Diurnal Mountain Winds

Diurnal mountain winds develop over complex topography of all scales, from small hills to large mountain massifs and are characterized by a reversal of wind direction twice per day. As a rule, winds flow upslope, up-valley, and from the plain to the mountain massif during daytime. During nighttime, they flow downslope, down-valley, and from the mountain massif to the plain. Diurnal mountain winds are strongest when skies are clear and winds aloft are weak. Diurnal mountain winds are produced by horizontal temperature differences that develop daily in complex terrain. The resulting horizontal pressure differences cause winds near the surface of the earth to blow from areas with lower temperatures and higher pressures toward areas with higher temperatures and lower pressures. The circulations are closed by return, or compensatory, flows higher in the atmosphere. Four wind systems comprise the mountain wind system, which carries air into a mountain massif at low levels during daytime and out of a mountain massif during nighttime. The slope wind system (upslope winds and downslope winds) is driven by horizontal temperature contrasts between the air over the valley sidewalls and the air over the center of the valley. The along-valley wind system (up-valley winds and down-valley winds) is driven by horizontal temperature contrasts along a valley’s axis or between the air in a valley and the air over the adjacent plain. The cross-valley wind system results from horizontal temperature differences between the air over one valley sidewall and the air over the opposing sidewall, producing winds that blow perpendicular to the valley axis and toward the more strongly heated sidewall. The mountain-plain wind system results from horizontal temperature differences between the air over a mountain massif and the air over the surrounding plains, producing large-scale winds that blow up or down the outer slopes of a mountain massif. The mountain-plain circulation and its upper level return flow are not confined by the topography but are carried over deep layers of the atmosphere above the mountain slopes. Because diurnal mountain winds are driven by horizontal temperature differences, the regular evolution of the winds in a given valley is closely tied to the thermal structure of the atmospheric boundary layer within the valley, which is characterized by a diurnal cycle of buildup and breakdown of a temperature inversion.

Weather Maps, Forecasts, and Data

Weather maps prepared by the National Weather Service summarize and synthesize weather data to provide a comprehensive picture of weather conditions at a given time. They are the basis of weather maps used on television to show precipitation, high and low pressure centers, and fronts. Weather maps are produced using both surface data and data from specified pressure levels. Data are plotted and contoured by computer, and analysts use satellite photos, satellite video loops, weather forecast models, and extrapolations from previous frontal and pressure system analyses to locate fronts and pressure centers. An example of a surface weather chart is presented in figure 9.1. A 500-mb chart for the same date and time was presented in figure 5.1. Symbols are used on weather maps to indicate synoptic-scale features. High and low pressure weather systems (highs and lows) are indicated by the letters H and L, with isobars labeled in millibars. Lines indicating frontal positions (section 6.2) represent the position on the ground of boundaries between air masses. Additional meteorological variables, such as temperature, are often analyzed on the same map using dashed or colored lines. Pressure, temperature, and other data from the reporting stations are plotted in coded form at the station locations. A station model specifies the positions in which different types of data are plotted relative to the station location. Figure 9.1 used an abbreviated station model. A complete station model is shown in figure 9.2. Figures 9.3 — 9.7 show additional symbols used in station models to indicate total sky cover, winds, pressure tendency, cloud types, and present weather types, respectively. A surface weather chart, with the symbols indicating fronts and high and low pressure centers and the information included in the station model, provides a snapshot of synoptic-scale conditions at ground level. By overlaying charts for several pressure levels (section 5.1.3), changes with altitude can be identified and the three-dimensional structure of the atmosphere at a given point in time can be visualized. By comparing consecutive charts, the rate of movement of fronts and the rates of development of high and low pressure centers can be determined.

