Prioritize...
Water vapor imagery can be a challenging topic! At the completion of this section, you should be able to...
- describe what is displayed on water vapor satellite imagery and correctly interpret water vapor images.
- explain the difference between using a wavelengths between roughly 6 and 7 microns versus 10-13 microns.
- explain what is meant by the term "effective layer" and discuss the implications of a warm versus cold effective layer.
- explain what information is not obtainable from a water vapor image and what features are almost never observed on such images.
As with the other sections on satellite imagery, it is important that you be able to differentiate a water vapor image from visible, traditional IR, and radar imagery. You should be able to point to certain clues that tell you that you are looking at a water vapor image and not one of the other types.
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Our look at visible and infrared imagery has hopefully shown you that using a variety of wavelengths in remote sensing is helpful because this approach gives us a more complete picture of the state of the atmosphere. Meteorologists can use visible and infrared imagery to look at the structure and movement of clouds because these types of images are created using wavelengths at which the atmosphere absorbs very little radiation (so radiation reflected or emitted from clouds passes through the clear air to the satellite without much absorption). Now, what if we took the opposite approach? What if we looked at a portion of the infrared spectrum where atmospheric gases (namely water vapor) absorbed nearly all of the terrestrial radiation? Water vapor imagery uses this exact approach.
In case you didn't catch it in the paragraph above, let me be clear: Water vapor imagery is another form of infrared imagery, but instead of using wavelengths that pass through the atmosphere with little absorption (like traditional infrared imagery, which utilizes wavelengths between roughly 10 and 13 microns), water vapor imagery makes use of slightly shorter wavelengths between about 6 and 7 microns. As you can tell from our familiar atmospheric absorption chart, these wavelengths are mostly absorbed by the atmosphere, and by water vapor in particular. Therefore, water vapor strongly emits at these wavelengths as well (according to Kirchoff's Law). Thus, even though water vapor is an invisible gas at visible wavelengths (our eyes can't see it) and at longer infrared wavelengths, the fact that it emits so readily between roughly 6 and 7 microns means the radiometer aboard the satellite can "see" it.
This fact makes the interpretation of water vapor imagery different than traditional infrared imagery (which is mainly used to identify and track clouds). Unlike clouds, water vapor is everywhere. Therefore, you will very rarely see the surface of the earth in a water vapor image (except perhaps during a very dry, very cold Arctic outbreak). Secondly, water vapor doesn't often have a hard upper boundary (like cloud tops). Water vapor is most highly concentrated in the lower atmosphere (due to gravity and proximity to source regions like large bodies of water), but the concentration tapers off at higher altitudes.
The fact that water vapor readily absorbs radiation between roughly 6 and 7 microns also raises an interesting question: Just where does the radiation that ultimately reaches the satellite originate from? The answer to that question is the effective layer, which is the highest altitude where there's appreciable water vapor. Above the effective layer, there is not enough water vapor to absorb the radiation emitted from below, nor is there enough emission of infrared radiation to be detected by the satellite. Any radiation emitted below the effective layer is simply absorbed by the water vapor above it.
In our previous discussion of traditional infrared imagery, I'm not sure if you realized that the radiation detected by the satellite only came from one distinct level in the atmosphere at a given point. If the column was clear, then the surface was detected; however, if the column contained clouds, then only the top-most layer of clouds was observed. The surfaces that emit the radiation that the satellite "sees" (highest cloud tops or the ground in the case of traditional IR imagery) are the "effective layers." A universal property of an effective layer is that only emissions from this layer are observed by the satellite. For a visual, consider emissions at a representative wavelength useful for traditional infrared imagery (10.7 microns, for example) from a cloudy atmospheric column (toward the left on the schematic below).
In the column with clouds, radiation emitted from the top of the cloud reaches the satellite because no appreciable liquid water or ice exists above the cloud, giving the radiation a "free pass" to the satellite. Below the observed cloud layer (that is, the effective layer), any emissions from liquid water and ice are absorbed by the cloud layer that lies above them. Of course, if the air column is free of clouds, then the ground is the effective layer at longer infrared wavelengths, because the emissions that the satellite radiometer sees are coming from the ground (column farthest to the left in the graphic above).
Now let's carry this idea over to water vapor imagery (refer to the right portion of the above schematic). At the wavelengths used for water vapor imagery (between roughly 6 and 7 microns), water vapor very effectively absorbs and emits radiation. Another way to think about it is that at a wavelength like 6.7 microns (the sample wavelength used in the schematic), water vapor radiates just like liquid water and ice do at 10.7 microns. So, water vapor is an invisible gas at visible wavelengths and longer infrared wavelengths, but it "glows" at wavelengths around 6 to 7 microns.
