Hadley Cell
Overview
The Hadley Cell is a global scale tropical atmospheric circulation that features air rising near the Equator, flowing poleward at a height of 10 to 15 kilometers above the Earth's surface, descending in the subtropics, and then returning equatorward near the surface. This circulation creates the trade winds, tropical rain-belts and hurricanes, subtropical deserts and the jet streams.
Discovery and Naming
The Hadley Cell was first postulated by George Hadley in 1735, as a response to the problem of understanding why the trade winds blow consistently from the east in the tropics. Hadley's theory, while not entirely correct, was a significant contribution to the field of meteorology.
Structure and Dynamics
The structure of the Hadley Cell is characterized by the movement of air. At the Equator, solar heating causes air to rise through convection, leading to low pressure at the surface. This rising air cools and loses its moisture, leading to high altitude clouds and heavy rainfall. As the air moves towards the poles, it cools and descends, creating a region of high pressure. The descending air warms and absorbs moisture, leading to arid conditions in the subtropics.
The dynamics of the Hadley Cell are driven by thermal energy. The sun heats the Earth's surface, causing air to rise at the Equator. This air then cools and descends at around 30 degrees latitude in both hemispheres. This cycle of rising and falling air creates a consistent pattern of atmospheric circulation.
Role in Climate
The Hadley Cell plays a crucial role in the Earth's climate. It helps to distribute heat from the Equator towards the poles, moderating temperature extremes. The cell's dynamics are responsible for the creation of the world's deserts, located in the subtropical high pressure zones, and the wet and dry seasons in the tropics.
Variability and Change
The strength and position of the Hadley Cell can vary on a range of timescales, from seasonal to geological, due to changes in solar heating and the distribution of land and sea. In recent decades, there is evidence that the Hadley Cell has been expanding poleward, which could have significant implications for subtropical climate and ecosystems.