Tag Archives: Fire Temperature RGB

Fires near the hottest city in the Arctic

June 30, 2020UncategorizedArctic, Day Land Cloud Fire RGB, fire, Fire Temperature RGB, GeoColor, Himawari-8, resolution, Siberia, Snow RGB, True Color RGBCurtis Seaman

Something incredible happened in the Arctic a few days ago. Rather than type it out, I will let the World Meteorological Organization (WMO) explain it:

In case you’re wondering where Verkhoyansk is, you can look it up on Google Maps.

If that temperature is verified, it would be the hottest temperature ever recorded north of the Arctic Circle. To put that 38 °C into perspective, that was only 2 °C off from the high temperature in Phoenix, AZ on 20 June 2020 – a place where 40 °C is normal in the summer. It’s also worth reiterating how unusual it is for any location to average 8-10 °C (15-20 °F) above normal for an entire month. Russia as a whole – by far the largest country on Earth – has averaged 8 °C above normal for the entire first half of 2020!

If you clicked that last link, you saw an excerpt of the video above, plus more information on the unusual impacts of this heatwave. The clouds of mosquitoes. The collapsing buildings due to melting permafrost. (One of the largest oil spills ever in the Arctic happened in May, caused by melting permafrost.) And, an even more alarmist impact of the heat: “zombie fires“.

That’s right – if you didn’t have enough with the coronavirus or the murder hornets or the melting Arctic, you can now panic about zombie fires. In all seriousness, the silly name has been applied to the phenomenon of fires in peat bogs never really being fully extinguished, and continuing to smolder deep down below the ice and snow that covered it up all winter. Then, when a heatwave happens the next summer, the smoldering turns to re-ignition of the fire, and it once again appears on the surface.

Fires have been happening on a massive scale throughout Siberia this summer (and they’re probably not all zombie fires). But, we have a tool to observe them from satellite: Polar SLIDER.

I’m sure there may be a few of you who are already familiar with the website. For those who aren’t, here’s a brief synopsis: Polar SLIDER is designed to show the most recent VIIRS imagery available anywhere on Earth in as close to real-time as possible. Images from individual orbits from both Suomi-NPP and NOAA-20 are stitched together to create hemispheric composites that always feature the most recent imagery on top. The way the orbits work, when Suomi-NPP is crossing over into the Southern Hemisphere, NOAA-20 is crossing into the Northern Hemisphere (and vice versa). By combining imagery from both satellites, there is a ~50 minute refresh over the poles, giving a quasi-geostationary satellite view of each pole. Imagery is available at six different zoom levels, separated by factors of 2, so you can zoom in to see full resolution VIIRS imagery anywhere on Earth.

Here’s an example of Polar SLIDER, reduced in size to play well with this blog software, showing our GeoColor product (True Color imagery during the day, blended with the Day/Night Band and a low cloud detection algorithm at night) over Siberia on 23-24 June 2020:

VIIRS GeoColor animation (23-24 June 2020)

How much smoke can you see? Did you count the plumes? Did you see the swirl in the smoke at about 70°N, 140°E? (For reference, Verkhoyansk is near 67°N, 133°E.)

That loop covers approximately 30 hours in the Arctic and, since we’re so close to the summer solstice, you can estimate the location of the Arctic Circle, even though it isn’t plotted.

Even those of you who have heard of (or seen) Polar SLIDER before might not be aware of the recent upgrades made in May 2020. For the first 18 months of its existence, Polar SLIDER had all 22 VIIRS channels (DNB, 16 M-bands, 5 I-bands) plus GeoColor (as shown in the loop above) and, occasionally, the I-band Natural Color (aka Day Land Cloud RGB) product. Now, we have added 10 new products, including the most popular RGB composites (the ones that are available from VIIRS, anyway) and two new RGBs for snow monitoring that utilize the 1.24 µm band that are not available on any geostationary satellite. (More information on those is available here.) We’ve also fixed the issues with the Natural Color RGB, making it a permanent fixture, rather than an anomaly.

