Hadley cells are large-scale atmospheric circulation patterns that occur in tropical regions. They form due to the intense heating of the Earth's surface by the sun, causing warm air to rise near the equator. As this air rises, it cools and eventually descends around 30 degrees latitude, creating areas of high pressure. These cells play a crucial role in global weather patterns and influence trade winds. The descending air in Hadley cells leads to dry conditions, contributing to the formation of deserts in regions like the Sahara Desert and Australian Outback.