The bottom line is that, the effective layer is the source region for the radiation detected by the satellite. It's the highest layer of appreciable water vapor, and above the effective layer, there is not enough water vapor to generate a signal to be observed by the satellite. And as with clouds in the traditional IR example, any radiation emitted below the effective layer is simply absorbed by the water vapor above it. Therefore, the satellite measures the radiation coming only from the effective layer, and like traditional infrared imagery, this radiation intensity is converted to a temperature, which means water vapor imagery displays the temperature of the effective layer of water vapor, although not all images you'll find online will contain a specific color temperature scale. Commonly, water vapor imagery uses shades of gray, with warmer (lower) effective layers shown as dark and colder (higher) effective layers shown in white. Many sites will add color enhancements to identify key temperatures like with traditional infrared imagery, but color schemes vary from website to website.
You may hear on television or see other online explanations that suggest water vapor imagery measures the water vapor content of the atmosphere, but that's not really true. We can infer certain things about the moisture profile of the atmosphere based on the temperature of the effective layer, but the satellite isn't actually measuring the amount of water vapor present in order to create water vapor images, and it tells us nothing about water vapor below the effective layer. So, what can we infer by knowing the temperature of the effective layer? Check out the short video (2:43) below:
Click here for a transcript of the video.
PRESENTER: We have here a color-enhanced water vapor image, and we’re going to see how to interpret this image. First, let’s get our bearings with the color scale along the bottom. Lower temperatures are color coded in pinks, blues, greens, and purples. Meanwhile, higher temperatures are either in shades of gray or in orange or red for the highest temperatures on this particular image – color schemes can vary, though from website to website.
If we make the same assumption we did with traditional infrared imagery – that temperature decreases with increasing height in the troposphere, then we can make meaning out of these temperatures. Basically, a colder effective layer means the effective layer is higher in the troposphere, and if we know the height of the effective layer, we can infer the depth of the dry air above it. With water vapor imagery, we can’t assume anything about what lies below the effective layer because all of the emissions from below are being absorbed by the effective layer.
So, let’s start with one of the warmer effective layers on this map – over eastern Texas in the dark gray shading. Our color scale tells us that the temperature of the effective layer is approaching -20 degrees Celsius. Using another tool, I looked up the temperature profile in this region at the time, and this temperature corresponded to a height a little above 20,000 feet, which is in the middle part of the troposphere. So, we can infer that the upper troposphere was dry here because all the meaningful water vapor was roughly 20,000 feet and below.
Now let’s pick a point here in eastern Kansas, where there’s more of a grayish white shading, which corresponds to about -35 degrees Celsius. Again, looking up the temperature profile, this temperature corresponded to a height of almost 30,000 feet, which is the upper troposphere, so we can conclude that there was more water vapor in the upper troposphere over eastern Kansas than there was over east Texas.
This area near the Kansas / Nebraska border has some of the lowest temperatures on the map – a very cold effective layer of around -60 degrees Celsius. On this date, that temperature was up near 40,000 feet, at the very top of the troposphere. Such a cold, high effective layer can only be caused by high ice clouds typical of the tops of cumulonimbus clouds. I should point out that at such low temperatures very little water exists in the vapor phase. However, ice crystals also have a fairly strong emission signature between 6 and 7 microns, so if you see such cold effective layers (say less than about -45 degrees Celsius or so), you are most likely looking at ice clouds (like cirrus, cirrostratus, or cumulonimbus tops) rather than at just water vapor. And, in fact in this case, this was an area of budding thunderstorms.
In the video, did you notice that the highest effective layer we observed was at the top of the troposphere, near 40,000 feet, and was actually most likely emissions from ice crystals (ice crystals also emit very effectively between 6 and 7 microns) in the tops of cumulonimbus clouds? Meanwhile, the lowest effective layer that we observed was near 20,000 feet? That's not uncommon. Because emissions from water vapor near the earth's surface are absorbed by water vapor higher up, it's often impossible to detect features at very low altitudes. In other words, low clouds (stratus, stratocumulus, nimbostratus, and fair weather cumulus) are rarely observable on water vapor imagery.
To see what I mean, check out the pair of satellite images below (infrared on the left, water vapor on the right). The yellow dot represents Corpus Christi, Texas, which was shrouded in low clouds (gray shading on the infrared image -- check out the meteogram for Corpus Christi). Now examine the water vapor image. This image uses traditional grayscale, so the dark shading on the water vapor image indicates a warm effective layer located in the middle troposphere. However, we can't see even a hint of low clouds! In this case, the effective layer (located above the low clouds) absorbed all of the radiation emitted from below, rendering the low clouds undetectable on the water vapor image. For another example of low clouds not appearing on water vapor imagery, check out the Case Study section below.
How Low Can Water Vapor Imagery Go?