Among the new products available on Polar SLIDER is what we call Natural Fire Color (and the National Weather Service calls “Day Land Cloud Fire RGB“), made from VIIRS bands I-1 (0.64 µm, blue), I-2 (0.86 µm, green) and I-4 (3.7 µm, red). As it is made from VIIRS I-bands, it is available at 375 m resolution around the globe. Here’s what it shows from 22-23 June 2020 over this part of Siberia:

VIIRS Day Land Cloud Fire RGB animation (22-23 June 2020) – click to play

This animation is too large for WordPress. You have to click on it to get it to play. But, I couldn’t resist showing the full resolution imagery. Also, a note about the timestamps on Polar SLIDER: it takes ~50 min for each satellite to cover each hemisphere, and the image times displayed on Polar SLIDER represent the Equator-crossing time as the satellite leaves the hemisphere, which is most likely not the time the satellite was viewing the area you’re looking at.

The Natural Fire Color/Day Land Cloud Fire loop covers a ten hour period from ~ 8:00 AM to 6:00 PM local time (depending on where you are in the scene, it might be 7:00 AM to 5:00 PM), during which time there were 11 consecutive VIIRS overpasses over this region between the two satellites. This is a textbook example of how fires typically die down at night (or, at least, when the sun is hovering over the horizon) and intensify during the heating of the day.

Of course, you can get a better idea of the intensity of the fires by looking at the Fire Temperature RGB, which is also now on Polar SLIDER:

VIIRS Fire Temperature RGB animation (22-23 June 2020) – click to play

Once again, you have to click on the animation to get it to play.

The Fire Temperature RGB is made with VIIRS M-bands (750 m resolution), so the fires don’t look as crisp when viewed at the 375 m zoom level. But, since it uses more information from fire-sensitive bands in the shortwave IR, it provides a qualitative estimate of fire intensity, not just the locations of the active hot spots. (As fires become more intense, their color changes from red to orange to yellow to white in the Fire Temperature RGB.)

Other differences to note between the two loops are: the Natural Fire Color RGB shows the reddish-brown burn scars more clearly amongst a background of green vegetation; it shows the bluish smoke more clearly; and it shows ice in the Arctic Ocean, which appears nearly black in the Fire Temperature RGB. We’ve covered all of this before, both here and elsewhere. We’ve also covered the importance of VIIRS’ high resolution (compared to geostationary satellites) when it comes to fires before. But, it’s worth looking at again. Compare the loops above with the view from the Advanced Himawari Imager (AHI) on Himawari-8:

AHI Day Land Cloud Fire RGB animation (23 June 2020) – click to play

AHI Fire Temperature RGB animation (23 June 2020) – click to play

You can find a loop of the AHI GeoColor showing the smoke plumes here.

It’s difficult to identify any fires in the AHI Natural Fire Color/Day Land Cloud Fire RGB, given the resolution of the 3.9 µm channel is 2 km at the Equator (more like 3-6 km in this part of the world) – not 375 m like the VIIRS version. Hot spots show up better in the AHI Fire Temperature RGB this far north, because this combination of channels makes the background surface appear darker relative to the pixels with active fires in them, whereas the background is brighter in the Natural Fire Color RGB.

Lastly, because I mentioned new RGBs for snow on Polar SLIDER, one of them has an interesting artifact when it comes to fires. The Snow RGB originally developed by MétéoFrance utilizes the 2.25 µm band as the blue component, making hot spots appear blue:

VIIRS MeteoFrance Snow RGB composite of channels M-11, M-8 and M-7 (23 June 2020) — VIIRS MétéoFrance Snow RGB composite of channels M-11, M-8 and M-7 (23 June 2020)

Of course, if you’re looking for fires, don’t reach for the Snow RGB. But, someone, somewhere is going to be looking at the Snow RGB when they spot a couple of bright blue pixels and wonder, “What’s going on here?” And, I’m here to say, “Those are moderately intense fires.”

A four-birds-eye-view of fires in Alberta

May 29, 2019UncategorizedAlberta, Canada, fire, Fire Temperature RGB, GOES-16, GOES-17, resolution, smoke, True Color RGBCurtis Seaman

It has been three years since the devastating Fort McMurray Fire – the costliest natural disaster in Canada’s recorded history. The city is still rebuilding. And, I’m sure the people of Alberta don’t want to be reminded of it. I hate to be the bearer of bad news but, here we go again! This time, it is the town of High Level, Alberta that has been evacuated. This is one of four “out of control” fires currently burning in Alberta:

Map of known fires from Alberta Wildfire (24 May 2019)

The red flame icons represent “out of control” fires, while the green ones are “under control”. This map was produced by Alberta Wildfire on 24 May 2019. The northernmost red flame represents the fire near High Level. According to Alberta Wildfire, the southern two fires started on 18 May 2019, while the other two started on 11-12 May 2019. But, as you shall see, they became out of control on 18 May.

