# A New Methodology for Estimating Surface Albedo in Heterogeneous Areas from Satellite Imagery
## 2 Concept of Albedo  
The albedo is the coefficient of reflection of a surface and is defined as the relation between the radiation reflected on the surface and the incoming radiation. With this definition in mind, albedo and reflectance are two similar terms, but not the same. As [PERSON] and [PERSON] [40] state, single-wavelength albedo is the same as reflectance, but when we talk about albedo, we generally refer to a certain range of wavelengths. The wavelengths used for the computation of albedo are those of the shortwave region of the spectrum of solar light, generally between 0.25 and 3.0 \(\upmu\)m [40]. With these, the bands of high absorption and low reflection are discarded from the process because the incoming solar radiation at the wavelength beyond 0.25-3.0 \(\upmu\)m approaches zero. Albedo values are in the range from 0 to 1, 0 being the albedo of a black body (no reflectivity) and 1 the albedo of a white body (total reflectivity). Examples of albedo values for different surfaces on the Earth are 0.04 for vegetal coal; 0.17 for bare soil; 0.25 for green grass; 0.40 for desert sand; 0.50 for concrete; between 0.50 and 0.70 for ocean ice and glaciers; and between 0.80 and 0.90 for fresh snow [41], the latter being the highest albedo value on Earth. There are currently two main methodologies for the computation of surface albedo: the indirect estimation from images of surface reflectance; and the direct measurement with sensors. The indirect estimation of albedo is based on the measurement of reflectance in different spectral bands from satellite or UAV platforms, and the application of methodologies to estimate albedo from the reflectance values. The direct measurement can be performed with sensors called albedometers, incorporated to meteorological stations. Both measurements are presented in this paper. Albedo can be subclassified into different types, but according to the type of solar radiation taken into account, there are the following types:  
* Black-sky albedo, also known as directional-hemispherical reflectance. Black-sky albedo is the albedo in the absence of diffuse radiation and depends on the solar zenith angle. It can be considered as the direct radiation component of the total albedo;
* White-sky albedo, also known as bi-hemispheric reflectance. White-sky albedo is the albedo for circumstances with no direct radiation and considering the diffuse radiation as isotropic (uniform reflectance properties). It can be understood as the diffuse radiation of the total albedo;
* Blue-sky albedo: this is the surface or real albedo. There are different methodologies for its calculation from satellite observations [42], but the most commonly used methodology is the integration of the BRDF (Bidirectional Reflectance Distribution Function) through the black-sky albedo, the white-sky albedo, and the diffuse fraction of sky light. That is, blue-sky albedo is the albedo value computed under real conditions with a combination of direct and diffuse light. The computation of blue-sky albedo is the objective of this work. The value of blue-sky albedo (hereafter called surface albedo) mainly depends on the following parameters [5; 43]: the environment; the material; the point of view of the observer; the solar angle; the time of the day; the state of the sky; the geometry of the environment; and the shadows. Albedo also changes with the season: as an example, the surface can be covered by snow in winter (albedo 0.80-0.90) and with grass in spring (albedo 0.25); and changes in vegetation that happen during the year, consequently changing their surface albedo value. As an example, [PERSON] et al. [44] show that vegetation greenness in Inner Mongolia has a positive correlation with surface albedo from May to September (growing season), this correlation being negative the rest of the year.