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The planetary boundary layer is the lowest part of the atmosphere. It is also known as the atmospheric boundary layer. The part of the atmosphere that the planetary boundary layer resides in is troposphere. This portion of the Earth’s atmosphere is in direct contact with the Earth surface. Due to this, the planetary boundary layer is highly susceptible to the friction created between the air and the foliage and surface of the Earth. This leads to this region of the atmosphere being one of the turbulent regions.
The PBL is the main reason for gust of winds and turbulence on certain days. This is caused by the surface friction of vegetation and topography. The effects of geography on the planetary boundary layer are almost immediate causing changes in the flow of air within hours. The reason it is called a boundary layer is because of how air acts above this region. The mass of air above the region is more calm and uniform in movement and velocity. Winds above the PBL are called geostrophic. This means that the pressure gradient and Coriolis force are balanced.
The planetary boundary layer is also affected by temperatures. One thing that scientists have determined about the source of weather on Earth and other planets is that solar radiation absorbed by the surface of a planet is eventually reflected or radiated back into the atmosphere. The absorption and release of heat in the atmosphere is what creates the conditions that make weather happen. The planetary boundary layer plays an important role as it is the first part of the atmosphere that comes in contact with this rising heat. The amount of heat it gains or loses directly affects its thickness. During the day when solar radiation is being reflected from the ground the thickness of the PBL increases. When it is night, the loss in heat causes the PBL to contract.
Another function of great heat is that more moisture is absorbed in the atmosphere. This is why when the temperature of the PBL increases it becomes more unstable. Understanding the Planetary Boundary Layer is important for many reasons. The first is to understand one of the underlying mechanics of weather to make weather forecasting more predictable. Another reason is that engineers need to know the factors affecting the PBL in order to design more effective aircraft. A poorly designed plane that doesn’t take the conditions of the PBL into account is more likely to be torn apart by wind shear.
We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.