Pressure and Winds

Atmospheric pressure at a given point in the atmosphere is the weight of a vertical column of air above that level. Differences in pressure from one location to another cause both horizontal motions (winds) and vertical motions (convection and subsidence) in the atmosphere. Vertical motions, whether associated with high and low pressure centers or with other meteorological processes, are the most important motions for producing weather because they determine whether clouds and precipitation form or dissipate. The location of high and low pressure centers is a key feature on weather maps, providing information about wind direction, wind speed, cloud cover, and precipitation. Pressure-driven winds carry air from areas where pressure is high to areas where pressure is low. However, the winds do not blow directly from a high pressure center to a low pressure center. Because of the effects of the rotation of the earth and friction, winds blow clockwise out of a high pressure center and counterclockwise into a low pressure center in the Northern Hemisphere. These wind directions are reversed in the Southern Hemisphere. The strength of the wind is proportional to the pressure difference between the two regions. When the pressure difference or pressure gradient is strong, wind speeds are high; when the pressure gradient is weak, wind speeds are low. As air flows out of a high pressure center, air from higher in the atmosphere sinks to replace it. This subsidence produces warming and the dissipation of clouds and precipitation. As air converges in a low pressure center, it rises and cools. If the air is sufficiently moist, cooling can cause the moisture to condense and form clouds. Further lifting of the air can produce precipitation. Thus, rising pressure readings at a given location indicate the approach of a high pressure center and fair weather, whereas falling pressure readings indicate the approach of a low pressure center and stormy weather. The vertical motions caused by the divergence of air out of a high pressure center or the convergence of air into a low pressure center are generally weak, with air rising or sinking at a rate of several cm per second, and they cannot be measured by routine weather observations.

Terrain-Forced Flows

Winds associated with mountainous terrain are generally of two types. Terrain-forced flows are produced when large-scale winds are modified or channeled by the underlying complex terrain. Diurnal mountain winds are produced by temperature contrasts that form within the mountains or between the mountains and the surrounding plains and are therefore also called thermally driven circulations. Terrain-forced flows and diurnal mountain winds are nearly always combined to some extent. Both can occur in conjunction with small-scale winds, such as thunderstorm inflows and outflows, or with large-scale winds that are not influenced by the underlying mountainous terrain. Terrain forcing can cause an air flow approaching a mountain barrier to be carried over or around the barrier, to be forced through gaps in the barrier, or to be blocked by the barrier. Three factors determine the behavior of an approaching flow in response to a mountain barrier: •the stability of the air approaching the mountains, •the speed of the air flow approaching the mountains, and •the topographic characteristics of the underlying terrain. Unstable or neutrally stable air (section 4.3) is easily carried over a mountain barrier. The behavior of stable air approaching a mountain barrier depends on the degree of stability, the speed of the approaching flow, and the terrain characteristics. The more stable the air, the more resistant it is to lifting and the greater the likelihood that it will flow around, be forced through gaps in the barrier, or be blocked by the barrier. A layer of stable air can split, with air above the dividing streamline height flowing over the mountain barrier and air below the dividing streamline height splitting upwind of the mountains, flowing around the barrier (figure 10.1), and reconverging on the leeward side (section 10.3.2). A very stable approaching flow may be blocked on the windward side of the barrier (section 10.5.1). Moderate to strong cross-barrier winds are necessary to produce terrain-forced flows, which therefore occur most frequently in areas of cyclogenesis (section 5.1) or where low pressure systems (figure 1.3) or jet streams (section 5.2.1.3) are commonly found. Whereas unstable and neutral flows are easily lifted over a mountain barrier, even by moderate winds, strong cross-barrier winds are needed to carry stable air over a mountain barrier.

Air Masses and Fronts

An air mass is a regional-scale volume of air with horizontal layers of uniform temperature and humidity. Air masses form during episodes of high pressure when weak winds allow air to remain for several days over a flat area with uniform surface characteristics. The characteristics of the underlying surface determine the characteristics of the air mass, which is given a two-letter identifier. Air masses are identified by their locations of origin (maritime “m” or continental “c”) and by their characteristics (tropical “T” or polar “P”). Tropical air masses form in high pressure areas in warm, tropical regions. When a tropical air mass is formed over oceans (mT), it is warm, moist, and usually unstable. When formed over land (cT), it is hot and dry, with unstable air near the surface and stable air aloft. Polar air masses form in high pressure areas in the polar and subpolar regions. A polar air mass that forms over water (mP) is cool, moist, and unstable. A polar air mass that forms over land (cP) is cold, dry, and stable. An extremely cold polar air mass that forms in winter over arctic ice and snow surfaces is called an arctic air mass (cA). The distinction between arctic and polar air masses is not always clear because an arctic air mass that travels over a warm surface may be warmer near the surface than a polar air mass, although it is still colder aloft. Source regions for air masses and typical trajectories affecting North America are shown in figure 6.1. Polar air masses that originate over the flat, ice- and snow-covered regions east of the Rocky Mountains in northern and central Canada and Alaska, and arctic air masses that originate over the ice-covered Arctic Ocean influence winter weather. The midlatitudes are not a good air mass source region. The exposure to traveling weather systems is too great, the range of temperature and humidity too wide, and, in the United States, the topography is too varied. Instead, the midlatitudes are a region where clashing air masses meet. Cold air masses are usually driven southward from the subpolar regions, whereas warm air is forced northward from tropical regions.