This being Alberta, satellites are often the first source for spotting fires in these remote areas. Plus, Alberta is in a unique position: it is far enough east to be within the view of GOES-16 and far enough west to be within view of GOES-17. It is also far enough north to get several overpasses from VIIRS on both Suomi-NPP and NOAA-20 each day. So, what do these satellites have to say about these fires? Let’s take a look.

We’ll begin with GOES-16, which is now operational as “GOES-East” and has been since December 2017. Since April 2019, the GOES-16 ABI has been running in Mode 6 operations. This means full disk images every 10 minutes. Here is the GeoColor loop (link goes to PDF file) from GOES-16, starting at 1700 UTC on 18 May 2019 and running until sunset (~0400 UTC 19 May 2019):

For the uninitiated, GeoColor is a blend of true color imagery during the day with a low cloud/fog detection product at night. True color is particularly useful for detecting smoke, as we have seen before. So, did you see it in the above loop?

I’ll admit, the clouds make it difficult to see, but there are two smoke plumes visible in that video. (It helps to view the video in full screen mode.) The smoke plume for the fire near High Level shows up first (around 1940 UTC), then the smoke from the pair of fires northeast of Lesser Slave Lake appears around 2150 UTC and become obvious around 0000 UTC on 19 May.

Here is the Fire Temperature RGB from GOES-16 ABI over the same time period:

This product is sensitive to hot spots – not smoke – but it still has an issue with clouds, which block the signal. Notice that the northernmost fire near High Level first appears in the Fire Temperature RGB around 1840 UTC – an hour before the smoke is really evident. The first fire northeast of Lesser Slave Lake is visible starting at 2200 UTC (10 min. after the smoke is visible), and the second one first appears at 2320 UTC (about 20 min. after its smoke plume appears). The fourth fire (the westernmost of all of them, and the one in the middle north-to-south) first appears at 2100 UTC, although it is in and out of the clouds, and its smoke plume is never very visible.

Compare that with GOES-17 ABI, which is now operational as “GOES-West” (since February 2019), and is also running Mode 6 operations:

These videos cover the same time period as the GOES-16 versions.

Moving clockwise starting at the fire near High Level, the first sighting of smoke according to GOES-17 is around 1900 UTC, 2200 UTC (first Lesser Slave Lake fire), 0000 UTC (second Lesser Slave Lake fire), and 2310 UTC (westernmost fire). This is 40 min. earlier, similar to, similar to, and not easily comparable to GOES-16. (Since GOES-16 never got a clear look at the smoke from the westernmost fire, it’s difficult to compare times against GOES-17, which was able to detect smoke from that fire. Also, I say a 10 minute difference is “similar to”, since it is a judgement call as to which image an analyst would confidently say they first saw the smoke or the hot spot.)

Comparing the hot spots in the same clockwise fashion, the time of first detection was 1820 UTC, 2140 UTC, 2320 UTC and 2050 UTC. This is 20 min. earlier, 20 min. earlier, similar to, and similar to GOES-16. If you don’t believe me, compare them frame by frame in this video:

GOES-16 is on the right and GOES-17 is on the left. When you can directly compare them like this, a few things jump out. 1) The fire near High Level appears to grow more in GOES-17 than it does in GOES-16. 2) The two fires near Lesser Slave Lake appear so close together in GOES-17 that they are difficult to distinguish as separate fires, although they are clearly separate fires according to GOES-16. 3) The westernmost fire is the most difficult to see due to the presence of clouds.

Here, it is important to note a few things. A) Alberta is generally closer to GOES-17 than it is to GOES-16. B) The position the clouds and the motion of the fire relative to the lines of sight of the satellites lead to significant differences between GOES-16 and GOES-17 in this case. C) The plural of “anecdote” is not “data”. The exact circumstances that apply here are not going to apply in the future but the concepts still will.

