Atmospheric mixing is the turbulent exchange of heat, moisture, and momentum through a layer of the atmosphere, driven by eddies that carry properties from one level to another until the layer's profile becomes more uniform. In the boundary layer this stirring is usually strong enough to reset the low-level temperature and dewpoint every afternoon.
How mixing works
Two forces drive daytime mixing. Buoyancy lifts warm surface parcels into cooler air aloft, and wind shear over the ground breaks flow into eddies. Together they produce turbulent motion that transports properties vertically without needing organized ascent.
As eddies move up and down, a warm parcel that rises carries heat with it, and a cool parcel that sinks brings cooler air down. Averaged over many eddies, the result is a net upward heat flux and a temperature profile that tends toward the dry adiabatic lapse rate through the mixed layer.
How forecasters read it
Mixing shows up on a morning sounding as a shallow surface layer that will grow taller as heating deepens it. Forecasters estimate an afternoon mixed layer depth by finding where the surface temperature intersects the morning temperature profile along a dry adiabat, then use that depth to reason about how surface parcels and moisture will be redistributed.
The same technique explains why afternoon dewpoints often fall when a moist surface layer is thin. Once mixing reaches into drier air above, that dry air is stirred down and the whole column ends up drier at the surface than the morning observation suggested.
Where it misleads
Mixed layer growth assumes the atmosphere above is roughly dry adiabatic and free to mix. Where a strong cap or elevated inversion sits close to the top of the boundary layer, mixing slows against it and moisture can pool rather than dilute. Where the layer above is unusually moist, mixing actually adds moisture at the surface.
Mixing also depends on cloud cover, wind, and land surface. A cloudy morning delays mixing by cutting the surface heat flux. Wet soil holds more sensible heat back as latent heat, producing a shallower and moister mixed layer than dry soil under the same sun.