Pressure Gradient Force

We live at the bottom of an ocean of air. Currents in this ocean move mass*es of gas around and this movement creates the weather we experience every day.

Video Overview

This ocean of air is mostly molecules of diatomic nitrogen and oxygen with smaller amounts of argon, helium, and carbon dioxide. The air above us also contains varying amounts of water vapor.

All of these molecules have mass which is pulled towards the Earth by the force of gravity.

The pull of gravity on all of the molecules in the column of gas above each of us applies a force on us that we experience as atmospheric pressure.

Air pressure is greatest at the ground and decreases with altitude both because at higher altitudes the column of air gets shorter and the density* of the gas particles decreases meaning there are fewer gas particles per unit volume* at high altitude compared to ground level.

Density changes with height

As individual gas molecules warm, they take up more space causing gas to be less dense. All else being equal, air pressure decreases with increasing air temperature. The amount of water vapor present also affects atmospheric pressure. Increasing water vapor concentration*s decreases atmospheric pressure.

Warm air is less dense than dry air. Humid water vapor-containing air is less dense than dry air, so warm wet air is less dense than cool dry air.

relationship between pressure temperature and moisture

Uneven heating and varying amounts of water vapor in the atmosphere results in regions of high and low pressure. This pressure differential causes air to move along the ground from regions of high pressure to regions of lower pressure. We experience these air currents as wind.

wind caused by pressure gradient

The force driving these currents caused by the difference in pressure between the two regions. This is called the pressure gradient force.

Air masses moved by pressure gradient force transfer heat by convection as cooler gas particles move into regions containing warmer gas particles. This is similar to the smaller more contained convention cycles present in a room heated from one side. Although it is not as simple in the atmosphere because the system is more open, so the rising and falling air masses cause movement in many directions.

The moment of air up around regions of low-pressure cause air masses to converge near the ground and diverge high up in the troposphere. The opposite happens around regions of high pressure where air currents diverge near the ground and converge in the upper atmosphere.

Air movements

Because we live on a rotating planet, winds don’t blow in straight lines directly between regions of high and low pressure. Actual wind direction is determined by how air movements driven by pressure gradient forces are deflected by the Coriolis Effect.