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Structure and movement of air: composition, pressure, winds, air masses, and fronts

The air is a mixture of gases formed by atoms organized in different groupings. Atmospheric pressure varies with altitude, being higher at sea level due to the greater thickness of the layer of air above and decreasing at higher altitudes.

Global wind map shows arrows representing air currents | Photo: Earth


Air is a mixture of gases formed by atoms organized in different groupings. As a matter, it occupies space and has mass. This characteristic allows air to resist the movement of bodies immersed in it, an effect that can be felt when extending a hand out of a moving car: the resistance of the air slows the hand down, generating a counterforce. Additionally, because it has mass, air exerts pressure on the bodies around it, known as atmospheric pressure, which can be measured with a barometer. Atmospheric pressure varies with altitude, being greater at sea level due to the thicker layer of air above, and decreases at higher altitudes.

Atmospheric pressure results from the weight of the air on surfaces. This pressure is measured with a barometer, an instrument that indicates pressure variations with changes in altitude. Regions at sea level record higher atmospheric pressure because there is a thicker layer of air over these areas. At higher altitudes, the pressure is lower due to the lesser amount of air above. ✅ Click and follow Elementary Science on Instagram

When air receives heat, the particles agitate, occupying a larger volume and making the air less dense. This phenomenon causes warm air to rise, creating a void that is soon filled by the colder air around it, generating what are known as air currents. This movement of air is what we refer to as wind.

Air masses are large extensions of air with uniform properties that form when air remains in an area long enough to acquire local temperature and humidity characteristics. These movements of air masses occur due to differences in atmospheric pressure, usually caused by variations in surface temperature.

Fronts form at the meeting of air masses with different characteristics. Warm fronts occur when a warm air mass advances over a cold air mass, moving above it and causing condensation of moisture, leading to cloud formation and precipitation. In a cold front, a cold air mass, being denser, moves beneath the warm air mass, also causing moisture condensation, which typically results in heavy rain and thunderstorms.

Soil and water have different capacities for heating and cooling: water, with its high specific heat, absorbs and releases heat slowly, while soil changes temperature rapidly. This behavior can be observed on beaches, where at noon the warm sand heats the air above it, while the water remains cooler. By late afternoon, the sand cools quickly, while the water still retains the heat absorbed throughout the day.

During the day, the heated air over the sand rises, and the cooler air from the sea moves in to fill the space left, generating a sea breeze that blows from the sea to the land. At night, the process reverses: the still-warm seawater heats the air above it, and the cooler air from the land moves toward the sea, creating a land breeze.

Due to the uneven heating of the Earth, greater in tropical zones and lesser in polar regions, a global circulation of air occurs. The heated air in tropical regions becomes less dense and rises, moving in the upper part of the troposphere toward the poles. As it cools at higher altitudes, this air descends and returns toward the equator. This cycle creates the moist trade winds, which blow from the tropics toward the equator and cause rainfall. The subtropical highs, in turn, are dry winds that blow from the equator to the tropics and are associated with desert areas.

This circulation of winds, driven by differences in pressure and temperature, contributes to the global distribution of thermal energy, balancing heat between regions of the globe. This exchange process carries warm air from the tropics to temperate zones and cold air from the poles to warmer areas.

 

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