Mountain Climates of North America

The basic climatic characteristics of the major mountain ranges in the United States—the Appalachians, the Coast Range, the Alaska Range, the Cascade Range, the Sierra Nevada, and the Rocky Mountains—can be described in terms of the four factors discussed in chapter 1. The mountains of North America extend latitudinally all the way from the Arctic Circle (66.5°N) to the tropic of Cancer (23.5°N) (figure 2.1). There are significant differences in day length and angle of solar radiation over this latitude belt that result in large seasonal and diurnal differences in the weather from north to south. Elevations in the contiguous United States extend from below sea level at Death Valley to over 14,000 ft (4270 m) in the Cascade Range, the Sierra Nevada, and the Rocky Mountains. Several prominent peaks along the Coast Range in Alaska and Canada (e.g., Mount St. Elias and Mount Logan) reach elevations above 18,000 ft (5486 m). Denali (20,320 ft or 6194 m) in the Alaska Range is the highest peak in North America. The highest peak in the Canadian Rockies is Mt. Robson, with an elevation of 12,972 ft (3954 m). The climates of the Coast Range, the Cascade Range, and the Sierra Nevada, all near the Pacific Ocean, are primarily maritime. The Appalachian Mountains of the eastern United States are subject to a maritime influence from the Atlantic Ocean and the Gulf of Mexico, but they are also affected by the prevailing westerly winds that bring continental climatic conditions. Only the climate of the Rocky Mountains, far from both the Pacific and Atlantic Oceans, is primarily continental. Each of the mountain ranges is influenced by regional circulations. For example, the Appalachians are exposed to the warm, moist winds brought northward by the Bermuda-Azores High and to the influence of the Gulf Stream. Similarly, the Coast Range feels the impact of the Pacific High, the Aleutian low, and the Japanese Current. A mountain range, depending on its size, shape, orientation, and location relative to air mass source regions, can itself affect the regional climate by acting as a barrier to regional flows.

Aerial Spraying

Aircraft are used in a number of resource management operations, including fire suppression, seeding and fertilizing operations, and the application of pesticides to agricultural, forest, and rangelands. The objectives of any aerial application are to apply the material, either liquid or solid, to the target area safely, efficaciously, and economically and to avoid drift, that is, off-target displacement of the agents. Barry (1993) is a general reference for aerial spraying of forests. Picot and Kristmanson (1997) provide an overview of all aspects of this topic. Bache and Johnstone (1993) give a detailed description of spray meteorology. The emphasis in this chapter is on the role of meteorology in the aerial application of liquid pest control agents to manage plant, fungal, and animal pests in mountainous forested areas. The effectiveness of a spray operation depends on the timing of the operation relative to phenological conditions, the characteristics of the forest canopy or rangeland being targeted, the spray formulation, pilot skills and attitude, the aircraft type and spray equipment used, and weather conditions. Pest control agents are regulated by federal, state, and local agencies. Restrictions on the use of agents are specified on the product label and may include weather conditions. Drift reduces the efficacy of a spray operation and can have unintended and undesirable impacts on nontarget species, residences, and public areas near the target area. Although there is a driftable component in every spray operation, the drift potential is generally greater for liquids than for solids because the size of liquid droplets becomes smaller after release into the atmosphere, depending on the volatility of the substance itself, the aircraft and spray equipment used, and the meteorological conditions at the time of spraying. The smaller the droplets, the greater the potential for drift. Weather conditions have a significant impact on drift because wind speed and direction, temperature, humidity, and atmospheric stability affect the transport, diffusion, evaporation, settling, and deposition of both solid particles and liquid droplets. The collection of meteorological data and the use of professional weather forecasts are thus an integral part of a spraying operation.

Fire Weather and Smoke Management

Wildland fires consume large areas of forest and grasslands every year. Fires are described in terms of fire behavior, which includes rate of spread and fire intensity. A fire that spreads rapidly burns less of the available fuel per square unit of area than a fire that moves slowly and allows the flaming front a longer residence time. A fire with flames that reach only two feet above the ground produces less heat and is less destructive than an intense fire that crowns, that is, has long flames and burns at the top (i.e., crown) of the forest canopy (figure 13.1). Fire suppression activities are initiated when a wildfire threatens people, property, or natural areas that need protection. These activities include dropping water or chemicals on a fire and establishing a fire line around the fire. A fire line is a zone along a fire’s edge where there is little or no fuel available to the fire. Roads, cliffs, rivers, and lakes can be part of a fire line, or land can be cleared by firefighters. Backfires may be set within the fire line to burn toward the fire, widening the fire line and reducing the likelihood of the fire spreading beyond it (figures 13.2 and 13.3). Fires can cross a fire line if the intensity is high or if spotting occurs, that is, if the wind carries burning material (firebrands) beyond the fire and across the fire line (figure 13.4). A wildland fire can be very destructive, but it can also be beneficial and may be used by land resource managers to accomplish specific ecological objectives. For example, smaller fires can reduce the danger of a large catastrophic fire by burning off underbrush. Fire can also be used to prepare land for planting, to control the spread of disease or insect infestations, to benefit plant species that are dependent on fire, to influence plant succession, or to alter the nutrients in the soil. When a fire is used to manage land resources, it is called a prescribed fire.

