This prairie, bigger than India, has become a desert.

This article comes from Weixin Official Accounts: Earth Knowledge Bureau (ID: diqiuzhishiju), authors: Xiaozhe, Kelxi, proofreading: Gu Hanying, editor: chestnut

Earth, what color is it? It was sea blue, dark green, and khaki.

A third of the earth's land mass is covered in a barren yellow, home to a billion people, and the lack of water constitutes the "dry part" of the earth, which is so large that it looks like a dune world embedded in the earth from space.

Find the largest arid area on the surface (at a glance) ▼

It seems as if a dune planet is embedded in the earth (horizontal screen view) ▼

Today, we will explore the causes of the largest arid areas on the earth's surface through the principles of meteorology, ocean currents and Earth's orbital parameters, and get a glimpse into the mysteries of global drought.

Sahara from Space (NASA)

What is an arid zone? The ratio of an area's "average precipitation" to its "average potential evapotranspiration," known as IA (the index of aridity), falls below 0.65 and crosses the threshold of "semi-humid and semi-arid areas," where precipitation is less than 65 percent of potential evapotranspiration.

Four grades can be distinguished: semi-humid and semi-arid areas (0.65-0.50 ), semi-arid areas (0.50-0.20), arid areas (0.20-0.05) and extremely arid areas (0.20-0.05).