The fire near High Level moves perpendicular to the GOES-17 line of sight, and moves parallel to the GOES-16 line of sight (away from the satellite on a curved surface to boot!) with slightly more coarse resolution in GOES-16. That explains the differences in apparent motion between the two views. The differences in viewing angle also explain why the fires near Lesser Slave Lake appear separate (GOES-16) or together (GOES-17). And, clouds blocking the line of sight explain why the fire near High Level appears later in GOES-16, while GOES-17 had a clear view of the fire 20 minutes earlier. The differences in perspective offered by the two satellites also make it tough to tell which fire is furthest west (the one I’ve been calling “westernmost”).

Both geostationary satellites are viewing Alberta at a high angle. What about VIIRS on Suomi-NPP and NOAA-20, which flies more-or-less directly overhead? Since these satellites are polar-orbiting, they can’t provide 10 min. imagery of the fires. But, they do combine to provide ~50 min. imagery of the fires for a few orbits in the early afternoon and early morning hours, and they have much better spatial resolution.

The first VIIRS overpass was from NOAA-20 at around 1920 UTC, and here’s what it saw (according to the Fire Temperature RGB):

NOAA-20 VIIRS Fire Temperature RGB (1920 UTC, 18 May 2019)

This image is about an hour after GOES-17 first spotted the fire near High Level, which is the most obvious fire in the scene. In fact, the westernmost fire is hidden under the clouds and the fires near Lesser Slave Lake haven’t started yet. The closest time matched images with both GOES are provided below.

GOES-17 (left) and GOES-16 (right) Fire Temperature RGB (1920 UTC 18 May 2019)

Approximately 50 min. later, Suomi-NPP passed overhead and provided this view:

S-NPP VIIRS Fire Temperature RGB (2011 UTC, 18 May 2019) — S-NPP VIIRS Fire Temperature RGB (20:11 UTC, 18 May 2019)

And the GOES view from approximately the same time:

GOES-17 (left) and GOES-16 (right) Fire Temperature RGB (2010 UTC 18 May 2019)

Here the fire near High Level is quite a bit more intense, as you could see if you were to toggle back and forth between the two VIIRS images. This is a definite sign that the fire is a dangerous one! Still no sighting of the Lesser Slave Lake fires, which didn’t get going for another 110 minutes or so, according to both GOES. There was another NOAA-20 overpass at 2101 UTC, then a Suomi-NPP overpass that caught the western portion of the scene at 2151 UTC. Combine all four overpasses in an animation, and you get this:

Animation of VIIRS Fire Temperature RGB images (18 May 2019)

Remember to click on the image to get the animation to play. If you look closely, you can see both of the fires northeast of Lesser Slave Lake in the NOAA-20 image from 2101 UTC – a full 50 minutes earlier than either GOES was able to detect them. But, they are just single pixels at this point, making them likely too small for ABI to detect them (particularly at such a high viewing angle).

As for the smoke, it’s not any easier to detect with VIIRS given all the clouds overhead (click to play):

Animation of VIIRS True Color images (18 May 2019)

You might have noticed in the above animation that Lake Athabasca is still covered in ice, which is not apparent in the Fire Temperature RGB. That means fire season in Alberta started before all the ice had melted!

Over the next few days, clouds weren’t as much of a problem so the hot spots and smoke plumes were easily visible from all four satellites. Here are direct comparisons between GOES-16, GOES-17 and the two VIIRS for 19 May 2019:

NOAA-20 VIIRS Fire Temperature RGB (1901 UTC, 19 May 2019)

GOES-17 (left) and GOES-16 (right) Fire Temperature RGB (1900 UTC, 19 May 2019)

S-NPP VIIRS Fire Temperature RGB (1959 UTC, 19 May 2019)

GOES-17 (left) and GOES-16 (right) Fire Temperature RGB (2000 UTC, 19 May 2019)

We can even do better than that and really zoom in:

Comparison between GOES-17 ABI, NOAA-20 VIIRS, and GOES-16 ABI Fire Temperature RGB images (1900 UTC, 19 May 2019) zoomed in at 400%

Comparison between GOES-17 ABI, S-NPP VIIRS, and GOES-16 ABI Fire Temperature RGB images (2000 UTC, 19 May 2019) zoomed in at 400%

Here, the previous images have been cropped to the fire near High Level, and zoomed in 400%. The difference in resolution between VIIRS and ABI at this latitude is obvious.

