While hot air does rise due to its lower density compared to colder air, the Earth's atmosphere is not infinitely expanding because it is held in place by gravity. Gravity acts as a force that pulls the air molecules towards the Earth's surface, counteracting the upward movement of the hot air.
Here's a more detailed explanation:
Gravity: Gravity is the force of attraction between objects with mass. On Earth, gravity pulls all objects, including air molecules, towards the center of the planet. The force of gravity increases as you move closer to the Earth's surface.
Buoyancy and Density: When air is heated, it expands and becomes less dense than the surrounding cooler air. This lower density air has a tendency to rise due to buoyancy. However, as the air rises, it moves into regions of lower pressure where the surrounding air is also expanding, causing the density to decrease further.
Equilibrium: As the hot air rises, it encounters colder air in the upper atmosphere. This encounter causes heat transfer through convection and conduction, resulting in the hot air losing some of its heat energy to the surrounding air. As a result, the hot air cools down and eventually becomes denser than the surrounding air.
Atmospheric Layers: The Earth's atmosphere is divided into several layers, such as the troposphere, stratosphere, mesosphere, and thermosphere. These layers have different characteristics and temperature profiles. The troposphere, where weather occurs, is the lowest layer and extends to an altitude of about 8-15 kilometers on average. In the troposphere, the temperature generally decreases with increasing altitude.
Stable Atmosphere: Due to the temperature decrease with altitude in the troposphere, known as the temperature gradient, the atmosphere becomes more stable. This stability inhibits the continuous rise of hot air, as the cooler air above prevents it from ascending indefinitely. Eventually, the rising hot air reaches a level where its density matches the density of the surrounding air, resulting in equilibrium.
In summary, while hot air rises due to buoyancy, the force of gravity acting on the Earth holds the atmosphere in place. The temperature gradient and the stabilization of the atmosphere limit the expansion of the air. The equilibrium between the rising hot air and the surrounding cooler air prevents the atmosphere from infinitely expanding.