At 30 degrees north or south of the equator, air descending to Earth's surface creates steady high-pressure zones.

Here's why:

* Hadley Cells: The Earth's atmosphere is divided into large circulation cells called Hadley cells. Air rises at the equator (low pressure) and travels towards the poles. As the air moves, it cools and sinks at around 30 degrees north and south. This sinking air creates high pressure.

* Dry Conditions: As the air descends, it compresses and warms. This warmer, drier air inhibits cloud formation, leading to generally dry conditions in these regions.

* Subtropical Highs: These high-pressure zones are known as subtropical highs. They contribute to the formation of major deserts like the Sahara Desert in Africa and the Atacama Desert in South America.

In summary, the descending air at 30 degrees north and south creates steady high-pressure zones, leading to dry conditions and the formation of subtropical highs.