finents form warm areas of low pressure and inblowing winds, much Ymoisture is brought inland from the oceans. As the moisture-laden air approaches the center of low pressure it gradually rises and finally produces clouds and rain. Look at Figs. 78 and 79 and see how heavy the rainfall is during the summer when inblowing winds prevail.
Because continental interiors receive more rain in summer than at any other season, central Kansas, for example, gets 16 inches of rain in the six months from April to September and only 4 during the other six months. If the Kansas rainfall were evenly distributed throughout the year it would not be enough for agriculture. V Luckily the continental low pressure causes the winds to blow toward the interior more strongly in summer than at any other season, and hence as far west as central Kansas the crops are usually well watered. In winter, on the other hand, the low temperature, high pressure, and outblowing winds cause the interiors of the continents to be very dry. Thus though the Dakotas are colder than New York State in winter, they have far less snow. This is an advantage in some J ways, for it permits cattle to pasture all winter, while in the spring the ground is not covered with a layer of snow which would delay the melting of the frost and so prevent early plowing and planting.
How the Great Seasonal Contrasts of Asia Cause Monsoons.— The most extreme effect of a continent upon winds and rainfall is seen in Asia. Because of the great size of Asia the extreme difference of temperature_betweep summer and winter may amount to in Siberia north of the Arctic Circle, and to even in southern Persia and western India. The differences between the low pressure of summer and the high pressure of winter are correspondingly extreme. The variations of pressure naturally give rise to stronginblowing winds in summer and outblovving winds in Winter. These are best developeTin India and are called monsoons.1/In Figs. 73 and 74 notice that in January_the_winds in the southern half of Asia all blow more or less from the north. Since they comefrom the dry interior ' there is practically no winter rain in larga_parts of India and China.
In s_pring when Asia grows hot and the high pressure of the interior gives place to low, the outblowing windsweaken and are replaced by irregular winds and calms. Then the air begins to move in from v the ocean, and soon strong south or southwest winds are established. In the July map the arrows point in almost the opposite direction from those of January, and the winds of southern Asia all blow in ward.
In India the alternation between the northeast monsoon in winter and the southwest monsoon in summer is remarkably regular. The people count upon this regularity so much that formerly when crude sailing vessels were the chief carriers of commerce, native boats from India used to sail to Zanzibar and the African coast each winter with the wind at their backs. Then they spent several months in trading or in idleness waiting for the southwest monsoon to be well established, and finally with the wind blowing toward India they merrily came back with the wind as favorable as on the outward voyage. Even now the natives of the coral islands west of India depend on the monsoons to take them to the mainland and back again. Their sail ing boats are so primitive that they cannot make headway against a wind.
How the Southwest Monsoons Bring Prosperity and Health.— The most important effect of the monsoons is the rain.'in winter only the southern tip of India gets much rain, for that is the only part of the country where the northeast trades blow from the sea to the land. Elsewhere drought prevails month after month and the people long for the summer monsoon. Before it comes the air is burning hot, the ground is parched, and almost nothing will grow. Then the southwest wind begins to blow, the clouds gather, the sky is full of lightning, the thunder crashes, rain falls, the air is cooled, and except in the Indus desert the land is ready for the seed. The people welcome the rains with feasting and rejoicing, for their timely coming usually gives promise of good crops. Moreover, the mon soon rains restore the health of the sick, check epidemics, and bring the most healthful part of the year.