Here is a four day loop of True Color RGB images from both VIIRS (click to play):

Animation of VIIRS True Color images (18-21 May 2019)

And, click on these links for similar loops of GeoColor from GOES-16, GOES-17 and a side-by-side comparison of the two. (The videos are too large to embed here.)

And, finally, here is a four day loop of Fire Temperature RGB images from both VIIRS (click to play):

Animation of VIIRS Fire Temperature RGB images (18-21 May 2019)

And links to MP4 video files from GOES-16, GOES-17 and a side-by-side comparison of the two.

The Land of 10,000 Fires

July 8, 2015UncategorizedAlaska, fire, Fire Temperature RGB, Natural Color RGB, optical effects, resolution, RGB composite, smoke, True Color RGBCurtis Seaman

Minnesota calls itself the “Land of 10,000 Lakes” – they even put it on their license plates. To an Alaskan, it seems funny to brag about that since Alaska has over 3,000,000 lakes. That’s like a Ford Escort bragging to a Bugatti Veyron that it can achieve highway speeds!

Alaska has Minnesota beat in one other area, and it’s one they’re definitely not putting on their license plates: the number of wildfires. OK, so it may not be 10,000 fires as my title implies, but there sure are a lot:

AICC Fire Map (28 June 2015) — Map of known wildfires (23 June 2015), courtesy Alaska Interagency Coordination Center

That map shows the number of known wildfires in Alaska on 23 June 2015 and was produced by the Alaska Interagency Coordination Center (AICC). To say that 2015 has been an active fire season in Alaska is an understatement. That would be like saying a Bugatti Veyron is a vehicle capable of achieving highway speeds! (By the way, if anyone in the audience works at Bugatti, and would like to compensate me for this bit of free advertising by, say, giving me a free Veyron, it would be much appreciated.)

Imagine being the person responsible for keeping track of all these fires! (That’s what the good folks at AICC do on a daily basis. There’s also a graduate student at the University of Alaska-Fairbanks working on this very same problem, who has come up with this solution.) This is being called the worst fire season in Alaska since, well, the beginning of recorded history. 2004 was the worst on record, but 2015 is on pace to shatter that. By 26 June 2015, Alaska’s fires had burned over 1.5 Rhode Islands worth of land area (or, alternatively, 0.8 Delawares). By 2 July, the total burned acreage had achieved 3 Rhode Islands (1.6 Delawares; 2/3 of a Connecticut). On 7 July, the total hit 3,000,000 acres (1 Connecticut; 2.5 Delawares; 5 Rhode Islands). (2004 ended at 2 Connecticuts worth of land burned, so there is a chance the pattern could switch and Alaska will get enough rain to fall short of the record, but at this rate, that seems unlikely.)

It’s interesting to see how this came to be, given that there were only a couple of fires burning in the middle of June:

VIIRS Fire Temperature RGB composite of channels M-10, M-11 and M-12 (21:28 UTC 15 June 2015)

If you followed this blog last year, you should know about this RGB composite, called “Fire Temperature.” If not, read this and this. Click to the full size image and see if you can see the six obvious fires. (Two are in the Yukon Territory.) Now, count up the number of fires you see in this image from just one week later:

VIIRS Fire Temperature RGB composite of channels M-10, M-11 and M-12 (22:16 UTC 23 June 2015)

Why so many fires all of a sudden? Well, it has been an abnormally dry spring following a winter with much less snow than usual. Plus, there have been a number of dry thunderstorms that produced more lightning than rain. You can see them in the image above as the convective clouds, which appear dark green because they are topped with ice particles. (Ice clouds appear dark green in this composite. Liquid clouds appear more blue.) A number of thunderstorms filled with lightning formed on the 19^th of June, and a lot of fires got started shortly after. Here is an animation of Fire Temperature RGB images from 15-25 July (showing only the afternoon VIIRS overpasses):

Animation of VIIRS Fire Temperature RGB images (15-25 June 2015)

It’s difficult to see the storms that led to all these fires, because the storms don’t last long and they typically form and die in between images. Plus, some of the fires may have started from hot embers of other fires that were carried by the wind.

