Key Points

Atmospheric pressure is the weight of a column of air over a unit area, measured in millibars (mb). The average pressure at sea level is 1,013.2 mb, and it decreases with increasing altitude.

The difference in atmospheric pressure creates a pressure gradient force, which causes air to move from high-pressure to low-pressure areas. This movement of air is known as wind.

Caused by the Earth's rotation, the Coriolis force deflects wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The force is strongest at the poles and absent at the equator.

Near the surface, horizontal winds are controlled by the combined effect of three forces: the pressure gradient force, the frictional force, and the Coriolis force.

In the upper atmosphere where friction is negligible, the pressure gradient force is balanced by the Coriolis force. This results in wind blowing parallel to the isobars, which is known as geostrophic wind.

The world has four major pressure belts: the Equatorial Low, the Subtropical Highs (around 30 degrees N/S), the Subpolar Lows (around 60 degrees N/S), and the Polar Highs.

Global atmospheric circulation is driven by three cells in each hemisphere: the Hadley Cell in the tropics, the Ferrel Cell in the mid-latitudes, and the Polar Cell near the poles.

The ITCZ is a low-pressure zone near the equator where the trade winds from the Northern and Southern Hemispheres converge, causing air to rise and form clouds.

An air mass is a large body of air with uniform temperature and humidity, formed over a homogenous source region like a vast ocean or plain. They are classified as continental (c), maritime (m), tropical (T), or polar (P).

A front is the boundary zone that forms when two different air masses meet. The process of front formation is called frontogenesis, and it leads to abrupt weather changes.

There are four types of fronts: Cold, Warm, Stationary, and Occluded. Each type brings distinct changes in temperature, pressure, and precipitation.

These weather systems develop in the mid and high latitudes along the polar front and have a distinct frontal system. They cover a large area and generally move from west to east.

These are violent storms that originate over warm tropical oceans. They are known as Hurricanes in the Atlantic, Typhoons in the Western Pacific, and Cyclones in the Indian Ocean.

Favorable conditions include a large sea surface with a temperature higher than 27 degrees Celsius, the presence of the Coriolis force, and a pre-existing weak low-pressure area.

A mature tropical cyclone has a calm center called the eye, surrounded by an eye wall where winds are strongest and rainfall is heaviest. The storm dissipates upon reaching land due to the cutoff of its moisture supply.

These are local winds caused by the differential heating of land and sea. During the day, a cool sea breeze blows towards the land, and at night, a cool land breeze blows towards the sea.

Thunderstorms are local storms caused by intense convection, producing thunder, lightning, and sometimes hail. A tornado is a violently rotating column of air descending from a thunderstorm to the ground.