Precipitation

Precipitation is often the primary weather factor affecting outdoor activities. Precipitation that is of an unexpected type or intensity or that comes at an unexpected time or recurs more frequently than expected can disrupt both recreational and natural resource management plans. Heavy rain or snowfall can interfere with travel and threaten safety. Precipitation is water, whether in liquid or solid form, that falls from the atmosphere and reaches the ground. Table 8.1, adapted from Federal Meteorological Handbook No. 1 (National Weather Service, 1995), describes the different types of precipitation particles, collectively called hydrometeors. International guidelines for the reporting of precipitation do not include a category for sleet. Meteorologists in the United States use the term to describe tiny ice pellets that form when rain or partially melted snowflakes refreeze before reaching the ground. These particles bounce when they strike the ground and produce tapping sounds when they hit windows. Colloquial usage of the term, often used by the news media, coincides with British usage, which defines sleet as a mixture of rain and snow. Snow pellets, or graupel, are common in high mountain areas in summer. Graupel are low density particles (i.e., not solid ice) formed when a small ice particle (an ice crystal, snowflake, ice pellet, or small hailstone) falls through a cloud of supercooled (section 8.4) water droplets. The tiny droplets freeze as they impact the larger ice particle, building it into a rounded mass containing air inclusions (figure 8.1). This coating of granular ice particles is called rime, and the particle is said to be rimed. Graupel is usually produced in deep convective clouds that extend above the freezing level. Whereas graupel reaches the ground at high elevations, it usually melts to form rain before reaching the ground at lower elevations. As falling snow accumulates, a snowpack develops that can be described in terms of water content and density. The water content of snow is usually expressed as specific gravity, a number obtained in this application by dividing the water-depth equivalent of snow by the actual snow depth.

Atmospheric Scales of Motion and Atmospheric Composition

Weather phenomena occur over a very broad range of scales of space and time, from the global circulation systems that extend around the earth’s circumference to the small eddies that cause cigarette smoke to swirl and mix with clear air. Each circulation can be described in terms of its approximate horizontal diameter and lifetime. Large-scale weather systems, such as hemispheric wave patterns called Rossby waves, monsoons, high and low pressure centers, and fronts, are called synopticscale weather systems. Temperature, humidity, pressure, and wind measurements collected simultaneously all over the world are used to analyze and forecast the evolution of these systems, which have diameters greater than 200 km (125 mi) and lifetimes of days to months. Mesoscale weather events include diurnal wind systems such as mountain wind systems, like breezes, sea breezes, thunderstorms, and other phenomena with horizontal scales that range from 2 to 200 km (1 to 125 mi) and lifetimes that range from hours to days. Mesoscale meteorologists use networks of surface- based instruments, balloon-borne sounding systems, remote sensing systems (e.g., radar, lidar, and sodar), and aircraft to make observations on these scales. Microscale meteorology focuses on local or small-scale atmospheric phenomena with diameters below 2 km (1 mi) and lifetimes from seconds to hours, including gusts and turbulence, dust devils, thermals, and certain cloud types. Microscale studies are usually confined to the layer of air from the earth’s surface to an altitude where surface effects become negligible (approximately 1000 feet or 300 m at night and 5000 feet or 1500 m during the day). A fourth and less rigorously defined term, the regional scale, denotes circulations and weather events occurring on horizontal scales from 500 to 5000 km (310 to 3100 mi). The regional scale is thus smaller than synoptic scale, but larger than mesoscale. The term is often used to describe events that occur within more or less homogeneous physiographic provinces (e.g., the Pacific Northwest region). Major mountain ranges impact the weather on the synoptic scale. They anchor large-scale pressure systems in the Northern Hemisphere, cause low and high pressure weather systems to form, and produce large-scale seasonal wind systems in Asia and North America.