Of course, when there’s smoke there’s fire. I mean – when there’s fire there’s smoke. Lots of it, which you’d never be able to tell from the Fire Temperature RGB. The Fire Temperature RGB uses channels at long enough wavelengths that it sees through the smoke as if it weren’t even there. But, the True Color RGB is very sensitive to smoke. Here’s a similar animation of True Color images:

Animation of VIIRS True Color RGB images (16-25 June 2015)

Look at how quickly the sky fills with smoke from these fires. And also note that the area covered by smoke by the end of the loop (25 June 2015) is too large to be measured in Delawares – units of Californias might be more useful.

The last frame in each animation comes from the VIIRS overpass at 21:30 UTC on 25 June 2015. It’s nice to know that you can still detect fires in the Fire Temperature RGB even with all that smoke around.

Another popular RGB composite to look at is the so-called “Natural Color”. This is the primary RGB composite that can be created from the high-resolution imagery bands I-1, I-2 and I-3. The Natural Color RGB is sort-of in-between wavelengths compared to the Fire Temperature and True Color. The True Color uses visible wavelengths (0.48 µm, 0.55 µm and 0.64 µm), the Fire Temperature uses near- and shortwave infrared wavelengths (1.61 µm, 2.25 µm and 3.7 µm), and the Natural Color spans the two (0.64 µm, 0.87 µm and 1.61 µm). This means the Natural Color is not as sensitive to smoke and not as sensitive to fires – except in the case of very intense fires and very thick smoke plumes.

Well, guess what? These fires in Alaska have been intense and have been putting out a lot of smoke, so they do show up. Here’s a comparison between the True Color and Natural Color images from 19 June 2015:

Animation comparing the True Color and Natural Color RGB composites (21:50 UTC 19 June 2015)

Thin smoke is invisible to the Natural Color, but thick smoke appears blueish (because the blue component at 0.64 µm is the most sensitive to it). As you go from visible wavelengths to near-infrared wavelengths, the smoke’s influence on the radiation transitions from Rayleigh scattering to Mie scattering, and the light is scattered more in the forward direction. This makes the smoke much more visible in the Natural Color composite when the sun is near the horizon, as in this image from 13:47 UTC on 24 June:

VIIRS Natural Color RGB composite of channels I-1, I-2 and I-3 (13:47 UTC 24 June 2015)

Notice the red spot in the clouds at 154 °W, 65 °N? (Click on the image to zoom in.) Here, the smoke plume is optically thick at 0.64 µm (blue component) and 0.87 µm (green component), but transparent at 1.61 µm (red component). It’s like the smoke is casting a shadow in two of the three wavelengths, but is invisible in the other. It’s the combination of large smoke particles and large solar zenith angle creating a variety of Rayleigh and Mie scattering effects leading to this interesting result.

A more dramatic example of this can be seen from the 12:57 UTC overpass on 21 June:

VIIRS Natural Color RGB composite of channels I-1, I-2, and I-3 (12:57 UTC 21 June 2015)

Notice the reddish brown band of clouds just offshore along the coast of southeast Alaska.

But, that’s not all! Really intense fires may be visible at 1.6 µm, so it’s possible the Natural Color composite can see them. Here’s the Natural Color composite from 22:09 UTC on 4 July zoomed in on an area of intense fires:

VIIRS Natural Color RGB composite of channels I-1, I-2 and I-3 (22:09 UTC 4 July 2015)

Notice the salmon- and red-colored pixels at the edges of some of the smoke plumes? Those are very intense hot spots showing up at 1.6 µm (I-3). In fact, the fires were so intense that they saturated the sensor at 3.7 µm (I-4) and this lead to “fold-over“:

VIIRS shortwave IR (I-4) image (22:09 UTC 4 July 2015). The color scale highlights pixels with a brightness temperature above 340 K.

Fold-over is when the sensor detects so much radiation above its saturation point, the hardware is “tricked” into thinking the scene is much colder than it is. In the image above, colors indicate pixels with a brightness temperature above 340 K. The scale ranges from red at 340 K to orange to yellow at 390 K. Channel I-4 reaches its saturation point at 368 K. Notice the white and light gray pixels inside the hot spots: the reported brightness temperature in these pixels is ~ 210 K – much colder than everything else around – even the clouds! This is an example of “fold-over”.

