watts per square meter (W/m2). Since the absorption varies with location as well as with diurnal, seasonal and annual variations, the numbers quoted are multi-year averages obtained from multiple satellite measurements. Of the ~340 W/m2 of solar radiation received by the Earth, an average of ~77 W/m2 is reflected back to space by clouds and the atmosphere and ~23 W/m2 is reflected by the surface albedo, leaving ~240 W/m2 of solar energy input to the Earth's energy budget. This amount is called the absorbed solar radiation (ASR). It implies a value of about 0.3 for the mean net albedo of Earth, also called its Bond albedo (A): Outgoing longwave radiation Outgoing longwave radiation (OLR) is usually defined as outgoing energy leaving the planet, most of which is in the infrared band. Generally, absorbed solar energy is converted to different forms of heat energy. Some of this energy is emitted as OLR directly to space, while the rest is first transported through the climate system as radiant and other forms of thermal energy. For example, indirect emissions occur following heat transport from the planet's surface layers (land and ocean) to the atmosphere via evapotranspiration and latent heat fluxes or conduction/convection processes. Ultimately, all of outgoing energy is radiated in the form of longwave radiation back into space. The transport of OLR from Earth's surface through its multi-layered atmosphere follows Kirchoff's law of thermal radiation. A one-layer model produces an approximate description of OLR which yields temperatures at the surface (Ts=288 Kelvin) and at the middle of the troposphere (Ta=242 Kelvin) that are close to observed average values: In this expression σ is the Stefan-Boltzmann constant and ε represents the emissivity of the atmosphere. Aerosols, clouds, water vapor, and trace greenhouse gases contribute to an effective value of about ε=0.78. The strong (fourth-power) temperature sensitivity maintains a near-balance of the outgoing energy flow to the incoming flow via small changes in the planet's absolute temperatures. Earth's internal heat sources and other small effects The geothermal heat flow from the Earth's interior is estimated to be 47 terawatts (TW) and split approximately equally between radiogenic heat and heat left over from the Earth's formation. This corresponds to an average flux of 0.087 W/m2 and represents only 0.027% of Earth's total energy budget at the surface, being dwarfed by the 173,000 TW of incoming solar radiation. Human production of energy is even lower at