>> Lecture Three: Satellite Images, Radiation and Temperature <<


This lecture covers various aspects of electromagnetic radiation in respect to meteorology - including satellite images, absorptivity, the solar radiation budget and the impact of the seasons on solar radiation.


 >> Lecture Three Outline <<

Satellite Images

>> Visible: Senses reflected visible light

Figure 1: This image shows a basic visible satellite loop over Pennsylvania.  The satellite obtained this loop by recording the amount of visible light reflected from the surface features and clouds.


Visible satellite images are very useful tools that allow meteorologists to view various atmospheric features through the use of reflected visible light.  Since these satellite sensors use reflected visible light in order to produce their images, visible satellite images can best be used during the daytime when solar radiation is reflected back toward the satellite.  A visible satellite loop can be seen in Figure 1 to the right.

In addition to sensing reflected visible light, these satellites can also determine cloud thickness by figuring out how much visible light a cloud reflects, or its albedo.  A thicker cloud generally has a higher albedo, and therefore, reflects more light.


>> Infrared: Senses terrestrial radiation emitted through the atmospheric window

Infrared satellite sensors, instead of detecting reflected solar radiation from the surface of the Earth, rely on the detection of terrestrial radiation emitted through the atmospheric window.  Since terrestrial radiation is constantly being emitted from the surface and from atmospheric features, infrared satellite images can be used 24 hours a day.  An infrared satellite loop can be seen in Figure 2 below.

Please download the Quicktime plug-in to view this satellite loop.

Figure 2: An infrared satellite loop.  This image shows terrestrial radiation emitted through the atmospheric window.


Figure 3: The image above illustrates the relation between cloud top temperature and height.




Infrared satellite images can also be used to calculate the temperature, and therefore height, of cloud tops.  This is done through the use of the Stefan-Boltzmann Law, which relates the amount of radiation emitted by a body and its temperature.  This process is illustrated in Figure 3 to the left.






>> Water Vapor: Senses infrared radiation absorbed/emitted (Kirchoff's Law) by water vapor.

Figure 4: A water vapor image.  This satellite image shows the movement of water vapor in the atmosphere by detecting 6.7 micron radiation emitted by the vapor.




Water vapor satellite images are obtained by the detection of radiation emitted at 6.7 microns in wavelength - the primary wavelength at which water vapor absorbs and emits radiation.  Since water vapor is always present in the atmosphere, water vapor satellite images allow meteorologists to see large scale atmospheric motions even when there are no clouds present.  An example of a water vapor satellite loop can be found to the right in Figure 4.



Absorptivity: Transparent versus Opaque

>> See Lecture Two Outline for details

Distribution of Solar Radiation

>> The sun warms the ground and the ground warms the air!

The incoming solar radiation warms the ground, which in turn warms the air through conduction and convection.  Conduction is the process by which heat energy is transferred through direct contact.  This process can be seen below in Figure 5.  Convection, on the other hand, is the transfer of heat energy due differential heating of a substance.  To view a movie demonstrating convection, click here (you must have the Quicktime plug-in to view this movie).

Please download the Macromedia Shockwave plug-in to view this demonstration.

Figure 5: The application above demonstrates the process of conduction.  In order to see how heat is transferred through direct contact, click the Flame On! button.

Annual Cycle: Tilt of the Earth's Axis

>> See Lecture Two Outline for details

Seasonal Temperature Lag: Heat Capacity

>> It takes time for land temperature to respond to solar radiation
>> Water is slower to warm than land

Figure 6: The image above shows the difference in the absorption of incoming solar radiation by water and land.



It takes much more solar radiation to warm one kilogram of water than it does to warm the same amount of land.  In addition, solar radiation can penetrate further into water than it can into land, effectively spreading the heating through a much larger vloume.  Also, the solar radiation that is absorbed by the ocean can easily be mixed by ocean currents and waves.  Figure 6, to the left, illustrates the differences between the absorption of solar radiation by land and water.





2004-2017 by Thomas P. Harrington