Microwave Image
These infrared and microwave images were created with data retrieved by
the Atmospheric Infrared Sounder (AIRS) on NASA's Aqua satellite, and show
the remnants of the former Hurricane Felix over Central America.
Infrared Images
Because infrared radiation does not penetrate through clouds, AIRS
infrared images show either the temperature of the cloud tops or the
surface of the Earth in cloud-free regions. The lowest temperatures (in
purple) are associated with high, cold cloud tops that make up the top
of the storm. In cloud-free areas the AIRS instrument will receive the
infrared radiation from the surface of the Earth, resulting in the warmest
temperatures (orange/red).
Microwave Images
In the AIRS microwave imagery, deep blue areas in storms show where
the most precipitation occurs, or where ice crystals are present in the
convective cloud tops. Outside of these storm regions, deep blue areas
may also occur over the sea surface due to its low radiation emissivity.
On the other hand, land appears much warmer due to its high radiation
emissivity.
Microwave radiation from Earth's surface and lower atmosphere penetrates
most clouds to a greater or lesser extent depending upon their water vapor,
liquid water and ice content. Precipitation, and ice crystals found at the
cloud tops where strong convection is taking place, act as barriers to
microwave radiation. Because of this barrier effect, the AIRS microwave
sensor detects only the radiation arising at or above their location in the
atmospheric column. Where these barriers are not present, the microwave
sensor detects radiation arising throughout the air column and down to the
surface. Liquid surfaces (oceans, lakes and rivers) have "low emissivity"
(the signal isn't as strong) and their radiation brightness temperature is therefore
low. Thus the ocean also appears "low temperature" in the AIRS microwave
images and is assigned the color blue. Therefore deep blue areas in storms
show where the most precipitation occurs, or where ice crystals are present
in the convective cloud tops. Outside of these storm regions, deep blue areas
may also occur over the sea surface due to its low radiation emissivity. Land
appears much warmer due to its high radiation emissivity.
Visible/Near-Infrared Images
The AIRS instrument suite contains a sensor that captures radiation in four
bands of the visible/near-infrared portion of the electromagetic spectrum.
Data from three of these bands are combined to create "visible" images
similar to a snapshot taken with your camera.
The Atmospheric Infrared Sounder Experiment, with its visible, infrared,
and microwave detectors, provides a three-dimensional look at Earth's
weather. Working in tandem, the three instruments can make simultaneous
observations all the way down to the Earth's surface, even in the presence
of heavy clouds. With more than 2,000 channels sensing different regions
of the atmosphere, the system creates a global, 3-D map of atmospheric
temperature and humidity and provides information on clouds, greenhouse
gases, and many other atmospheric phenomena. The AIRS Infrared Sounder
Experiment flies onboard NASA's Aqua spacecraft and is managed by NASA's
Jet Propulsion Laboratory, Pasadena, Calif., under contract to NASA. JPL
is a division of the California Institute of Technology in Pasadena.