INTERACTION OF EM WITH ATMOSPHERE Course: Introduction to RS & DIP Mirza Muhammad Waqar Contact: EXT:2257 RG610

Outlines  Remote Sensing System  The Energy Source  The Atmosphere  The Energy-Matter Interaction at the Earth’s Surface  The Sensor  The data processing and supply system  The multiple data users  An ideal Remote Sensing System

Remote Sensing System 3

Electromagnetic Radiations  c = f λ  c = 3x10 8 m/sec  Either f, λ can be used for characterization of EM radiations.

Electromagnetic Radiation  Remote sensing is based on detecting electromagnetic (EM) energy.  Since the speed of light is constant, wavelength and frequency are inversely related to each other.

Electromagnetic Radiation 6

Black Body Radiations

Energy Interactions in the Atmosphere 8  Irrespective of its source, all radiations detected by remote sensors passes through some distance of atmosphere  The path length can vary widely depending upon  Source of illumination elevation angle  Sensor view angle  Following phenomena can take place, while interaction of energy in atmosphere.  Scattering  Absorption

Scattering  Scattering occurs when particles or large gas molecules present in the atmosphere interact with and cause the electromagnetic radiation to be redirected from its original path.  Type of Scattering  Raleight Scattering  Mie Scattering  Non Selective Scattering

Rayleigh Scatter  Rayleigh scatter is common when radiation interacts with atmospheric molecules and other tiny particles that are much smaller in diameter than the wavelength of interacting radiation.  The effect Rayleigh scatter is inversely proportional to the fourth power of wavelength.  Hence shorter wavelengths will scatter more by Rayleigh scatter.

The Blue Sky ~  A “blue” sky is a manifestation of Rayleigh scatter.  In the absence of scatter, the sky would appear black.  Sunlight interacts with the earth’s atmosphere,  It scatter the shorter (blue) wavelength more dominantly than the other visible wavelengths.  Consequently, we see a blue sky.

Mie Scattering  Mie scattering occur when atmospheric particle diameters essentially equal the wavelengths of the energy being sensed.  Water vapors, dust particles are major causes of Mie scatter.  Mie scatter influence longer wavelengths as compared to the Rayleigh scatter.

Non-Selective Scattering  It occurs when the diameter of the particles causing scatter are much larger than the wavelengths of the energy being sensed.  Water droplets, for example causes such scatter.  This scattering is “nonselective” with respect to wavelength.  Hence in visible, equal quantities of blue, green and red light are scatter, hence fog and cloud appear white in satellite images.

Absorption  Atmospheric absorption results in the effective loss of energy.  The most efficient absorbers of solar radiation in atmosphere are  Water vapor  Carbon dioxide  Ozone  The wavelength range in which the atmosphere is particularly transmissive of energy are referred to as atmospheric windows.

Atmospheric Windows 15  The wavelength ranges in which the atmosphere is particularly transmissive of energy are referred to as atmospheric windows.

Questions & Discussion