Lesson 1
Meteorological Satellite Orbits
Lesson 2
Review of Radiative Transfer
Lesson 3
Visible Image Interpretation
Lesson 4
Infrared Image Interpretation
Lesson 5
Multispectral Image Interpretation
Lesson 6
Fires & Aerosols
Lesson 7
Lesson 8
Lesson 9
Fog and Stratus
Lesson 10
Lesson 11
Lesson 12
Lesson 13
Global Circulation
Lesson 14
Synoptic Scale
Lesson 15
Global Circulation
Lesson 16
Satellite Oceanography
Lesson 17

Lesson 3: Surface Radiative Properties

Remote Sensing the Surface

In remote sensing the surface of the Earth from a satellite, we select spectral regions, or channels, in which the atmosphere is transparent. These are called atmospheric windows. In the solar spectrum, or the shortwave, very little absorption occurs in the visible region; however, Rayleigh scattering is large. Rayleigh scattering is well understood and can be handled via modeling as described in the previous chapter. There are also windows in the near-infrared spectral regions where Rayleigh scattering is smaller.

The surface albedo is defined as the ratio of the radiation reflected from the surface to the radiation incident on the surface. The surface albedo varies from approximately 5% for calm, deep water to over 90% for fresh snow. The surface albedo over land depends on the type and condition of the vegetation. Thus, over land, the surface albedo varies from location to location and with time. Over water, the surface albedo is also a strong function of solar zenith angle.

The surface albedo depends on the wavelength of the incident radiation. Figure 6 is an example of spectral reflection of various surfaces. Snow is very reflective at visible wavelengths (.4 to .7 mm) and less reflective an the near-infrared wavelengths. Plants have higher reflectances in the near-infrared than in the visible. Photosynthesis is effective at absorbing visible energy. When plants dry out, their chlorophyll content decreases and the reflectance at visible wavelength increases.

The figure below show typical values of spectral albedo as a function of surface type.

Spectral reflecantance image

Analysis of the spectral reflectance of different surfaces generally shows a distinct difference between the visible and near IR regions. Observations in both the visible (such as 0.58- 0.68mm) and near-infrared (such as 0.725-1.1 mm) are useful for monitoring surface conditions. Vegetation regions generally have reflectances in the near-infrared (NIR) that range from 20-40%, while visible reflectances generally range from 5-15%. Soils also have a higher reflectivity in the NIR than in the visible while the opposite is true for snow.

  1. Vegetation Index - NDVI
  2. Snow Index - NDSI


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