Among the secondary causes affecting climate, probably none is of greater importance than the direction of prevailing winds. The currents of air are warm or cold, wet or dry, according as they have had their origin in warm or cold latitudes, and have traversed inland tracts, or the expanse of ocean, in their advancing course. With us, and in the northern half of the globe in general, north and east winds are cold and dry, while south and west winds are warm, and often accompanied by moisture. Within the Southern Hemisphere these conditions are reversed, and the hottest currents of air come from a northwardly direction. The prevailing winds of western Europe are from the west and southwest; and it is to this fact that we must mainly ascribe the high winter temperature, as well as the comparative freedom from extremes of heat and cold which distinguishes the countries of western Europe. The same cause explains the abundant moisture which belongs to those regions in general, and which distinguishes the western shores of our own islands in a remarkable degree. Such winds have traversed the immense expanse of the Atlantic, and come to the western seaboard of Europe laden with the moist vapors gathered on their course. These vapors, condensed upon the high grounds which line the western side of the British Islands, or, further to the northward, upon the long chain of the Scandinavian Mountains, fall to the earth in copious torrents of rain. In the process of condensation, a vast quantity of latent heat is disengaged, and the temperature is correspondingly raised. Warmth and moisture are, indeed, speaking generally, concomitant conditions of European climate, and are especially so in the case of western Europe. Even in the case of lands which nearly approach the tropic, the influence of prevailing winds in raising or lowering the temperature is strikingly seen. At New Orleans, bordering on the Mexican Gulf, and throughout the adjacent portions of the United States, the winters are often of excessive severity. Cold winds, generated in the higher latitudes of the New World, and blowing for weeks in succession from the northern quarter of the sky, are the cause of this. The generally level interior of the North American Continent--a vast lowland plain, bounded only to the east and west by the Alleghanies and the Rocky Mountains--presents no obstacle to the advance of these cold northerly blasts. The middle and eastwardly parts of North America are subject to like influences, in this regard, to the plains of eastern Europe. To the westward of the Rocky Mountains, on the other hand, the conditions affecting climate present greater analogy to those that belong to western Europe. In the case of many countries, some local wind, of occasional prevalence, forms a marked characteristic of climate. The most remarkable of these local winds are the simoon, the sirocco, the föhn, the harmattan, and the mistral.