The importance of the weather factor is usually considered by historians when recognizing causes and results of historical developments. They acknowledge how weather has played a part in determining routes of early explorations, discoveries, settlement locations, migrations of people, past trade relations, and specialized economic patterns worldwide. Weather has had a unique and significant role in shaping the outcomes of battles and military campaigns and in forging national destinies. The weather rationale is also used to explain the basis of how many cultures and institutions, like slavery, started in the United States.
American Indians are basically Mongoloid, though considerable variation is found. It is generally agreed that they migrated into northern America from Asia during prehistoric times of glacial advances when there was a land and ice bridge connecting the two continents at the present site of the Bering Strait, a mere distance of forty-five miles.
Some historians have theorized that the birth and growth of early civilization may have begun along the Nile River oases. It was there that the hot desert climate provided a year-round growing season augmented by the deposit of fresh soils brought to the region from elsewhere by the annual floods. This, in turn, gave rise to early irrigation that bolstered farm yields.
Similar geographic features contributed to an emerging civilization that sprung up around 3500 B.C. on the flood plains of the lower Tigres and Euphrates Rivers in Mesopotamia.
It is theorized that the prevailing winds and currents of the Atlantic Ocean were key factors in determining land claims based on early explorations and discoveries. The trade winds drove sailing vessels westward from the northeast to the West Indies, and they returned to Europe via the steady blowing westerlies. The Norse sailors were delivered to America by the polar gales and northeasterly winds, thus explaining their probable presence in the higher latitudes of North America prior to 1492.