Why is the atmosphere stable in the morning, becoming unstable in the afternoon.

Overnight the ground cools due to radiative cooling. The ground cools the air by conduction, creating a layer of cool air close to the ground. So in the morning we have an atmosphere that is cool at the bottom. The cool, dense air is below warmer less dense air, so this situation is stable. A classic inversion.

As the sun comes up in the morning, the heat from the sun warms the ground. The lower atmosphere is largely transparent to this radiation, so isn't affected directly. Instead the warming ground heats the air by conduction.

When the air close to the ground becomes warmer than the air above, it becomes unstable. The lower air becomes less dense than the air above and starts trying to rise up. The temperature at which this starts is called the trigger temperature.

Once the trigger temperature has been met, we start getting convection.

Links

• Stablity
• Convection
• Conduction
• Radiative cooling

Briefing prepared by Phil Plane.

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