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Chapter 5 GENERAL CIRCULATION Local Fire-Weather Elements-Wind

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Chapter 5 GENERAL CIRCULATION Local Fire-Weather Elements-Wind Chapter 5 GENERAL CIRCULATION Local fire-weather elements-wind, temperature, moisture, and stability-respond continually to the varying patterns of pressure systems and to the changing properties of huge masses of air moving in generally predictable circulations over the earth's surface. These broadscale circulations determine the regional patterns of rapidly changing fire weather-long term trends resulting in periods of wetness or drought and above or below-normal temperatures, and in seasonal changes in fire weather. If we are to become acquainted with these variations in fire weather, we must understand how they are brought about, and the settings in which they take place. The response to overall airflow applies also to local fuel conditions, so an understanding of general air circulation within the troposphere is essential to a usable knowledge of wildland fire behavior. GENERAL CIRCULATION So far we have been concerned principally with heated uniformly, and the resultant unequal heating the static properties of the atmosphere-its of the atmosphere causes compensating air temperature, moisture, and pressure. In this motions, which tend to reduce the horizontal chapter we will begin a more detailed consideration temperature differences. of the dynamics of the atmosphere-its motion-which was introduced in chapter 1. The actual motions that are developed within the atmosphere are extremely complex and are not yet We learned in chapter 1 that the atmosphere is fully understood. Theories and models, which have a gaseous mantle encasing the earth held there by been derived, are not wholly accepted because gravity-and rotating with the earth. Within this huge they do not completely account for all of the envelope of air there are motions of a variable observed atmospheric motions. Most of the major nature. If forces were not present to act on the features of the global circulations are rather well atmosphere and upset its equilibrium, there would understood. Therefore, future modifications of be no atmospheric motion-no circulation. The present-day theories resulting from further research pressure exerted by the weight of the atmosphere will not seriously affect out understanding of the would be the same everywhere at a given level. But general circulation as it relates to fire weather. disturbing forces are present. The earth is not PRIMARY CIRCULATION In equatorial regions the earth’s surface Certainly this is a very hypothetical situation, but let receives more solar energy from the sun than it us accept it for the sake of development of our radiates back to space, and therefore acts as a discussion. We know that regions of warm heat source for the air in these regions. In polar surfaces, which warm the air overlying them, are regions the earth’s surface radiates more energy characterized by rising air. The warming air into space than it receives from the sun. Since expands and is forced aloft by the cooler, denser equatorial regions do not get hotter and hotter, and air flowing in from adjacent areas. since polar regions do not become progressively colder, there must be some net transport of heat In equatorial regions the warm air would rise to energy from equatorial to polar regions. Just near the tropopause, reach a level of the same air how this is accomplished is one of the major density, then spread out and flow both north and features of the general circulation that is not south. As it moved toward the poles, it would cool completely understood. This transport could be by radiation and sink as its density increased. In accomplished by closed horizontal “cells” with the polar regions it would descend and begin to north-south flow, by large-scale eddies, or, move toward the Equator. perhaps, by both methods. We will consider both methods. In this hypothetical case the transport of heat could take place by simple convective circulation. Convective Circulation At the earth’s surface there would be a permanent low-pressure belt around the earth at the Equator Let us suppose that the earth's surface was and a high-pressure area at each pole. uniform, that the earth did not rotate, and that it was uniformly heated around the entire Equator. heating mentioned above combined with the effect of the earth's rotation and the unequal partitioning of heat due to the uneven distribution of land and sea areas. Before we discuss the circulation on a rotating earth with a uniform surface, we will need to consider why and how the earth's rotation affects airflow. How the Earth's Rotation Affects Airflow: Coriolis Force If a maps of air, or any other body, moves in a straight line as viewed from a position in space, its path as viewed from a position on the earth is curved. The curvature indicates a deflection to the right in the Northern Hemisphere and a deflection to the left in the Southern Hemisphere. The reason for the deflection is that the earth, rotating toward the east on its axis, turns underneath the moving air or body. This deflective force is called the In a simple convective circulation, worm air expands and is Coriolis force. It is an apparent rather than a real forced aloft. As it cools, it descends and returns to the heat source. force, but since we are stationed on earth and view motions from the earth, the deflection is real from Since the earth does rotate, and since the sun all earthbound positions. is its single source of energy, this simple convective pattern cannot exist. The real circulation patterns To visualize the Coriolis force, let us consider a are the result of the unequal large disk or merry-go-round, rotating in a counterclockwise direction, as representing the Northern Hemisphere. A boy tossing a ball from the center outward would find that the ball made a straight path in space, but traced a curved path on the disk below showing a deflection toward the right. Although more difficult to visualize, it is a fact that if the boy were stationed at any place on the rotating disk and tossed the ball in any horizontal direction, the ball would trace a curved path on the disk with a deflection to the right. On the rotating earth, an air current in the Northern Hemisphere starting as a southerly wind, that is, moving toward the north, would be deflected to the right and become a southwest or west wind. Likewise, a north wind deflected to the right becomes a northeast or east wind. Since the northward airflow aloft just north of the equatorial region becomes nearly a true westerly flow, the northward movement is slowed and the air "piles If the general circulation could be represented by a simple up" at about latitude 30°N. The air also loses convective circulation, heated air at the Equator would rise to considerable heat by radiation. near the tropopause, then spread out both north and south. Toward the poles, it would cool, and finally descend and move back to the Equator. of the earth's rotation comes into play. The northward-flowing current is turned to the right and becomes the prevailing westerlies of middle latitudes. The southward-flowing current, also deflected to the right, becomes the northeast trades of the low latitudes. The air aloft that gradually moves northward continue to lose heat. In the polar regions it descends, gives up additional heat to the surface, and flows southward. This current is also turned to the right by the Coriolis force and becomes the polar easterlies of high latitudes. The cold air gradually pushes southward and finally meets the A ball tossed horizontally from the center (or, in fact any location) northward-flowing tropical air in what is referred to on a large, counterclockwise rotating disk will take a straight path as the polar front zone. The polar and tropical air in space, but, because of the Coriolis force, the path traced on the masses, which have different densities, tend to disk will show a deflection to the right. resist mixing. Instead, the lighter tropical air flows Because of the piling up and the heat loss, up and over the forward edge of the denser polar some of the air descends, producing a surface air. high-pressure belt, while the rest continues in the westerly current aloft. Air that has descended flows This type of cellular circulation causes air to both northward toward the pole and southward accumulate in the polar region. When the toward the Equator at the surface. Again, the effect On a rotating earth with a uniform surface, the general circulation of the Northern Hemisphere would be composed of the trade winds, prevailing westerlies, and polar easterlies. mass becomes great enough, the polar front zone is pushed southward, and the cold polar air penetrates to fairly low latitudes in a "polar outbreak". In this simplified circulation system, heat energy is carried northward by the airflow aloft, and cold air moves southward in cold outbreaks to maintain a balance of energy between equatorial and polar regions. This primary circulation system results in the formation of several well-defined major regional circulation patterns or wind belts, some of which we have already mentioned. These are known as: Doldrums, trade winds, horse latitudes, prevailing westerlies, polar front zone, and polar easterlies. The equatorial region of warm and moist rising air currents is referred to as the doldrums. It is a region of light surface winds, considerable cloudiness, and widespread shower activity. When The polar front zone is an area of storminess, cloudiness, and the doldrum belt moves north from the Equator, as precipitation, and its position is extremely variable.
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