Electromagnetic Radiation
The energy Earth receives from the sun is called electromagnetic radiation which travels through space in the form of a wave with both electric and magnetic characteristics. Although we are talking about light, most of the electromagnetic spectrum cannot be detected by the human eye. Even satellite detectors only capture a small portion of the entire electromagnetic spectrum.The full range of wave frequencies in solar radiation is called the electromagnetic spectrum, shorter wavelengths have more energy than longer wavelengths. Because the Sun is much hotter than the Earth, (surface temperature of 5,880 Kelvin, approximately 10,000 degrees Fahrenheit) it radiates short-wave energy. The Sun warms the Earth, and the Earth radiates energy in response. But because the Earth’s average surface temperature is only about 288 Kelvin (about 59 degrees Fahrenheit), the energy Earth radiates has a longer wavelength than solar radiation. Ultimately, electromagnetic radiation determines Earth’s climate, since the planet must shed as much energy as it absorbs.

Radiometers
Remote sensing instruments on satellites are called radiometers. They are designed to accurately measure electromagnetic energy radiating from the earth and atmosphere. And everything emits electromagnetic radiation. More specifically, all objects with a temperature above absolute zero emit radiation.

Radiometers measure radiation of different wavelengths in discrete intervals by using mirrors that scan a region and reflect digital data back to the satellite to be transmitted down to earth for processing. Satellite radiometers can “see” in a wide range of electromagnetic spectral intervals. These intervals are called channels or spectral bands. In this course we will focus on the three most common channels: visible light (0.6 microns), longwave infrared (10 to 12 microns), and a special channel near one of the infrared absorption bands of water (6.7 microns) that we call the “water vapor channel.”

The ART of electromagnetic radiation
As with all energy, radiation can change form, but it must be conserved. The art to remembering what happens when electromagnetic energy moves through the atmosphere is that it can either be absorbed, reflected, or transmitted when it interacts with other objects. Visible satellite images depend on the availability of reflected sunlight. Infrared satellite images (IR) result from radiometers that detect emitted electromagnetic energy so meteorologists rely on IR images to track storms overnight.

The Importance of Contrast
Collecting electromagnetic energy from an area usually results in an image that provides instant identification of a feature, however, if there isn't enough contrast between the feature and it's surrounding, scientists look at the same scene in multiple channels or apply enhancements to the image to create the contrast necessary to discern the feature they are trying to study.

Atmospheric Opacity
Radiation detected by satellites is comprised of both terrestrial and atmospheric sources, however, energy from the earth's surface must travel through the atmosphere before it reaches the satellite. Satellite sensors are designed to be particularly sensitive to those wavelengths of radiant energy that can be reflected or emitted back up through the atmosphere to space. By using the laws of radiation to calibrate radiometers and interpret details displayed on satellite images, scientists can measure the height, temperature, moisture content (and more) about nearly every feature of the earth’s atmosphere, hydrosphere, lithosphere, and biosphere.

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