REVIEW QUESTIONS. 99 Continual summer is found in the tropics; summer and winter of nearly equal length in the temperate zones; short, hot summers, followed by intensely cold winters, in the polar zones. The atmosphere is heated—1. Either by direct absorp- tion of the rays while passing through it; or 2. By con- tact with, or by radiation and reflection from, the heated earth. -Isothermal lines connect places whose mean temperature is the same. The mathematical zones are bounded by the parallels of latitude; the physical zones, by the isotherms. The mathematical and physical zones do not coincide— 1. Because of the unequal distribution of the land and water areas. 2. The irregularities in the surface of the land. 3. The distribution of the winds and moisture. 4. The ocean currents. 5. The difference in the rainfall. The temperature of the air decreases with the altitude ~ —1, Because the air receives most of its heat from the earth’s surface, so that it must grow continually colder the farther we go above the surface. 2, The decreased den- sity and humidity of the air prevent it from absorbing either the direct rays of the sun or those reflected or ra- diated from the earth. ‘ Places situated near the sea have a more equable, uni- form climate than those in the same latitude in the inte- rior of the continent. Whenever any part of the earth’s surface is heated more than the neighboring parts, ascending currents occur over the heated area, lateral surface currents blow in toward the heated area, and upper currents blow from the heated area. The general system of the atmospheric circulation con- sists mainly of the following currents: 1. The polar cur- rents, blowing from the poles toward the equator. 2. The equatorial currents, blowing from the equator toward the poles. The direction of these currents is modified by the rota- tion of the earth. Thus modified, the equatorial currents are south-west in the northern hemisphere, and north- west in the southern. The polar currents are north-east in the northern hemisphere, and south-east in the south- ern. When a wind at the surface blows in any direction, there is generally an upper current blowing in the opposite di- rection, The equatorial currents do not continue as upper cur- rents to the poles—1. Because they become cooled and fall. 2. From the contracted space of the higher latitudes when compared with that of the equator. We distinguish the following wind zones: the zone of calms; the zones of the trades; the zones of the calms of Cancer and Capricorn; the zones of the variables; and the zones of the polar winds. Land and sea breezes are caused by the unequal heating of the land and water during day and night; monsoons, by their unequal heating during summer and winter. Monsoons occur on the coasts of tropical countries within the limits of the trade zones. They are most frequent in the Indian Ocean, in the Gulf of Guinea, and in the Mex- ican Gulf and neighborhood. The Etesian Winds blow over the Mediterranean toward the Desert of Sahara. The Hot Winds caused by the deserts of Sahara and Arabia are the Harmattan, over Guinea; the Solano, over Spain; the Sirocco, over Italy; the Simoom, over Arabia, Nubia, and Persia; and the Khamsin, over Egypt. In most mountainous regions winds blow up the valleys toward the mountains during the day, and down the val- leys from the mountains during the night. Cyclones are caused by the wind blowing in from all sides toward an area of low barometer caused by the overheating of the area. The centrifugal force thus gen- erated increases both the size of the area and the differ- ence of pressure as compared with regions surrounding it. The fury of the storm is increased by the heat energy liberated by the condensation of the vapor in the uprush- ing air. Storms occur whenever the ordinary equilibrium of the atmosphere is violently disturbed by wind, rain, snow, hail, or thunder and lightning. Nearly all powerful storms are attended with a rotation of the wind. Such storms are known under the general names of Cyclones, Hurricanes, Typhoons, and Tornadoes. The north-easters and other great storms of the United States are species of cyclones. REVIEW QUESTIONS. ——-059500-——— Of what use is the atmosphere in the economy of the earth? Define meteorology. Describe the construction of a barometer. What proof have we that the greater part of the atmo- sphere, by weight, lies within a few miles of the earth’s surface ? Define hypsometry. Describe the construction of a thermometer. Why are the vertical rays of the sun warmer than the oblique rays? What is the characteristic climate of the tropics? Of the temperate regions? Of the polar-regions? In what different ways does the atmosphere receive its heat from the sun? State the boundaries of the mathematical torrid zone. Of the physical torrid zone. Of the mathematical and physical temperate zones. Of the mathematical and phys- ieal frigid zones. 12 In what parts of the eastern hemisphere is the greatest mean annual temperature found? In what parts of the western hemisphere? What influence is produced on the climate of high lati- tudes by a preponderance of moderately elevated land masses? On the climate of the tropics? Why should the temperature of the atmosphere decrease with the altitude? Name all the causes which prevent the mathematical climatic zones from coinciding with the physical climatic zones. ~ What is the origin of winds? Name the currents of which the atmospheric circulation principally consists. Explain the action of the rotation of the earth on the direction of the equatorial and polar currents. Name the causes which produce the shifting of the equa- torial and polar currents.