Climate and Weather
The term climate is used to express the combination of temperature and moisture which prevails at any particular place, or, in more familiar terms, the prevailing weather.
The most prominent causes of diversity of climate are the heat of the sun, the respective position of land and water, and the elevation of land above the level of the sea. To these may be added, as producing considerable though less marked effects, the nature of the soil, the prevailing winds, the position of mountain ranges, and the currents of the ocean.
The Supreme Influence of the Sun
The sun is the grand agent in diffusing heat over the earth’s surface. While the sun is above the horizon of any place, that place is receiving heat; and when the sun is below the horizon, it is parting with it by the process called “radiation.” Whenever therefore the sun remains more than twelve hours out of the twenty-four above the horizon of any place, and consequently less than twelve hours below, the general temperature of that place will be above average; and when the reverse occurs, it will be below average. If the temperature depended solely on the heat of the sun, then indeed a tolerably accurate view of the respective climates of the zones of the globe might easily be assumed; but it is so greatly modified by other circumstances, that considerable differences prevail in countries situated in the same parallels of latitude.
How Affected by Position of Land and Water
The relative position of the land and water is an essential cause of this diversity. The waters of the ocean are of very equal temperature, and have a tendency to moderate both heat and cold, wherever their influence extends. Thus when a cold wind passes over the sea, it becomes warmed, while a hot wind becomes cooled; and thus islands generally experience milder winters and more temperate summers than continents. Such countries are said to possess an insular climate. But when any region experiences great severity of cold in winter and a high degree of heat in summer, it is said to possess an extreme or excessive climate. The most striking instances of an extreme climate are drawn from places like Yakutsk, situated in the depths of Siberia, where the difference between the average temperature of winter and summer amounts to the astonishing sum of 101 degrees Fahrenheit.
THE LIFE-GIVING SUN SENDING HEAT AND LIGHT
The sun is the great life-giver of our earth. Its waves of light and heat and electricity come to the earth through a measureless ocean of ether and make it a living rather than a dead world. The above illustration shows how these waves are constantly bombarding the earth, and not only giving it life but contributing to it the glory of the seasons, the wonders of color, and the brilliant effects of light which we see in the skies and call Auroras, or Northern and Southern Lights.
Large illustration (347 kB)
Influence of Elevations
A gradual decrease in temperature takes place in the ascent from the sea to the line of perpetual snow. This line, which is called the snow-line, varies in different latitudes, and sometimes, owing to local causes, differs on the same latitude; as a general rule, however, a gradual decrease in elevation of the snow-line takes place as we recede from the equator north and south. The height of this line within the tropics varies from 16,000 to 17,000 feet above the level of the sea, and in the northern hemisphere meets the level at about the eightieth parallel.
Modifications by Prevailing Winds, Mountains and Ocean Currents
Countries where the prevailing winds sweep across a wide expanse of ocean are not subject to extremes of heat and cold. Thus the climate of oceanic islands is always moderate, and the climates of all coasts are more equable than in the interior of continents.
Climate is also modified greatly by the position of mountain ranges, especially when ridges extend east and west, screening it from the north or leaving it exposed unsheltered in that direction.
Thus the Carpathians screen Hungary from the cold blasts of the north; while Poland, to the north of that range, and therefore unprotected from those piercing winds, suffers from a very cold and humid atmosphere.
The currents of the ocean are likewise potent agents in the formation of climates, and render places which would otherwise be uninhabitable, fit for man’s habitation. Thus the Polar currents coming to the equatorial regions cool, and the Gulf Stream making its way to Polar regions warms, otherwise extreme temperatures.
Rainless and Rainy Regions of the Earth
In some parts of the Earth extensive tracts exist where rain is never known to fall, and if at all only at intervals, and then in small quantities. The rainless districts of the New World include the flat territories of northern Chili and Peru, some parts of Mexico, and some parts of California. In the Old World an extensive rainless band extends from the western shores of Africa to the central regions of Asia, including the Great Sahara Desert, Egypt, part of Arabia, and the Desert of Gobi. Countries so circumstanced, unless like Egypt rendered fertile by the irrigation of a great river, constitute the most arid and desolate regions of the earth.
The quantity of rain which falls in any region depends greatly on local causes, such as the variations of the surface, the prevailing winds or the proximity of the ocean. Rain is usually more copiously deposited in mountains and well-wooded islands than in any other description of surface.
In tropical regions the rains follow the sun, i. e., when the sun is north of the equator, the rains prevail in the northern tropic, and when south of that line in the southern tropic. This forms the rainy and dry seasons to which countries so situated are subject. This does not, however, apply to the whole intertropical regions, for in a zone extending from the fifth to the tenth parallels on each side of the equator there are two rainy and two dry seasons.
In the narrow belt called the variables, between the regions of the north and south trade-winds, rain is almost incessant, accompanied by thunder and lightning. In many parts of the intertropical regions during the rainy season the rain pours down in such torrents that a larger quantity falls in a few hours than in a whole month in temperate North America.
TRAVELERS GROUPED ON THE SANDS OF THE SAHARA, TERRORIZED BY AN APPROACHING SIMOON
The dreaded Simoon of the desert is a whirlwind of terrific force that raises great gyrating clouds of sand, and sweeps forward with suffocating effect upon both man and beasts. It frequently darkens the sky at midday, and sometimes lightning accompanies it caused by the friction of the sand and air, though no rain falls. The Simoon seldom lasts more than twenty minutes.