|WHAT IS A MIXED LAYER AND ATMOSPHERIC MIXING?
METEOROLOGIST JEFF HABY
Mixing and mixed is a term that can be frequently encountered in forecast discussions. Terms such as mixed layer, layer of air mixed out
and mixed PBL (Planetary Boundary Layer) use this term. When a layer of air is mixed, it is made more uniform by blending the air together. The
term convection can also mean a mixing of air. As an analogy, a pot of boiling water is mixing the water to help distribute the heat.
Air will be better mixed in the PBL when the ground is warmed. This is because air from the surface will rise and blend with air higher
aloft. The air rises since warmer air is less dense than cooler air and thus rises like a helium balloon. This helps create air with
more uniform properties. A case when air will not mixed as much is when the air at the surface is cold and the air above it is warm. Since
cold air is denser than warmer air it will resist rising and mixing with the air above it. In this case the lower atmosphere is not mixed.
The mixing of air is aided by convection and the wind. Air will better be able to mix if it is merging together through convective motions
and if wind is blowing the air together. The PBL will typically be better mixed in the afternoon since there are convective currents
and stronger wind as compared to the night hours.
A layer of air is well mixed if it has uniform properties. This can be thought of like an air mass. Air masses do not just exist at the
surface but can exist aloft also. For example, in severe weather season there can be warm and moist air near the surface with warm and
dry air aloft. Each of these air masses comes from a different source region. The dry air aloft is called the elevated mixed layer. It
is an air mass with uniform properties but it is located aloft since it was advected from a higher elevation region.