The reddish pixels in the Natural Color image match up very closely with the saturated, “fold-over” pixels in I-4:

Animation comparing the Natural Color RGB and I-4 images (22:09 UTC 4 July 2015)

What to do if we have fires saturating our sensor? Use M-13 (4.0 µm), which has a sensor designed to not saturate in these conditions:

VIIRS shortwave IR (M-13) image (22:09 UTC 4 July 2015)

Here, we have reached color table saturation (yellow is as high as it goes), but M-13 did not saturate. In fact, the “fold-over” pixels in I-4 have a brightness temperature above 500 K in M-13. That’s 130-140 K above the saturation point of I-4 (110-120 K above the top of the color table)! The lack of saturation is also why the hot spots appear hotter in M-13, even though it has lower spatial resolution:

Animation comparing VIIRS shortwave IR bands I-4 and M-13 (22:09 UTC 2015)

The fact that intense fires show up at 1.6 µm is part of the design of the Fire Temperature RGB. Most fires show up at 3.7 µm (red component). Moderately intense fires are also visible at 2.25 µm (green component) and will appear orange to yellow. Really intense fires, like these, appear at 1.6 µm (blue component) and will appear white (or nearly white):

VIIRS Fire Temperature RGB composite of M-10, M-11, and M12 (22:09 UTC 4 July 2015) — VIIRS Fire Temperature RGB composite of M-10, M-11, and M-12 (22:09 UTC 4 July 2015)

And, if you’re curious as to how all four of these images compare, here you go:

Animation comparing VIIRS images of intense wildfires (22:09 UTC 4 July 2015)

Shortwave infrared wavelengths are good for detecting fires, visible wavelengths are good for detecting smoke and the Natural Color composite, which uses wavelengths in-between, might just detect both – especially when intense fires exist.

Funny River Isn’t Laughing

May 23, 2014Uncategorizedfire, Fire Temperature RGB, I-4, Natural Color RGB, RGB composite, True Color RGB, twilightCurtis Seaman

Imagine you’re getting ready for bed. You take one last look out of your bedroom window and you see this:

Photo of Funny River Fire — Photo of the Funny River Fire, taken near 1:00 AM local time (0900 UTC), 21 May 2014. Courtesy Bill Roth/Alaska Dispatch.

Good luck sleeping!

That is the light and smoke from the Funny River Fire, which started on 20 May 2014 and rapidly grew to over 44,000 acres in under 48 hours. Rapidly expanding fires like this one burn through a lot of fuel and can create a lot of smoke. Enough smoke to be seen by radar:

And certainly enough smoke to be seen from space:

VIIRS "True Color" RGB Composite, taken 21:58 UTC 20 May 2014 — VIIRS “True Color” RGB Composite of channels M-3, M-4 and M-5, taken 21:58 UTC 20 May 2014

Look for the grayish plume arcing from the Kenai Peninsula out over the Gulf of Alaska. That is one impressive smoke plume!

The image above is what we like to call a “True Color” image. It is a combination of the red, green and blue visible-wavelength channels of VIIRS (M-5, 0.67 µm; M-4, 0.55 µm; M-3, 0.48 µm, respectively), so named because it represents the “true” color of objects as the human eye would see them. It is the most commonly used “RGB composite”, which is why a number of people simply refer to it as the “RGB”. To add more confusion, other people call it “Natural Color” because it is only an approximation of the “true” color and has to be corrected for atmospheric effects (i.e. Rayleigh scattering) to look right. However, we want to distinguish this from the EUMETSAT definition of “Natural Color”, which looks like this:

VIIRS "Natural Color" composite, taken 21:58 UTC 20 May 2014 — VIIRS “Natural Color” composite of channels I-1, I-2 and I-3, taken 21:58 UTC 20 May 2014

Notice that the smoke plume isn’t as easy to see in the Natural Color image. This is because we are looking at longer wavelengths {1.61 µm (I-3, red component), 0.87 µm (I-2, green component) and 0.64 µm (I-1, blue component)} and smoke scatters less solar radiation back to the satellite as the wavelength increases. This is also why the smoke appears blue – the only channel of the three really able to see the smoke plume is I-1 (the blue component). The fact that we are able to see the smoke at all in the Natural Color image is a testament to just how much smoke there is!

