EMR:INTRACTION WITH ATMOSPHERE

Remote sensing is the science of acquiring information about the Earth's surface without actually being in contact with it.

• Remote sensing is the observation and measurement of objects from a distance, i.e. instruments or recorders are not in direct contact with objects under investigation.

What is Remote Sensing?

How Remote Sensing Works

What is Remote Sensing?

Energy recorded by remote sensing systems undergoes fundamental interactions that should be understood to properly interpret the remotely sensed data.

For example, if the energy being remotely sensed comes from the Sun, the energy:

• is radiated by atomic particles at the source (the Sun), • propagates through the vacuum of space at the speed of

light,• interacts with the Earth's atmosphere,

• interacts with the Earth's surface,• interacts with the Earth's atmosphere once again, and • finally reaches the remote sensor where it interacts with various optical systems, filters, emulsions, or detectors.

ELECTRO MAGNETIC RADIATION

Electromagnetic Radiation

An electrical field (E)A magnetic field (M) oriented at right angles to the electrical field.

Energy Source or Illumination (A)

A body surface can1. absorb incident radiation,2. reflect incident radiation,

a) as a mirrorb) with spherical symmetry (Lambert)

3. transmit incident radiation,4. emit radiation.The characteristics are a function of the wavelength of the radiation.

LAW OF EMR

Wavelength

The wavelength is the length of one wave cycle

Electromagnetic Radiation

Color Wavelength (nm)

Red 780 - 622

Orange 622 - 597

Yellow 597 - 577

Green 577 - 492

Blue 492 - 455

Violet 455 - 390

Electromagnetic Radiation

Frequency

Frequency refers to the number of cycles of a wave passing a fixed point per unit of time.

Radiation and the Atmosphere

The energy travels from its source to the target, it will come in contact with & interact with the atmosphere it passes through. This interaction may take place a second time as the energy travels from the target to the sensor. All these interaction occurred by the following ways-

Interaction with the atmosphere:

• Scattering

• Absorption.

Interaction with target/surface:• Absorption.

• Transmission.• Reflection. Fig: Interaction with the Atmosphere

  . Fig: Interaction with the Atmosphere

Before radiation used for remote sensing reaches the

Earth's surface it has to travel through some distance of the Earth's atmosphere. Particles and gases in the atmosphere can affect the incoming light and radiation. These effects

are caused by the mechanisms of Scattering Absorption.

EMR: Interaction with the Atmosphere

Radiation-Target

Absorption

Transmission

Reflection

Interaction with the Target (C)

Absorption (A) Transmission (T) Reflection (R)

Reflectance

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.

How much scattering takes place depends on several factors including the wavelength of the radiation, the abundance of particles or gases, and the distance the radiation travels through the atmosphere. There are three (3) types of scattering which take place

1.Rayleigh Scattering.

2.Mie Scattering.

3.Non-selective Scattering.

Mie Scattering

Mie scattering occurs when the particles are just about the same size as the wavelength of the radiation.Dust, pollen, smoke and water vapor are common causes of Mie scattering which tends to affect longer wavelengths than those affected by Rayleigh scattering. Mie scattering occurs mostly in the lower portions of the atmosphere where larger particles are more abundant, and dominates when cloud conditions are overcast.

Non-selective Scattering This occurs when the particles are much larger than the wavelength of the radiation. Water droplets and large dust particles can cause this type of scattering. Nonselective scattering gets its name from the fact that all wavelengths are scattered about equally. This type of scattering causes fog and clouds to appear white to our eyes because blue, green, and red light are all scattered in approximately equal quantities (blue+green+red light = white light).The final scattering mechanism of importance is called Non-selective scattering.This occurs when the particles are much larger than the wavelength of the radiation.Water droplets & large dust particle can cause this type of scattering. This type of scattering causes fog& clouds to appear white to our eyes.

At sunset, solar radiation must traverse a longer path through the atmosphere. Viewing a setting sun, the energy reaching the observer is largely depleted of blue radiation, leaving mostly red wavelengths (Rayleigh). Dust/smoke adds additional scattering with a wavelength dependence that increases the red sky effect (Mie).

RED SKY AT NIGHT

Absorption

Absorption is the other main mechanism at work when electromagnetic radiation interacts with the atmosphere. In contrast to scattering, this phenomenon causes molecules in the atmosphere to absorb energy at various wavelengths. Ozone, carbon dioxide, and water vapor are the three main atmospheric constituents which absorb radiation.Ozone serves to absorb the harmful (to most living things) ultraviolet radiation from the sun. Without this protective layer in the atmosphere our skin would burn when exposed to sunlight.You may have heard carbon dioxide referred to as a greenhouse gas. This is because it tends to absorb radiation strongly in the far infrared portion of the spectrum - that area associated with thermal heating - which serves to trap this heat inside the atmosphere.

Absorption of EMR by atmosphere

Different molecules absorb different wavelengths of radiation:

• O2 and O3 absorb almost all wavelengths shorter than 300 nm.

• Water (H2O) absorbs many wavelengths above 700 nm, but this depends on the amount of water vapor in the atmosphere.

When a molecule absorbs a photon, it increases the energy of the molecule. We can think of this as heating the atmosphere, but the atmosphere also cools by emitting radiation.

Electromagnetic Radiation & Health

Electromagnetic radiation can be classified into two types: ionizing radiation and non-ionizing radiation, based on its capability of ionizing atoms and breaking chemical bonds. Ultraviolet and higher frequencies, such as X-rays or gamma rays are ionizing, and these pose their own special hazards: see radiation and radiation poisoning. Non-ionizing radiation, discussed here, is associated with electrical and biological hazards.

Electrical Hazard. Fire Hazard.

Biological Hazard. In many respects, remote sensing can be thought of as a reading process. Using various sensors, we remotely collect date that may be analyzed to obtain information about the objects, areas or phenomena being investigated. In most cases the sensors are electromagnetic sensors either airborne or space borne for inventorying.