How does global atmospheric circulation determine the location of arid and high-rainfall areas?

Introduction

Global atmospheric circulation is a key factor in determining the climate of different regions worldwide. By understanding how this system works, we can explain why certain areas experience arid conditions with high pressure while others have high rainfall with low pressure. This guide will explore the mechanisms of global atmospheric circulation and its impact on climate patterns.

What is Global Atmospheric Circulation?

Global atmospheric circulation refers to the large-scale movement of air that redistributes heat and moisture around the Earth. It is driven by the sun’s uneven heating of the Earth’s surface, which creates pressure differences that cause air to move.

The Three-Cell Model

The three-cell model of atmospheric circulation helps to explain the movement of air in different parts of the world. It consists of three primary circulation cells in each hemisphere: the Hadley Cell, the Ferrel Cell, and the Polar Cell.

1. Hadley Cell: Located between the equator and 30° latitude, this cell is driven by the intense solar heating at the equator. Warm air rises at the equator, creating a low-pressure zone and moving towards the poles. Around 30° latitude, the air cools and sinks, forming a high-pressure zone, which leads to arid conditions.
2. Ferrel Cell: Situated between 30° and 60° latitude, this cell operates opposite to the Hadley and Polar cells. Air in the Ferrel Cell is driven by the movements of the neighbouring cells, creating a zone of westerly winds.
3. Polar Cell: Found between 60° latitude and the poles, this cell is characterized by cold, dense air that sinks at the poles, creating high-pressure zones. This air then moves towards the equator, where it meets the Ferrel Cell, rises, and creates a low-pressure zone.

What is Atmospheric Pressure?

Atmospheric pressure is the force exerted by the weight of the air in the Earth’s atmosphere on a given surface area. It is measured in units of millibars (mb) or inches of mercury (inHg). Atmospheric pressure varies across different regions of the Earth and is influenced by factors such as altitude, temperature, and the presence of weather systems.

Atmospheric Pressure

High Pressure and Arid Regions

High atmospheric pressure, often called a “high-pressure system,” occurs when the atmospheric pressure at a location is higher than its surrounding areas. High-pressure zones are associated with descending air that inhibits cloud formation and precipitation. These areas are typically found at around 30° latitude north and south of the equator.

1. Subtropical Highs: The sinking air in the Hadley Cell creates subtropical high-pressure zones. These regions, including the Sahara Desert, the Arabian Desert, and the Australian Outback, experience low rainfall and arid conditions.
2. Polar Highs: Cold, sinking air forms polar high-pressure zones at the poles. These areas are cold deserts that receive little precipitation, contributing to their arid climate.

Low Pressure and High Rainfall Areas

Low atmospheric pressure, or a “low-pressure system,” occurs when the atmospheric pressure at a location is lower than its surrounding areas. Low-pressure zones are associated with rising air that cools and condenses to form clouds and precipitation. These areas are typically found at the equator and around 60° latitude north and south.

1. Equatorial Low (Intertropical Convergence Zone – ITCZ): At the equator, intense solar heating causes air to rise, creating a low-pressure zone known as the ITCZ. High rainfall and tropical rainforests like the Amazon and the Congo Basin characterise this zone.
2. Subpolar Lows: Around 60° latitude, the meeting of the warm air from the Ferrel Cell and the cold air from the Polar Cell creates low-pressure zones. These regions, such as the North Atlantic and the Southern Ocean, experience frequent storms and high rainfall.

The Role of the Coriolis Effect

The Coriolis effect, caused by the Earth’s rotation, influences the direction of wind flow within the atmospheric circulation cells. It causes moving air to be deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, contributing to the trade winds, westerlies, and polar easterlies.

1. Trade Winds: These easterly winds are found in the tropics, blowing from the subtropical highs towards the equatorial low. The Coriolis effect deflects them westward.
2. Westerlies: Found in the mid-latitudes, these winds blow from the subtropical highs towards the subpolar lows and are deflected eastward.
3. Polar Easterlies: These winds blow from the polar highs towards the subpolar lows and are deflected westward.