Prioritize...
This page is meant to summarize the characteristics and capabilities of the various types of satellite and radar images we've covered in this lesson. At this point, you should be able to decide which remote sensing product (visible, infrared, or water vapor satellite imagery, or radar imagery) is appropriate given a particular need.
Read...
We've covered several different types of remote sensing products in this lesson, ranging from images created by passive remote sensors aboard satellites to those created by active remote sensors located in ground-based radars. Before we wrap up, I want to quickly summarize the characteristics and capabilities of each product. Knowing the characteristics and capabilities of each type of imagery will help you choose the most useful type of remote sensing product given a particular need. For example, if you needed to know where the nearest area of rain was located, which type of imagery would you use? If you needed to know if skies were cloudy overnight, which type of imagery would you use? You should be able to answer those types of questions based on your knowledge of these products.
Visible satellite imagery...
- is based on the albedo of objects (the fraction of incoming sunlight that is reflected to the satellite).
- can tell you about the thickness of clouds (thicker clouds have higher albedos and appear brighter than thinner clouds, which have lower albedos).
- can be used to distinguish between snow cover and clouds, given that surface features such as lakes and rivers can be observed.
- is not able to detect clouds (or anything else) during the satellite's local night (visible imagery requires sunlight).
- is not useful for determining whether precipitation is present under the observed clouds.
Infrared satellite imagery...
- is based on the fact that measuring an object's infrared emission tells you something about its temperature.
- displays the temperature of either cloud tops or the earth's surface (if the sky is clear).
- can be combined with the assumption that temperature decreases with height to determine cloud-top heights. Colder cloud-tops (lower temperatures) mean higher clouds.
- is not able to give any direct indication of cloud thickness or the presence of precipitation (although inferences can be made in some cases).
Water Vapor imagery...
- uses infrared radiation; except unlike traditional infrared imagery, it uses wavelengths at which water vapor strongly emits and absorbs infrared radiation.
- displays the temperature of the effective layer. Warm effective layers mean that the middle to upper troposphere contains very little water vapor (that is, they're "dry"). By comparison, colder effective layers indicate a higher concentration of water vapor and/or ice clouds in the upper troposphere.
- is not able to give any measure of the atmospheric water vapor content below the effective layer.
- usually does not show the presence of low clouds or water vapor near the surface. These almost always lie below the effective layer.
- is used to trace air motions in the middle and upper troposphere, even in areas with no clouds.
Radar imagery...
- originates from ground-based sensors (not from satellites) that actively emit pulses of radiation.
- uses the microwave part of the electromagnetic spectrum (not the infrared).
- usually displays the variable "reflectivity" (units dBZ) which is the measure of the amount of signal returned to the radar from the original transmitted pulse.
- can help forecasters detect areas of precipitation (via reflectivity) as well as potential areas of severe weather via Doppler velocities and dual polarization data.
- cannot tell you anything about cloud top temperature, cloud height, or cloud thickness.