Lecture Notes for November 4, 2019

1. An infrared satellite photograph detects the 10 micrometer (μm) IR radiation actually emitted by the ground, the ocean and by clouds. You don't depend on seeing reflected sunlight, so the earth can be photographed during the day and at night. You may recall that 10 μm radiation is in the middle of the atmospheric window, so this type of radiation is able to pass through air without being absorbed. If clouds don't get in the way, you can see the ground on an IR photograph.

2. Clouds absorb 10 μm radiation and then emit 10 μm IR radiation of their own. The intensity of the cloud radiation will depends on the cloud's temperature. The top surface of a low altitude cloud will be relatively warm. Warmer objects emit IR radiation at a greater rate or at higher intensity (the Stefan Boltzmann law). This is shown as grey on an IR satellite photograph. A unimpressive grey looking cloud on an IR satellite photograph may actually be a low altitude, thick cloud that is producing a lot of rain or snow.

3. Cloud tops found at high altitude are cold and emit IR radiation at a lower rate or lower intensity. This will show up white on grey-scale IR photographs, but images are often color enhanced to show cold cloud tops (those that emit low amounts of IR radiation).

An example of an IR satellite photograph is shown below. The left side shows a grey scale image. The highest intensity radiation is colored black (warmest emitting temperature) with the lowest intensity radiation colored white (coldest emitting temperature). With this grey shade scale, clouds will generally appear white, since cloud tops are generally colder than the ground surface.
Slightly different shades of white or grey on IR satellite photographs can be difficult to distinguish with the naked eye. The satellite sensor on the other hand is able to precisely measure the intensity of the IR radiation it is photographing. The images are often color enhanced to bring out very small differences in intensity that are due, ultimately, to differences in cloud temperature. An example is shown on the right below.


IR image 17 Z Nov. 7, 2014	color enhanced image 17 Z Nov. 7, 2014

The right image above is an enhanced version of the image above at left (both images are from the National Oceanic and Atmospheric Administration Geostationary Satellite Server site). The intensity that corresponds to a particular color can be determined using the scale shown at the right edge of the image. The color scale is used for the portion of the intensity range expected for clouds. This allows one to see the subtle, but important variations in cloud top temperature throughout the image.

4. Two very different clouds (a thunderstorm and a thin, high altitude ice cloud) would both appear white on the satellite photograph and would be difficult to distinguish. Meteorologists are interested in locating tall thunderstorms because they can produce severe weather. Fortunately, as we will see, these two cloud types have very different appearances of visible satellite photographs, so this ambiguity can be resolved.

5. The ground changes temperature during the course of the day. On an infrared satellite animation you can watch the ground change from dark grey or black (during the afternoon when the ground is warmest) to lighter grey (early morning when the ground is cold) during the course of a day. In the sketch below the ground temperature varies between 80 F and 50 F during the day. Because of water's high specific heat, the ocean right alongside doesn't change temperature much during the day and remains grey throughout the day. The ocean remains 65 F throughout the day in the figure below.


Morning when ground is cool	Afternoon when the ground is warmer


early morning (14 Z = 7 am MST)	afternoon (21 Z = 2 pm MST)

Early morning (ground is cool) and afternoon (ground has warmed) photographs are shown above. Focus in on the center of the pictures (SW Arizona, S California, NW Mexico and the northern end of Baja California). There don't appear to be any clouds there so we are able to see the ground and ocean. Note how much darker the ground appears in the right (warm afternoon picture). I don't see a change in the images of the ocean west of the California/Mexico border in the two images

Here's a link to an IR satellite photograph loop. It is sometimes easier to see the changing appearance of the land surface as it warms and cools when the pictures are in motion.

Visible Imagery

A visible satellite photograph photographs sunlight that is reflected by clouds. You won't see clouds on a visible satellite photograph at night. Thick clouds are good reflectors and appear white. Thinner clouds don't reflect as much light and appear grey. The low altitude layer cloud and the thunderstorm above would both appear white on this photograph and would be difficult to distinguish.

If there are no clouds, then visible images show the reflectance of the underlying ground surface. Water and vegetated ground surfaces are good absorbers of visible light, in other words those surface are not good reflectors of visible light, so they appear dark. Snow covered surfaces are highly reflective of visible light, so they appear bright. Since snow and clouds both appear bright, it can be difficult to distinguish them on a single visible satellite image. But, if you look at a movie of satellite images, clouds tend to move, while snow does not (though you can see it melt away in time lapse movies of visible satellite images).

Specialized satellite imaging is used for much more than just imaging clouds. For example, things like soil moisture and the health of vegetation can be determined from a combination of different visible and infrared "channels," which are sensitive to different bands of electromagnetic radiation.

Combining IR and Visible Images to Distinguish Clouds

Low stratus or nimbostratus clouds, thick low altitude clouds, appear grey and white on infrared and visible photographs, respectively. Thunderstorms appear white on both types of satellite photographs.


1745 Z (10:45 am MST) Full Disk IR image	1745 Z Full Disk Visible image

A comparison of full disk INFRARED and VISIBLE images is shown above. Perhaps the most obvious difference is the area of clouds out over the Pacific Ocean and west of South America. The clouds are bright white on the VIS image but barely discernible on the IR image. These must be fairly thick low altitude clouds. In fact it is known that this region is persistently covered by low altitude (warm for IR emission), thick (high reflection in the visible) clouds.

There's another even more striking difference between the two images - the appearance of outer space. The earth appears on a white background on the IR image. Remember that white on an IR image indicates weak intensity IR emissions. In this case the satellite is not detecting any IR emissions coming from space - zero intensity IR radiation. The earth appears on a black background in the VIS image. The satellite is not seeing any visible light coming from space.

Geostationary orbit
The photographs above were taken from a satellite in geostationary orbit. The satellite is positioned above the equator at an altitude of about 22,000 miles altitude (by comparison the International Space Station is about 260 miles above the earth). At that altitude the satellite completes one orbit a day. The motion of the satellite and the spin of the earth are in the same direction. Thus the satellite remains above and is always viewing the same point on the ground.

A Link to a Good Place to View Satellite Imagery
There are many web sites that provide access to satellite imagery. One that I particularly like is the College of Du Page Satellite Imagery Page. We will spend a little time in class looking at some images.