If you look back carefully at our familiar atmospheric absorption spectrum, notice that absorption (and therefore emission) by water vapor isn't uniform in the range of wavelengths used for water vapor images (roughly 6 to 7 microns). Indeed, toward the higher end of the range, absorption is less than 100 percent, and using the different absorption and emission properties of water vapor near 7 microns allows satellites to "see" effective layers in different layers of the troposphere. Therefore, you'll sometimes find water vapor images labeled "upper-level", "mid-level" or "lower-level." While the altitude of the effective layer on any of these images varies based on the amount of water vapor in an air column (and how it's distributed), make sure that you're not fooled by these names. Even "lower-level" water vapor imagery typically detects effective layers between roughly 7,500 feet and 18,000 feet. In other words, most often, you're looking at emissions from effective layers of water vapor in the middle troposphere, even on so-called "lower-level" water vapor imagery.
Therefore, even "lower-level" water vapor imagery still can't often detect surface water vapor or the presence of low clouds. For example, check out this side-by-side comparison of a visible image and "lower-level water vapor" image. On this water vapor image, shades of yellow and orange mark regions with a warmer effective layer. Note that the lower-level water vapor image provides no indication of the presence of low clouds whatsoever (especially notable over Illinois and Indiana), because their tops were located below the effective layer at this time (their emissions were absorbed by water vapor higher up). The bottom line is that even on "lower-level" water vapor images, you cannot see near-surface water vapor, fog, or low clouds, unless the atmospheric is extremely dry higher up (which is only possible in very cold, dry Arctic air).
Now that we've discussed how to interpret water vapor imagery, what might we use it for? Forecasters most often use water vapor imagery to visualize upper-level circulations in the absence of clouds. This is because water vapor is transported horizontally by high-altitude winds and thus can act like a tracer, much like smoke from an extinguished candle (as in the photo on the right). Consider this enhanced IR satellite loop and focus your attention on the Southwest. Since there are no clouds present, we can't really tell how the air is moving over this region. Now, check out the corresponding loop of water vapor images and focus your attention on the same area. What do you see? Do you notice the ever-so-slight counter-clockwise circulation of the air off the California coast? Such upper-level circulations are in fact important, as we will learn later in this course. The lesson learned here is that we were able to identify this circulation only with the aid of water vapor imagery.
Water vapor imagery's ability to trace upper-level winds ultimately allows forecasters to visualize upper-level winds, and computers can use water vapor imagery to approximate the entire upper-level wind field. Here's an example of such "satellite-derived winds" in the middle and upper atmosphere at 12Z on September 28, 2022 (toward the left side of the image, you can see Hurricane Ian about to make landfall in Florida). Having such observations over the data-sparse oceans is extremely valuable to forecasters, and much of this information gets put into computer models so that they better simulate the initial state of the atmosphere, which leads to better forecasts than if we didn't have these observations.
This concludes our look at the three most common types of satellite imagery. Before moving on to radar imagery, take a moment to review the key points about water vapor imagery as well as the Case Study below.
Water Vapor satellite 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 of water vapor. Warm effective layers mean that upper troposphere and possibly parts of the middle troposphere are "dry" (they contain very little water vapor). 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.
Case Study...
You saw some cases above showing that water vapor imagery typically does not show the presence of low clouds or water vapor near the surface. Check out the short video below (2:03) for another example -- this time in an extremely moist low-level environment.
PRESENTER: It’s important to remember that water vapor imagery very rarely gives us insights about surface or near-surface moisture. For example, check out this water vapor image of North America and the western Atlantic Ocean in the image on the left, and focus in on the Caribbean Sea. Note the general dark shading in the region, indicating a relatively warm effective layer and a dry upper atmosphere. The zoomed in version on the right focusing on Puerto Rico, Hispaniola, and much of the Caribbean Sea gives us a better look at exactly where the dark shading is located. It certainly includes Puerto Rico and Hispaniola.
But, don’t let the dark shading cause you to conclude that the entire air column is dry. Adding surface station models to the water vapor image shows surface dew points of 72 degrees at these stations in the Dominican Republic and Puerto Rico. So, concentrations of water vapor near the surface are quite high – the low-level air mass is moist, but you would never know it from the appearance of the water vapor image because radiation from the large amounts of water vapor near the surface is absorbed by water vapor higher up in the middle regions of the atmosphere.
Furthermore, the station models indicate varying degrees of partly cloudy skies. The clouds that were present were fair-weather cumulus clouds – shallow puffy clouds that often dot the tropical sky. They usually have tops that are only several thousand feet above the ground, and radiation from the tops of these clouds was being absorbed by water vapor above, which cloaks these low-topped clouds from the satellite radiometer’s view.
Rare exceptions do occur, when water vapor from the lower troposphere does appear on water vapor images. That can sometimes occur when columns of air are extremely dry, and there’s not enough water vapor in the middle or upper troposphere to absorb emissions from water vapor near the surface or from the tops of low clouds, but typically indications of water vapor near the surface or low topped clouds do not appear on water vapor images.