Climatic dry line
As a dry line in is geography a climatic boundary designated areas with precipitation surpluses from such separated by precipitation deficits. At the dry line is the annual average of the deposit - equal to the amount of evaporation .
The term often describes the boundary between semi-humid (semi-humid) tropics and semi-arid (semi-arid) tropics. There are six humid (wet) and six arid (dry) months; the water balance is balanced. That is, the precipitation is equal to the evaporation.
The dry line is particularly pronounced in North America at about 100 ° west longitude (e.g. Denver , 105 ° w. L.). At the dry line, the annual precipitation limit is around 500 mm. On the eastern side of the dry line you can find high, even precipitation of over 500 mm / year. This is due to the influence of the Gulf of Mexico as well as the Atlantic. Humidity increases from the Great Plains inland to the east coast. The parts of the country located to the west of the so-called dry line have low, punctual precipitation of less than 500 mm / year.
This is based on the damming effect of the coastal cordillera on the west coast, which lies in the windward direction of the Rocky Mountains (Los Angeles, San Francisco). The damming effect means that the coastal region receives abundant precipitation, but the intramontaneous basins in the lee of the Pacific mountain system have extremely little precipitation and therefore have the character of deserts or semi-deserts. This limit is of particular importance for US agriculture because it also represents the limit of so-called rain - fed agriculture .
The dry line on the southwest coast of South America , where the world's driest desert - the Atacama - has emerged directly behind the coastal cordillera, is locally even more pronounced .