|Direction:There are two important directions when dealing with the Earth’s radiation budget. Energy can either be incoming (down) or outgoing (up).|
|Wavelength: There are two principal wavelength ranges. (Image from UCAR) Energy from the Sun includes X-rays to radio waves, but the bulk of the energy is in the visible wavelength range (with some UV and infrared). We call this SOLAR radiation. Photosynthetically active radiation (PAR) (400 – 700 nm) is a subset of the SOLAR radiation that is important for photosynthetic activity. Energy from the Earth is in the infrared range. We also call these two ranges shortwave (SW) and longwave (LW).|
|Location: There are two main locations where we can measure the radiation budget: at the surface (using surface instrumentation or satellite techniques) and at the top of the atmosphere (TOA; using satellites). In principal you can measure the radiation flow at any level between those two, but it’s not so easy.|
|Albedo: For SW radiation, albedo tells what fraction of the incoming (downward) radiation is reflected (upward). The albedo of the surface is NOT the same as the albedo at the TOA.|
|Net radiation: This gives the amount of energy actually added to the system. It is easy to calculate:
Net = Energy in – Energy out.
But there are multiple options, both at TOA and at the surface:
SW_Net = SW_Down – SW_UP.
LW_Net = LW_Down – LW_UP.
Total_Net = SW_Net + LW_Net = SW_Down – SW_UP + LW_Down – LW_UP.
|Time interval:The components of the radiation budget change with time. It is not really practical to keep an infinitely detailed record. We typically have information for time periods like hourly, daily, monthly average, seasonal average, annual average (the numbers in the picture are an average over several years – a climatology).|