, but is jimited in the winter, Inversions, which trap
<br />pollutants within a layer of air, are formed under stable
<br />conditions. Moderate summer inversions are typical during
<br />the evening and dissipate at dawn; however, winter ioversions
<br />are stronger and last longer. Inversions are enhanced by
<br />weak pressure gradients, cold clear oights, soowcover, and
<br />
<br />lower elevations. Seasonal stability data are presented in
<br />Table 2-2.
<br />
<br />Table 2-2
<br />SELECTED ATMOSPHERIC
<br />DISPERSION DATA,
<br />ALAMOSA, COLORADO
<br />
<br />Season
<br />
<br />Stability Frequency
<br />(percent)
<br />
<br />Unstable Neutral Stable
<br />
<br />Approximate
<br />Mixing Depth (10)
<br />
<br />Morning Afternoon
<br />
<br />Annual
<br />Winter
<br />Spring
<br />Summer
<br />Fall
<br />
<br />29
<br />21
<br />26
<br />39
<br />29
<br />
<br />34
<br />27
<br />47
<br />31
<br />33
<br />
<br />37
<br />52
<br />27
<br />30
<br />38
<br />
<br />350
<br />300
<br />450
<br />350
<br />250
<br />
<br />2,300
<br />1,300
<br />2,900
<br />3,200
<br />2,000
<br />
<br />Source: PEDCO Environmental, Ine, (1981).
<br />Note: Mixing depths are statewide averages.
<br />
<br />AIR QUALITY
<br />
<br />The existing air quality throughout the San Luis Resource
<br />Are:' can only be surmised, sioce no monitoring data are
<br />available for most pollutants. The air quality of the study
<br />ar~, h<;,wever, is believed to be typical of undeveloped
<br />regIOns 10 the western United States; ambient pollutant levels
<br />are usually near or below the measurable limits, Locations
<br />vulnerable to decreased air quality from extensive
<br />development include the immediate operation areas (milling
<br />operallons, power plants, elc,) and local population centers
<br />(farm tilling, residential woodsmoke, etc.).
<br />
<br />-,
<br />
<br />Air Quality Regulations
<br />
<br />National ambient air quality standards (Table 2-3) limit
<br />the total amounts of specific pollutants allowed in the
<br />atmosphere: carbon monoxide (CO), lead, nitrogen dioxide
<br />(NO,), owne, sulfur dioxide (SO,), particulate matter (total
<br />suspended particulates- TSP, and inhalable particulates-
<br />PM1Q). State standards include these parameters but may
<br />also be more stringeot (i.e" the 3-hour SO, stanw.'rd). These
<br />standards were established to protect public health (primary
<br />standards) and public welfare (secondary standards).
<br />
<br />2-2
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