Here’s another Natural Color image from a few orbits before, which happened to be right after sunrise:

VIIRS "Natural Color" composite, taken 13:48 UTC 20 May 2014 — VIIRS “Natural Color” composite, taken 13:48 UTC 20 May 2014

The smoke plume is as optically thick as a cloud, and is even casting shadows!

Now, the Funny River Fire is the perfect opportunity to introduce another RGB composite being developed at CIRA for use with VIIRS, which we call the “Fire Temperature RGB”. This RGB composite uses the near-IR and shortwave-IR channels to highlight fires. The blue component is M-10 (1.61 µm), the green component is M-11 (2.25 µm) and the red component is M-12 (3.70 µm). Here’s what the Fire Temperature RGB looks like for the 21:58 UTC overpass:

VIIRS "Fire Temperature" composite, taken 21:58 UTC 20 May 2014 — VIIRS “Fire Temperature” composite of channels M-10, M-11 and M-12, taken 21:58 UTC 20 May 2014

This is yet another example of just how large a fire this is!

The Fire Temperature RGB provides information on how hot (or how “active”) the fire is. This is due to the fact that fires generally show up best around 4 µm. At shorter wavelengths, the amount of background solar radiation increases, so fires need to be hotter to be visible. This means that relatively cool or small fires will only show up in M-12 and appear red. Hotter fires will show up in M-11 and M-12 and appear yellow. The hottest, most active fires will be detected in all three channels and show up white. Also, there’s very little sensitivity to smoke, as you can see, so the imagery provides useful information even with such a thick smoke plume. Due to radiative differences between liquid droplets and ice particles, ice clouds tend to appear dark green, while liquid clouds appear more blue.

At night, M-11 doesn’t produce valid data (although there is a push from several user groups to change that), but M-10 and M-12 still provide valuable information. In fact, the only thing you can see at night in M-10 are fires and gas flares. Even without M-11 at night, we can use the Fire Temperature RGB to monitor the fire around the clock (whenever VIIRS is overhead) and here’s an animation to prove it:

Animation of VIIRS Fire Temperature RGB images of the Funny River Fire (2014).

In the first frame, the fire is obscured by clouds (we still can’t see through those) but, after that, you can see the fire was pushed to the shores of Tustumena Lake. With nowhere else to go, the fire expanded east and west. The fire slowly loses intensity until the last two frames, when activity picks up on the north side. (Although, clouds block the view of the east flank of the fire at that time, so we can’t say how active it is there.) Also notice on the second and third nights (11:00 to 13:00 UTC) that the fire appears less intense. This is probably due to the combination of reduced fire activity at night and the presence of clouds that are not visible at night in this RGB composite.

Here’s what the Funny River Fire looked like in the high-resolution fire detection channel (I-4, 3.74 µm) for the same times:

VIIRS channel I-4 images of the Funny River Fire (2014)

For this image, cooler pixels appear light, while warmer pixels appear dark. Pixels with a brightness temperature above 340 K have been colored. This channel by itself shows the clouds over the fire at night, but it can be ambiguous during the day because liquid clouds are highly reflective at this wavelength, so they also look warm.

It has already been demonstrated that the Day/Night Band is capable of detecting fires at night (click here and here for examples). So, why not just use it here? I tell you why: the day/night terminator is already encroaching on the nighttime overpasses. This makes it difficult to see fires, since the light from the fire is competing with light from the sun. This was a particularly intense fire on the first night though, so the Day/Night Band was able to see it (as evidenced by this Near Constant Contrast [NCC] image):

VIIRS NCC image, taken 12:09 UTC 20 May 2014

As you can see, the NCC product shows both the fire and the smoke plume. Notice also that you can’t see any city lights, even though it’s still nighttime over the fire because there is enough twilight to drown out the signal. That makes fire detection with the Day/Night Band tricky when the terminator is so close. It’s only because the Funny River Fire was so intense that we are able to see it. Of course, since it was so intense, we are able to see the smoke easily even if we can’t see the light from the fire.

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Tag Archives: Fire Temperature RGB

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