What is the boundary layer and its typical depth?

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The boundary layer is the lowest part of the atmosphere, directly affected by the Earth's surface through friction, heating, and moisture exchange. Turbulent mixing from wind shear and buoyancy keeps this layer well mixed, especially during daytime heating. Its depth changes with stability and weather: typically around 1 to 2 kilometers, but it can extend to several kilometers when strong convection stirs the air, such as on hot, sunny days or in windy, unstable conditions. At night or in stable conditions, the layer becomes shallower as cooling near the surface stabilizes the air and limits mixing. The boundary layer often has a surface layer near the ground, a deeper mixed layer, and a top entrainment region where air from the free atmosphere begins to blend in. It’s not the layer above the stratosphere, nor the upper troposphere where jet streams reside, nor simply the dry surface layer, as those describe different parts of the atmosphere.