December Fluff

Posted on December 19, 2016 by Curtis_Seaman

By now, you probably know the drill: a little bit of discussion about a particular subject, throw in a few pop culture references, maybe a video or two, then get to the good stuff – high quality VIIRS imagery. Then, maybe add some follow-up discussion to emphasize how VIIRS can be used to detect, monitor, or improve our understanding of the subject in question. Not today.

You see, VIIRS is constantly taking high quality images of the Earth (except during orbital maneuvers or rare glitches). There isn’t enough time in a day to show them all, or go into a detailed discussion as to their relevance. And, nobody likes to read that much anyway. So, as we busily prepare for the upcoming holidays, we’re going to skip the in-depth discussion and get right to the good stuff.

Here then is a sample of interesting images taken by VIIRS over the years that weren’t featured on their own dedicated blog posts. Keep in mind that they represent the variety of topics that VIIRS can shed some light on. Many of these images represent topics that have already been discussed in great detail in previous posts on this blog. Others haven’t. It is important to keep in mind… See, I’m starting to write too much, which I said I wasn’t going to do. I’ll shut up now.

Without further ado, here’s a VIIRS Natural Color image showing a lake-effect snow event that produced a significant amount of the fluffy, white stuff back in November 2014:

VIIRS Natural Color RGB composite of channels M-5, M-7 and M-10 (18:20 UTC 18 November 2014)

As always, click on the image to bring up the full resolution version. Did you notice all the cloud streets? How about the fact that the most vigorous cloud streets have a cyan color, indicating that they are topped with ice crystals? The whitish clouds are topped with liquid water and… Oops. I’m starting to discuss things in too much detail, which I wasn’t going to do today. Let’s move on.

Here’s another Natural Color RGB image using the high-resolution imagery bands showing a variety of cloud streets and wave clouds over the North Island of New Zealand:

VIIRS Natural Color RGB composite of channels I-1, I-2 and I-3 (02:55 UTC 3 September 2016)

Here’s a Natural Color RGB image showing a total solar eclipse over Scandinavia in 2015:

VIIRS Natural Color RGB composite of channels M-5, M-7 and M-10 (10:06 UTC 20 March 2015)

Here’s a VIIRS True Color image and split-window difference (M-15 – M-16) image showing volcanic ash from the eruption of the volcano Sangeang Api in Indonesia in May 2014:

VIIRS True Color RGB composite of channels M-3, M-4 and M-5 (06:20 UTC 31 May 2014)

VIIRS split-window difference (M-15 – M-16) image (06:20 UTC 31 May 2014)

Here’s a VIIRS True Color image showing algae and blowing dust over the northern end of the Caspian Sea (plus an almost-bone-dry Aral Sea):

VIIRS True Color RGB composite of channels M-3, M-4 and M-5 (09:00 UTC 18 May 2014)

Here is a high-resolution infrared (I-5) image showing a very strong temperature gradient in the Pacific Ocean, off the coast of Hokkaido (Japan):

VIIRS I-5 (11.45 um) image (03:45 UTC 12 December 2016)

The green-to-red transition just southeast of Hokkaido represents a sea surface temperature change of about 10 K (18 °F) over a distance of 3-5 pixels (1-2 km). This is in a location that the high-resolution Natural Color RGB shows to be ice- and cloud-free:

VIIRS Natural Color RGB composite of channels I-1, I-2 and I-3 (03:45 UTC 12 December 2016)

Here’s a high-resolution infrared (I-5) image showing hurricanes Madeline and Lester headed toward Hawaii from earlier this year:

VIIRS I-5 (11.45 um) image (22:55 UTC 30 August 2016)

Here are the Fire Temperature RGB (daytime) and Day/Night Band (nighttime) images of a massive collection of wildfires over central Siberia in September 2016:

VIIRS Fire Temperature RGB composite of channels M-10, M-11 and M-12 (05:20 UTC 18 September 2016)

VIIRS Day/Night Band image (19:11 UTC 18 September 2016)

Here is a 5-orbit composite of VIIRS Day/Night Band images showing the aurora borealis over Canada (August 2016):

Day/Night Band image composite of 5 consecutive VIIRS orbits (30 August 2016)

Here is a view of central Europe at night from the Day/Night Band:

VIIRS Day/Night Band image (01:20 UTC 21 September 2016)

And, finally, for no reason at all, here’s is a picture of Spain wearing a Santa hat (or sleeping cap) made out of clouds:

VIIRS Natural Color RGB composite of channels M-5, M-7 and M-10 (13:05 UTC 18 March 2014)

There you have it. A baker’s ten examples showing a small sample of what VIIRS can do. No doubt it will be taking more interesting images over the next two weeks, since it doesn’t stop working over the holidays – even if you and I do.

Single-Purpose Flour

Posted on November 16, 2016 by Curtis_Seaman

Think of a snowflake. What happens when that snowflake hits the ground? Now, picture other snowflakes – millions of them – all hitting the ground and piling up on top of each other, crushing our first poor snowflake. Skiers love to talk (and dream) about “fresh powder.” But, what happens when the “powder” isn’t so fresh?

Those delicate, little snow crystals we imagine (or look at directly, if we click on links included in the text) undergo a transformation as soon as they hit the ground. Compression from the weight of the snow above, plus the occasional partial thaw and re-freeze cycle (when temperatures are in the right range), breaks up the snow flakes and converts the 6-pointed crystals into more circular grains of snow. As more and more snow accumulates on top, the air in between the individual snowflakes/grains (which is what helps make it a good insulator) gets squeezed out, making the snow more dense. If enough time passes and enough snow accumulates, individual snow grains can fuse together. These bonded snow grains are called “névé.” If this extra-dense snow can survive a whole summer without melting, then a second winter of this compaction and compression will squeeze out more air and fuse more snow grains, creating the more dense “firn.” After 20 or 30 years of this, what once was a collection of fragile snowflakes becomes a nearly solid mass of ice that we call a “glacier.” Glaciers can be made up of grains that are several inches in length.

But, you don’t need to hear me say it (or read me write it), you can watch a short video where a guy in a thick Scottish accent explains it. (Did you notice his first sentence was a lie? Snow is made of frozen water, so glaciers are made of frozen water, since they are made of snow. I think what he means is that glaciers aren’t formed the same way as a hockey rink, but the way he said it is technically incorrect.) At the end of the video, the narrator hints at why we are looking at glaciers today: glaciers have the power to grind down solid rock.

When a glacier forms on a non-level surface, gravity acts on it, pulling it down the slope. This mass of ice and friction from the motion acts like sandpaper on the underlying rock, converting the rock into a fine powder known as “glacial flour” or, simply, “rock flour.” In the spring and summer months, the meltwater from the glacier collects this glacial flour and transports it downstream, where it may be deposited on the river’s banks. During dry periods, it doesn’t take much wind to loft these fine particles of rock into the air, creating a unique type of dust storm that is not uncommon in Alaska. One that can be seen by satellites.

And, wouldn’t you know it, a significant event occurred at the end of October. Take a look at this VIIRS True Color image from 23 October 2016:

VIIRS True Color RGB composite of channels M-3, M-4 and M-5 (21:24 UTC 23 October 2016)

See the big plume of dust over the Gulf of Alaska? Here’s a zoomed in version:

Zoomed in version of above image.

That plume of dust is coming from the Copper River delta. The Copper River is fed by a number of glaciers in Wrangell-St. Elias National Park, plus a few in the Chugach Mountains so it is full of glacial sediment and rock flour (as evidenced by this photo). And, it’s amazingly full of salmon. (How do they see where they’re going when they head back to spawn? And, that water can’t be easy for them to breathe.)

Notice also that we have the perfect set-up for a glacial flour dust event on the Copper River. You can see a low-pressure circulation over the Gulf of Alaska in the above picture, plus we have a cold, Arctic high over the Interior shown in this analysis from the Weather Prediction Center. For those of you familiar with Alaska, note that temperatures were some 30 °F warmer during the last week in October in Cordova (on the coast) than they were in Glennallen (along the river ~150 miles inland). That cold, dense, high-pressure air over the interior of Alaska is going to seek out the warmer, less dense, low-pressure air over the ocean – on the other side of the mountains – and the easiest route to take is the Copper River valley. The air being funneled into that single valley creates high winds, which loft the glacial flour from the river banks into the atmosphere.

Now, depending on your preferences, you might think that the dust shows up better in the Natural Color RGB composite:

VIIRS Natural Color RGB composite of channels I-1, I-2 and I-3 (21:24 UTC 23 October 2016).

But, everyone should agree that the dust is even easier to see the following day:

VIIRS True Color RGB composite of channels M-3, M-4 and M-5 (21:01 UTC 24 October 2016)

VIIRS Natural Color RGB composite of channels I-1, I-2 and I-3 (21:01 UTC 24 October 2016)

You can also see a few more plumes start to show up to the southeast, closer to Yakutat.

Since Alaska is far enough north, we get more than one daytime overpass every day. Here’s the same scene on the very next orbit, about a 100 minutes later:

VIIRS Natural Color RGB composite of channels I-1, I-2 and I-3 (22:42 UTC 24 October 2016)

Notice that the dust plume appears darker. What is going on? This is a consequence of the fact that glacial flour, like many aerosol particles, has a tendency to preferentially scatter sunlight in the “forward” direction. At the time of the first orbit (21:01 UTC), both the sun and the dust plume are on the left side of the satellite. The sunlight scatters off the dust in the same (2-dimensional) direction it was traveling and hits the VIIRS detectors. In the second orbit (22:42 UTC), the dust plume is now to the right of the satellite, but the sun is to the left. In this case, forward scattering takes the sunlight off to the east, away from the VIIRS detectors. With less backward scattering, the plume appears darker. This has a bigger impact on the Natural Color imagery, because the Natural Color RGB uses longer wavelength channels where forward scattering is more prevalent. Here’s the True Color image from the second orbit:

VIIRS True Color RGB composite of channels M-3, M-4 and M-5 (22:42 UTC 24 October 2016)

The plume is a little darker than the first orbit, but not by as much as in the Natural Color imagery. Here are animations to show that:

Animation of VIIRS True Color images (24 October 2016)

Animation of VIIRS Natural Color images (24 October 2016)

There are many other interesting details you can see in these animations. For one, you can see turbid waters along the coast in the True Color images that move with the tides and currents. These features are absent in the Natural Color because the ocean is not as reflective at these longer wavelengths. You can also see the shadows cast by the mountains that move with the sun. Some of the mountains seem to change their appearance because VIIRS is viewing them from a different side.

The dust plumes were even more impressive on 25 October 2016, making this a three-day event. The same discussion applies:

VIIRS True Color composite of channels M-3, M-4 and M-5 (20:43 UTC 25 October 2016)

VIIRS True Color composite of channels M-3, M-4 and M-5 (22:26 UTC 25 October 2016)

VIIRS Natural Color RGB composite of channels I-1, I-2 and I-3 (20:43 UTC 25 October 2016)

VIIRS Natural Color RGB composite of channels I-1, I-2 and I-3 (22:26 UTC 25 October 2016)

Full disclosure, yours truly drove through a glacial flour dust storm along the Delta River on the north side of the Alaska Range back in 2015. Even though it was only about a mile wide, visibility was reduced to only a few hundred yards beyond the hood of my car. It felt as dangerous as driving through any fog. The dust event shown here was not a hazard to drivers, since it was out over the ocean, but it was a hazard to fisherman. Being in a boat near one of these river deltas means dealing with high winds and high waves. To forecasters, these dust plumes provide information about the wind on clear days (when cloud-track wind algorithms are no help), which is useful in a state with very few surface observing sites to take advantage of.

The last remaining issue for the day is one of terminology. You see, “glacial flour dust storm” is a mouthful, and acronyms aren’t always the best solution. (GFDS, anyone?) “Haboob” covers desert dust. “SAL” or “bruma seca” covers Saharan dust specifically. So, what should we call these dust events? Something along the lines of “rock flour”, only more proactive! And, Dusty McDustface is right out!

Watch for Falling Rock

Posted on July 11, 2016 by Curtis_Seaman

Q: When a tree falls in the forest and nobody is around to hear it, does it make a sound?

A: Yes.

That’s an easy question to answer. It’s not a 3000-year-old philosophical conundrum with no answer. Sound is simply a pressure wave moving through some medium (e.g. air, or the ground). A tree falling in the forest will create a pressure wave whether or not there is someone there to listen to it. It pushes against the air, for one. And it smacks into the ground (or other trees), for two. These will happen no matter who is around. As long as that tree doesn’t fall over in the vacuum of space (where there is nothing to transmit the sound waves and nothing to crash into), that tree will make “a sound”. (There are also sounds that humans cannot hear. Think of a dog whistle. Does that sound not exist because a human can’t hear it?)

What if it’s not a tree? What if it’s 120 million metric tons of rock falling onto a glacier? Does that make a sound? To quote a former governor, “You betcha!” It even causes a 2.9 magnitude earthquake!

That’s right! On 28 June 2016, a massive landslide occurred in southeast Alaska. It was picked up on seismometers all over Alaska. And, a pilot who regularly flies over Glacier Bay National Park saw the aftermath:

If you didn’t read the articles from the previous links, here’s one with more (and updated) information. And, according to this last article, rocks were still falling and still making sounds (“like fast flowing streams but ‘crunchier'”) four days later. That pile of fallen rocks is roughly 6.5 miles long and 1 mile wide. And, some of the rock was pushed at least 300 ft (~100 m) uphill on some of the neighboring mountain slopes.

Of course, who needs pilots with video cameras? All we need is a satellite instrument known as VIIRS to see it. (That, and a couple of cloud-free days.) First, lets take a look at an ultra-high-resolution Landsat image (that I stole from the National Park Service website and annotated):

Glacier Bay National Park as viewed by Landsat (courtesy US National Park Service)

Of course, you’ll want to click on that image to see it at full resolution. The names I’ve added to the image are the names of the major (and a few minor) glaciers in the park. The one to take note of is Lamplugh. Study it’s location, then see if you can find it in this VIIRS True Color image from 9 June 2016:

VIIRS True Color RGB composite image of channels M-3, M-4 and M-5 (20:31 UTC 9 June 2016), zoomed in at 200%.

Anything? No? Well, how about in this image from 7 July 2016:

VIIRS True Color RGB composite of channels M-3, M-4 and M-5 (21:42 UTC 7 July 2016), zoomed in at 200%

I see it! If you don’t, try this “Before/After” image overlay, by dragging your mouse from side to side:

That dark gray area in the image from 7 July 2016 that the arrow is pointing to is the Lamplugh Glacier landslide! If the “Before/After” overlay doesn’t work, try refreshing the page, or look at this animated GIF:

Animation of VIIRS True Color images highlighting the Lamplugh Glacier landslide

Of course, with True Color images, it can be hard to tell what is cloud and what is snow (or glacier) and with VIIRS you’re limited to 750 m resolution. We can take care of those issues with the high-resolution (375 m) Natural Color images:

Animation of VIIRS Natural Color images of the Lamplugh Glacier landslide

Make sure you click on it to see the full resolution. If you want to really zoom in, here is the high-resolution visible channel (I-1) imagery of the event:

Animation of VIIRS high-resolution visible images of the Lamplugh Glacier landslide

You don’t even need an arrow to point it out. Plus, if you look closely, I think you can even see some of the dust coming from the slide.

That’s what 120 million metric tons of rock falling off the side of a mountain looks like, according to VIIRS!

The Great Flood of 2015

Posted on January 5, 2016 by Curtis_Seaman

As we begin 2016, struggling to get back into the swing of things at work and vowing not to overeat or over-drink ever again, it’s appropriate to bid farewell to 2015 – not just for all the weird weather events that we covered on this blog over the year, but also for the weird, wacky weather that ruined many people’s holidays. I’m not sure of the exact number, but this article mentions 43 weather-related fatalities in the U.S. in the second half of December. Let’s see, between 23-30 December 2015, there were:

– 77 tornadoes (including 38 on the 23^rd and 18 on the 27^th);

– Parts of New Mexico and west Texas got over 2 ft (60 cm) of snow from a blizzard that created drifts upwards of 10 ft (3 m) on the 27^th;

– Record warmth was observed in the Northeast before and during Christmas and the site of Snowvember went until 18 December before the first measurable snow of the season;

– Chicago received almost 2″ of sleet (48 mm) on the 29^th when any accumulation of sleet is quite rare;

– And – what will be our focus here – St. Louis received over 3-months-worth of precipitation in three days (26-28 December), from a storm that flooded a large area of Missouri, Illinois and Arkansas. In fact, the St. Louis area had the wettest December on record, right after having the 7th wettest November on record, which put it over the top for wettest calendar year on record. Current estimates place 31 fatalities at the hands of this flooding, which caused the Mississippi River to reach its highest crest since the Great Flood of 1993.

What kind of satellite imager would VIIRS be if it couldn’t detect massive flooding on the largest river in North America? (Hint: not a very useful one. Or, a less useful one, if you’re not into hyperbole.) Hey, if it works in Paraguay, it works here – or it isn’t science!

I shouldn’t have to prove that the Natural Color RGB is useful for detecting flooding (since I have done it many, many, many, many, many, many times before), so we can go right to the imagery. Here’s what the Midwest looked like on 13 November 2015 – before the flooding began:

VIIRS Natural Color RGB composite of channels I-1, I-2, and I-3 (19:02 UTC 13 November 2015).

And, here’s what the same area looked like on New Year’s Day:

VIIRS Natural Color RGB composite of channels I-1, I-2, and I-3 (18:45 UTC 1 January 2016).

Notice anything different? This is actually the reverse of the last time we played “Spot the Differences” – we’re looking for where water is now that wasn’t there before, instead of searching for bare ground that used to have water on it.

Of course, the first thing to notice is the large area of snow covering Iowa, Nebraska and northwest Missouri that wasn’t there back in November. Next, we have more clouds over the southern and northern parts of the scene. Those are the easy differences to spot. Now look for the Missouri River in eastern Missouri, the Arkansas River in Arkansas, the Illinois River in Illinois, the Indiana River in Indiana… Wait! There is no Indiana River. I fooled you! (Although, there are rivers in Indiana that are flooded.)

The most significant areas of flooding are in northeast Arkansas and the “Bootheel” of Missouri (which I think looks more like a toe or a claw than a heel), and the Mississippi River along the border of Tennessee shows signs of significant flooding as well. (If only it were the Tennessee River!) Here’s a before and after comparison, zoomed in on that part of the region:

You may have to refresh the page to get this to work right.

There’s a lot more water in the image from 1 January 2016 than there was back in November 2015! Since we are looking at the high-resolution Imagery bands, our quick-and-dirty estimate of water volumes still applies like it did for California’s drought: multiply the number of water-filled pixels by the depth (in feet) of the flooding, and by 100 acres to get the floodwater volume in acre-feet. Then multiply that by 325,852 gallons per acre-foot to get the volume in gallons. Even though this estimate is not exact, you can see how the gallons of floodwater add up. And, if you live in California, you can dream of seeing that much water! If you live in Missouri and can think of an economical way to transport this water to California, you’d be rich.

Now, see how many other areas of flooding you can find when you compare the two images in animation form:

Click to view an animation of VIIRS Natural Color RGB images from 13 November 2015 and 1 January 2016.

You will have to click on the image to see the animation. You can click on the image again to see it in full resolution (with most web browsers).

One thing you might notice is that some of the floodwaters appear more blue than black. Take a look at the Arkansas River in particular. As we discussed with the Rio Paraná and Rio Paraguay, this is due to the increased sediment that increases the albedo of the water at visible wavelengths. In other places the floodwaters are shallow enough that VIIRS can see the ground underneath – again making the water appear more blue in this RGB composite.

Wouldn’t it be nice to identify areas of flooding without having to play a “Spot the Differences” game? Maybe something that would automatically detect flooded areas? Well, you’re in luck:

VIIRS-based Flood Map (18:48 UTC 1 January 2016). Image courtesy S. Li (GMU).

This image is an example of the VIIRS-based flood detection product being developed by the JPSS Program’s River Ice and Flooding Initiative. This initiative is a collaboration between university-based researchers and NOAA forecasters who use products like these to help save lives. Thanks to S. Li for developing the product for and providing the image!

If you want to know what the flooding looks like from the ground, here is a nice video. Or, you can look at some pictures here.

As a final note, the American Meteorological Society is holding its Annual Meeting in New Orleans next week. This event will be held at the Convention Center – right on the bank of the Mississippi River – right at the time the river is forecast to crest from these floodwaters. The world’s largest gathering of weather enthusiasts might be directly impacted by this flood. Let’s hope no one has to swim their way to any poster sessions or keynote speeches! (I don’t think local residents want to deal with any flooding, either.)

Goose Lake is Gone (Again)

Posted on July 24, 2015 by Curtis_Seaman

We’ve covered mysteries before on this website. Well, here’s one from 150 years ago:

The emigrants, coming west on the Applegate Trail to Oregon in the 1870s, were puzzled. The trail was, of course, a seemingly unending set of wagon-wheel ruts stretching from the jumping-off points in the Midwest over deserts and mountains and all sorts of obstacles that seemed insurmountable, but weren’t.

But this one seemed impossible. Had the wagons before them really plunged directly into the enormous lake that lay before them? The ruts led directly into the water, and there was no sign of them having come out again.

It was miles across – the other side lay almost invisible on the horizon, much too far to float a caulked wagon. And yes, it was deep – far too deep to ford.

There was nothing for it but a trip around the lake, since the western sky lay on the other side. And so, around they went – making a detour of something like 100 miles.

On the other side, they found the wagon ruts again. They emerged from the water and headed on westward toward the Cascades. Once arrived at the West Coast, none of the previous emigrants knew anything about any lake there.

Was it aliens who came down to Earth to put a lake where there was none before? Did the earlier emigrants have covered wagon submarine technology (and very short term memories)? Maybe it was a very localized, very short-term Ice Age – a glacier snuck down from the Cascades and into the valley in the middle of the night and then melted without anyone noticing. What about that?

SPOILER ALERT: None of those theories is true. Anyone who would come up with these ridiculous ideas should be ashamed of themselves. Oh, wait – I came up with them. Hmmm. What I meant to say is: those are all good theories that are worthy of scientific exploration. Unfortunately, VIIRS wasn’t around in the 1870s. Plus, this mystery has already been solved. As our source explains:

It remained a mystery until, several years later, a drought struck and the lake dried up again.

What we’re talking about is Goose Lake, which is at times the largest lake that’s at least partially in Oregon. (In terms of surface area, not volume.) It’s right on the border between Oregon and California. When Goose Lake is at its fullest, it has a surface area of 147 square miles (380 km²), but it’s only 26 ft (8 m) deep. Maybe, if the emigrants weren’t so cowardly, they could have walked across it (although they might have gotten stuck in the mud). It would have saved 100 miles of extra walking (although they might have gotten stuck in the mud).

As you are probably well aware, California and Oregon are under a long-lasting, extreme drought. So, if you live near Goose Lake, it’s probably no surprise that the lake has dried up again. And, since this is 2015, VIIRS can tell us something about it this time.

Have you ever played one of those “spot the differences” games? (Don’t play them at work, or you’ll never get anything done.) Well, here’s a “spot the differences” game you can play at work – at least if your work involves detecting evidence of drought.

Here’s what Goose Lake looked like three years ago, according to VIIRS Natural Color imagery:

VIIRS Natural Color RGB composite of channels I-1, I-2 and I-3 (20:40 UTC 15 July 2012)

Note that it’s not as dark in color as the other lakes because it is so shallow. Now, here’s the same scene just last week:

VIIRS Natural Color RGB composite of channels I-1, I-2 and I-3 (21:40 UTC 16 July 2015)

Notice anything different? Now, for this spot-the-differences game, we’re going to ignore clouds, because they are always going to be different between the two images, difficult to count, and irrelevant to this discussion. (Except that clouds can obscure the view of a lake and can cast shadows that look like lakes.)

Since I labelled Goose Lake on those images, you have no excuse for not spotting that difference. Besides, if you can’t see that 147 square miles of lake surface are missing from the second image, you have no hope to see any of the other differences.

I counted at least 20 lakes or reservoirs that are present in the 2012 image that have dried up and vanished in the 2015 image. Plus, there are about as many lakes or reservoirs that have noticeably shrunk since 2012. Can you spot them all? Can you see more than I did?

After you’ve declared yourself done, compare your results with mine:

Comparison of the above VIIRS Natural Color images of Goose Lake.

As always, click on it to see the full resolution image. I’ve marked with red arrows those lakes that are visible in the 2012 image that are not visible in the 2015 image. Yellow arrows indicate the lake has lost surface area (but not totally vanished) between 2012 and 2015. And, there are a few spots that look like surface water visible in the 2015 image that are not present in 2012 – I’ve marked those with green arrows. There are a couple of lakes visible in the 2012 image that are covered by clouds in the 2015 image. Those are left unmarked. I’ve also labelled a burn scar left over from a pretty big wildfire in south-central Oregon visible in 2012 that has since disappeared. That’s the main non-lake, non-cloud related difference between the two images.

Most notably, Upper Alkali Lake (southeast of Goose Lake) dried up, which you should have noticed without me pointing it out. Drews Reservoir on the northwest side of Goose Lake in Oregon appears to have dried up, as does New Year Lake right across the border from Upper Alkali Lake in Nevada. Thompson Reservoir (the northernmost red arrow) looks bone dry and Gerber Reservoir (west of Drews Reservoir) has very little water left. The eastern half of Clear Lake Reservoir is now empty and the western half is significantly reduced in size. Three big reservoirs (lakes) on the southern edge of the image have also lost quite a bit of water (Trinity Lake, Shasta Lake and Eagle Lake).

Even if you don’t care that a bunch of salty, alkaline lakes in rural Jefferson (as they might prefer you to call it) have dried up, you should care about the reservoirs. And not just for the boating and other water recreation activities, which are now hazardous. When towns run out of water, prime agricultural land lays fallow, and Tom Selleck gets in trouble with the law, you know things are serious.

The reservoirs closer to central California are down quite a bit as well, and these impact a lot of people. Use your honed-in spot-the-difference skills in these VIIRS I-2 (0.865 µm) images from the same dates and times as the above images:

VIIRS I-2 image (20:40 UTC 15 July 2012)

VIIRS I-2 image (21:40 UTC 16 July 2015)

I-2 is one of the components of the Natural Color imagery (the green component). What makes it good for this purpose is that land and, particularly, vegetation are highly reflective at this wavelength, so they appear bright. Water is absorbing, so it appears black (or nearly so if the water’s dirty or shallow). It also has 375 m resolution at nadir. If you click to the full resolution versions of the above images, you can see that most of the reservoirs have lost quite a bit of surface area between 2012 and 2015.

If you’re too lazy, or have poor eyesight, click on this image below to better compare the two images:

Comparison of VIIRS I-2 images from the same dates and times as above

One more point that needs to be made: 375 m resolution at nadir is good for weather satellites like VIIRS, but the fact that you can see the loss of water in these images is testimony to how bad this drought is!

As you may or may not know, the resolution of VIIRS in these images degrades from 375 m at nadir to 750 m at the edge of the swath. As a reasonable approximation, that’s means each pixel is a quarter mile to a half mile wide. That means each pixel of missing water represents between 40 and 160 acres. We’ll say 100 acres, given that these images were taken roughly halfway between nadir and edge of scan. If the water was only 1 foot deep in these pixels, that would be a loss of 100 acre-feet. That’s 32.5 million gallons of water. (By the way, the average household uses between 0.5 and 1 acre-foot per year in water.)

Multiply the number of pixels that have lost water by 100 to get the area in acres. Multiply that by the average depth of the water lost to get the volume in acre-feet. And then multiply that by 325,852 gallons per acre-foot and that’s a lot of gallons of missing water!

(In case you’re interested, this PDF document says the average depth of Goose Lake is 8 ft. At 147 sq. mi. of surface area, that’s 245 billion gallons of water gone, give or take.)

JPSS (SNPP and NOAA-20)

VIIRS Imagery and Visualization Team Blog

Tag Archives: I-1

December Fluff

Single-Purpose Flour

Watch for Falling Rock

The Great Flood of 2015

Goose Lake is Gone (Again)