What is the Planetary Boundary Layer?

From The Secret World of Weather:

The lowest part of the atmosphere is the only area strongly influenced by the sun’s radiative heating of the Earth’s surface. And the part that touches the ground is most strongly affected. It behaves differently from the higher parts. The low layer is known as the planetary boundary layer and it’s a region that experiences massive daily fluctuations in temperature, moisture and local winds.

It’s worth getting into the habit of spotting the level of the bottom of low clouds or the top of misty, smoky, or dusty air when you get the opportunity and noting that this is the top of the soup and that the weather above and below will be behaving differently.

For more on this and other weather clues, see the book: