Tag Archives: Clouds

Cloud-Free Heard Island

Composite, cloud-free satellite imagery of Heard Island, being produced in QGIS.  Image credit: Bill Mitchell (CC-BY), using USGS (Landsat 8, EO-1) data (public domain).
Composite, cloud-free satellite imagery of Heard Island, being produced in QGIS. Image credit: Bill Mitchell (CC-BY), using USGS (Landsat 8, EO-1) data (public domain).

Heard Island is a pretty cloudy place most of the time. However, there are occasional times when the weather clears, particularly on the southeastern (leeward) side of the island. On rare occasions, the northwest and southwest sides of the island come out from the clouds as a satellite passes over.

For the past two years, I have been watching Heard Island using true-color imagery from four satellites: Terra, Aqua, Landsat 8, and EO-1. I have posted previously about satellite imagery from these instruments. Although every image of the Island I have seen has clouds in it covering a portion of the island, I was curious whether or not I had accumulated clear imagery of the entirety of Heard Island.

In part, this question was spurred by a follower on Twitter asking about eruptive activity at Heard. I had to admit I didn’t really know whether the activity was low-level and continuous (like Kilauea) or more intermittent. Given that our knowledge of its eruptive activity is primarily from satellite observations, do the satellite “thermal anomalies” correspond to short eruptive events, or simply a cloud-free view of the volcano?

For high-resolution imagery of Heard Island, the datasets of interest are from EO-1 ALI, and Landsat 8 OLI. The two MODIS instruments (one on Aqua, one on Terra) are moderate-resolution, and while 250-m resolution is sufficient for some purposes, this one needs more. Looking through the archives, I was able to find EO-1 ALI data primarily for Mawson Peak and points southeast, and Landsat 8 OLI covered much of the island, particularly the northwest.

Not only is having cloud-free, high-resolution data important for me, but I want the data to be recent. There has been a retreat of up to 5.5 km for some of the glaciers since 1947, and the Google Maps imagery of that area (Stephenson Lagoon) is horribly outdated. Fortunately, I found most of the island covered in large swaths with images from 2014 onward, and mostly 2016. There was even good imagery from when I was on Heard Island! Our ship, the Braveheart, is visible as a few white pixels in Atlas Roads (just north of Atlas Cove), slightly closer to the Azorella Peninsula than to the Laurens Peninsula. The tents and campsite are too small and darkly colored to be visible on this image.

Braveheart in Atlas Roads and the campsite (non-contrasting) at Atlas Cove, Heard Island.  Satellite image pixels are 15 m across, and the Azorella Peninsula isthmus (along Campsite label) is 1 km wide.  Image credit: Bill Mitchell (CC-BY), using USGS (Landsat 8 OLI) data (public domain).
Braveheart in Atlas Roads and the campsite (non-contrasting) at Atlas Cove, Heard Island. Satellite image pixels are 15 m across, and the Azorella Peninsula isthmus (along Campsite label) is 1 km wide. Image credit: Bill Mitchell (CC-BY), using USGS (Landsat 8 OLI) data (public domain).

A small portion of the island between Atlas Cove and Mawson Peak was the most difficult to find. With the topography of the island, the steady stream of wind, and the humid air, the 2.5 km by 2.5 km region was cloudy pretty much all the time. Eventually, using the EO-1 ALI instrument and going back to early 2010, I found a reasonably clear image of it.

Once I had the images (after combining true-color and panchromatic brightness data in QGIS), I needed to stitch them together. Thanks to the wonderful QGIS training manual, I was able to create vector (polygon) layers which corresponded to the clear region of each image (plus some surrounding ocean). At this point the troublesome mostly-cloudy spot became evident, and the search was on for imagery to fill the void.

Creating polygons for clipping the satellite imagery using QGIS.  Four polygons are shown here, including the small polygon of much cloudiness.  A fifth dataset was subsequently incorporated.
Creating polygons for clipping the satellite imagery using QGIS. Four polygons are shown here, including the small polygon of much cloudiness. A fifth dataset was subsequently incorporated.

Finally, I tried to put them together. This turned out to be more trouble than it was worth for my purposes, having only five images. Several of the images had differing resolutions (10 m/pixel for EO-1 ALI, 15 m/pixel for Landsat 8 OLI). Additionally, since I was handling these in their raw format, color balances/exposures were not consistent across images. I decided it best, then, to leave them separate, and sent them around to the Heard Island Expedition team.

Soon I had an email from the expedition leader: he was very interested in the imagery, but it wasn’t opening in Google Earth. Some searching later, I found that Google Earth works best with a certain map projection (EPSG:4326), and when exporting the GeoTIFF, I needed to select “rendered image” rather than “raw data”. I re-exported the images, zipped them up, and tested it out on another computer: success! This Google Earth friendly imagery is now available here (17 MB zip).

One continuation of this project would be to keep looking through the documentation on GeoTiffs to find out how to make the rendered images use a transparent, not white, border where there is no data. That would likely let me create a virtual raster catalog to load all of them in one go, rather than having to load them separately.

Book Discussion: The End of Night

Moon over Berkeley, and a lot of stray light.  Image credit: laikolosse (CC-BY-NC).
Moon over Berkeley, and a lot of stray light. Image credit: laikolosse (CC-BY-NC).

Recently I’ve been reading an interesting book by Paul Bogard, The End of Night. It’s non-fiction, and is based around the increasing amounts of night-time light in the developed world—and why that may not be a great thing.

Just 200 years ago, before electric lights, the night sky—complete with the swath of the Milky Way, and other naked-eye observable galaxies—was spectacular on any clear night from anywhere on Earth. Today, however, few in the developed world see that a few times a year, let alone on every clear night. Instead, our nights look like they do in the photo of Berkeley above, with a milky haze of yellow-orange light from the 589 nm sodium D lines (admittedly there is some fog in the picture too).

Darkish summer skies in Canada; many stars are visible, and a hint of the Milky Way can be discerned.  Pale orange light is from the Sun being relatively near the horizon even in the middle of the night.  Image credit: laikolosse (CC-BY-NC).
Darkish summer skies in Canada; many stars are visible, and a hint of the Milky Way can be discerned. Pale orange light is from the Sun being relatively near the horizon even in the middle of the night. Image credit: https://secure.flickr.com/photos/laikolosse (CC-BY-NC).

There were four key messages I took from the book.

First, straightforwardly, is that a dark night sky is incredibly beautiful, and we should preserve that beauty. Seeing the Milky Way clearly with the naked eye can be a powerful experience especially for those who have rarely or never have. Unfortunately, very few places in western Europe (or the eastern half of the US, or populated areas in the western US) are near dark skies. Bogard cites a statistic that 80% of children born in the US today will never see a truly dark sky. In my experience as a teaching assistant at Berkeley going on a geology field trip to Bishop, CA and the eastern Sierras, I can attest to a large proportion of the students being quite surprised by all the stars visible from such dark skies.

Adding light doesn’t make it easier to see. Sure it makes it easier to see initially when you first go from the light into the dark, but if the light isn’t in the right places, it actually makes it harder to see. I have biked at night on roads where the streetlights made it very difficult for me to see the road, because the lights were so bright and everything else so dark. Glare from bad lighting makes the lighting less effective. It’s also light going places it isn’t needed or wanted, which is wasted energy (and money, and CO2 where electricity comes from fossil fuels). The picture at top has a number of lights shining directly into the camera, miles away; all that light is wasted and unneeded. Bedrooms have light streaming in from outside even at night, which causes its own problems.

People are evolutionarily adapted to sleep at night, and our bodies expect for it to be dark at that time. Longer periods of light are decreasing the amount and quality of sleep we get (particularly night-shift workers), and those have significant detrimental public health consequences including increased risk of cancer.[1 and references therein] Turning off or dimming the lights at night—especially blue light and light in the bedroom—could help us sleep better and be healthier for it. Minimizing the number of people who are working at all hours of the night and thus exposed to the health risks of doing so would also be good, both from a moral and economic perspective.

Finally, it was made clear throughout that the point here isn’t to turn off all the lights and go back to the stone age, but rather to be thoughtful and deliberate about our outdoor lighting. Making light fixtures which put light where it should go (i.e. down on the ground, not out to the sides or up) and using them only when needed is fairly simple. How much light is really needed at a car dealership, ice rink, or empty parking lot in the middle of the night? Gas stations are often extremely brightly lit, yet most of that light isn’t needed to pump gas or wash the bugs off the windshield. In fact, pulling out of a bright gas station at night can be dangerous since your eyes will have adjusted to the brighter environment and it will take a few minutes for them to dilate again and bring your vision back to its optimum level.

Supposing I turn off my 60 W worth of porch lights (sadly not the dark-sky friendly type) for an average of 10 hours/day year round (3650 h), that reduces my electricity consumption by 219 kWh (780 MJ). At a residential electric rate of $0.08/kWh, that translates to a savings of $17.52 annually, as well as a CO2 savings of 150 kg (3400 moles). It also makes my bedroom darker.

I went outside one night recently when it was clear (albeit humid). I have an urban view, and can only see ~25% of the sky. From my deck, I counted twenty stars. That’s right, I could only see 20 stars. Extrapolating for the full-sky view gets me up to 80, and if we want to be generous we could round that to 100. Compare that to the dark(ish) sky pictured above, and you can see that there’s really a huge difference. In this neighborhood, too, there’s enough stray light running around (even in the summer when leaves block it) that turning off my outside lights isn’t making it hard to get around.

Night can be a pretty neat time. In college, I would occasionally go cross-country skiing at night. Sure, I would have a headlamp with me, but on a clear night I typically didn’t use it. Even cloudy nights, thanks to some local light pollution, were easy to ski without the aid of the headlamp. Being outside at night in the crisp, quiet solitude of a snowy winter was amazing. It’s part of what I missed while in graduate school in a bright, noisy city where it never snows.

After reading this book, I’m looking forward to the Heard Island expedition even more, because the southern ocean, like Antarctica, is home to pristine skies free from artificial lights. Of course, unlike Antarctica, the likely sky condition is mostly cloudy or cloudy, so getting a clear night may be very rare. That will only serve to make the moment more special, if and when it happens. I will bring a camera, and I will try to get a picture of such an event. But that picture will be but a still, lifeless version of the magic at Heard Island.

[1] Hansen, J. J Natl Cancer Inst (2001) 93, p. 1513–1515. DOI: 10.1093/jnci/93.20.1513

Gravity Waves and Vortices at Heard Island

Gravity waves and Von Karman vortices at Heard Island, May 1, 2015.  Resolution is 250 m/pixel.  Image credit: NASA Aqua/MODIS.
Gravity waves and Von Karman vortices at Heard Island, May 1, 2015. Resolution is 250 m/pixel. Image credit: NASA Aqua/MODIS.

Last week at Heard Island, a pair of interesting atmospheric phenomena occurred and were nicely captured in the image above: gravity waves, and Von Karman vortices. Von Karman vortices have been mentioned here previously, and we will explore them in a little more depth later in this post.

Gravity waves are phenomena which occur when a parcel of air is moved out of equilibrium (e.g. lofted too high by a mountain) and then moves back toward equilibrium. The momentum of the air parcel will cause it to overshoot equilibrium (on both sides), oscillating back and forth across the equilibrium level until the energy is dampened and dissipated. This is similar to the wake of a boat, which will bring the water up and down until eventually it restores itself to an equilibrium level.

In the image, you can see the gravity waves formed by Mt. Dixon on the Laurens Peninsula, on the northwest corner of Heard Island (you won’t see the mountain, but that is where the waves begin). In the atmosphere, if the waves happen to take water through condensing/evaporating levels, clouds will form at the peaks and disappear in the troughs. The very nearest waves to Mt. Dixon are punctuated by these clear troughs, while further downwind there are still clouds in the troughs.

Another striking feature of the image is the Von Karman vortex street downwind of Big Ben, the volcano on Heard Island. Von Karman vortices are formed when the flow on the leeward side of the obstruction (here the volcano) becomes turbulent. The turbulence leads to eddy formation. Here, the eddies are particularly visible because, like with the gravity waves, some areas are evaporative and have no clouds, while others are condensing and do have clouds. As the vortex moves downwind from the island, gradually the eddies are slowed by viscosity and dissipate. Equilibrium moisture levels are also reached further downwind from the island, visible in the increased cloudcover.

Geoscientist’s Toolkit: Science on a Sphere

An educator from the Denver Museum of Nature and Science presents about sea floor spreading at the Science on a Sphere network meeting in Long Beach, CA (2012).  Image credit: Bill Mitchell.
An educator from the Denver Museum of Nature and Science presents about sea floor spreading at the Science on a Sphere network meeting hosted by the Aquarium of the Pacific in Long Beach, CA (2012). Image credit: Bill Mitchell.

One of the coolest tools I’ve had an opportunity to work with in the course of my research, outreach, and explorations of science centers, is the Science on a Sphere (SOS), which can be found at any of more than a hundred museums, educational institutions, and science centers worldwide.

Developed by NOAA, Science on a Sphere is a 1.7-meter diameter globe, surrounded by four projectors, which can display animated digital maps of the globe. Because the display is actually spherical, the maps do not have edges or strongly distorted projections, which can make looking at rectangular maps confusing.* On this display, it makes perfect sense why a flight from Los Angeles to Beijing would go near or over the Aleutian Islands of Alaska.

Many, many maps are available, covering a variety of topics including biology, geology, meteorology, planetary science, oceanography, and geography. There are maps of temperature changes going forward a hundred years, generated for each of the scenarios in the IPCC’s latest report. There are near-real-time maps of clouds seen (in infrared) by NOAA’s GOES and POES weather satellites. A near-real-time feed of earthquakes, coded by size and depth, is provided by the USGS. Following a tsunami, NOAA provides computer model output showing the wave heights as the waves travel across the ocean.

You can see the paths of commercial airplanes, the Earth at night, agricultural regions and their productivity, and the locations of volcanoes worldwide. Even the Moon, other planets (particularly Mars), and the Sun have their own visualizations.

Through the marvels of modern technology, these datasets can be overlain on top of each other, paused, backtracked, and even marked up like a sportscaster.

I spent many hours with the SOS at the Lawrence Hall of Science while I was living in Berkeley. Not only did I have fun talking with visitors about science, but I brought my favorite dataset to Science on a Sphere: near-real-time true-color imagery from the MODIS instrument aboard NASA’s Aqua satellite.

Aqua/MODIS image of Earth, March 30, 2015.  Can you spot Heard Island peeking out from the clouds?  Image credit: NASA.
Aqua/MODIS image of Earth, March 30, 2015. Can you spot Heard Island peeking out from the clouds? Image credit: NASA.

You should see if your local science center has a Science on a Sphere exhibit. The Science Museum of Minnesota does!

* For the curious, SOS maps are stored in an equirectangular (i.e. lat-long) format.

Geoscientist’s Toolkit: Trained Eyeballs

A squall line approaches.  Image credit: Laikolosse (CC-BY-NC).
A squall line approaches. Image credit: Laikolosse (CC-BY-NC).

Last weekend, I attended a (free) SKYWARN training class in my area, and have become a trained severe weather spotter [NOT a storm chaser!].* In the class, we covered topics such such as safety around storms, storm development, and identification of cloud formations indicative of severe or intensifying storms.

Despite the many advances in technology—from geosynchronous weather satellites to dual-polarization radar to networked automated weather stations across the country—there are times when there is little substitute for human eyes.

Humans are very good at pattern recognition, and with a little training can identify different types of clouds, rocks, or observe that a valley was carved by a glacier. You might want to go outside sometime and take a close look at something, be it a cloud, tree, rock, or animal. What do you notice about it?

I’ll describe a few of the things I notice in the photograph above. There’s a low, dark cloud base, and toward the center-left are some disorganized clouds beneath the base. Rain is visible along the left side beyond the hill. A stiff breeze is blowing directly toward the camera, as indicated by the wavelets on the water.

With those observations in mind, I would interpret the scene in this way: a cold front is passing through, and these clouds are part of a squall line. The front has nearly reached the photographer’s location. Warm, humid air toward the right is buoyantly rising over the colder air. As it cools upon rising, the water vapor condenses and precipitates as rain. While it’s not clear from this photograph, the low-level clouds may be part of a shelf cloud, or there may be a shelf cloud above the frame of the picture. The primary hazards here would be high winds, lightning, heavy rain, and possibly some hail.

Some people I’ve talked with have suggested that when taking notes and observations in the field, that the left-hand pages of the notebook should be used for observations, and the right-hand pages for interpretation. Keeping descriptions and interpretations distinct can help when an alternate interpretation is brought up, or when writing a paper with separate results and discussion sections. The results are strictly the measurements and observations, and the discussion then describes the interpretation off them.

Happy science-ing!

* If you live in the US and are interested in severe weather, you should check with your local weather office to see when and where they offer training.**

** Some places, such as the San Francisco Bay Area, don’t really have weather, so classes are few and far between. For more excitement in your meteorology, you should go to the Midwest. You could also take the online course (not necessarily accredited in your area).

Meteorology and its Application to Heard Island

Cloud vortices off Heard Island. Image credit: NASA GSFC (Terra/MODIS, 9/19/2011 for you satellite imagery junkies).

Meteorology is an important consideration for any outing, be it a trip to the grocery store or an extended expedition. You will prepare differently if the temperature is hot enough to melt butter in the shade than you would if it’s cold enough to flash-freeze a pot of boiling water.

There are two different things we will need to keep in mind for these preparations: climate and weather. Although they are related, they are fundamentally different. Climate is long-term, whereas weather is short-term. For instance, the average high temperature* for Minneapolis for January 4th is 24 F. That’s the climatological average; if you’re planning a trip to Minneapolis for January 4, 2016, that’s a ballpark of what to expect. Weather is more variable, of course. In 2015, the high temperature on January 4th was 12 F; in 2014, it was 35 F. That’s weather. Here’s an analogy: take two six-sided dice. On average, when you roll them, their sum will be seven (that’s the climate). However, you shouldn’t be surprised with a roll of three!

Last month at the Five Thirty Eight blog, Nate Silver did a great job analyzing the amount of weather at various cities across the US, and in general exploring this distinction and the limits of its usefulness. Unfortunately, his title is a bit misleading:

“Which City Has The Most Unpredictable Weather?”

His use of unpredictable has nothing to do with the skills or abilities of the local meteorologists.** Instead, it is a reflection of the variability away from the climate average. That is, if you use the climate average as a forecast, how likely is it to be correct? In some areas, such as Phoenix, AZ, the climate average high temperature is a very good predictor. In others, like Minneapolis, the departures from average can be larger and more frequent, as exemplified above.

So, now that we have that discussion out of the way, and deferring things like uncertainty to another post, what do we know about Heard Island?

Expect cool temperatures, wet weather, and plenty of wind. The vast icy waters surrounding the island keep conditions from changing much. Temperatures remain near freezing year-round, with monthly average temperatures in summer still only 5.2 C (~41 F). The ocean provides plenty of moisture too; a research expedition from 1948-1954 recorded precipitation on 75% of days at Atlas Cove, one of the more accessible parts of the island.

The volcano can make its own weather by disrupting the west-to-east winds. Windward locations tend to see more fog and rain (like the coastal parts of San Francisco), while leeward areas see more sun and warmer temperatures (like Berkeley or Palo Alto). Lenticular clouds, formed when the humid air is pushed up around the mountain, are also found at Heard Island. Other conditions can cause cloud vortices when the smooth flow of wind is disrupted while passing the island, as seen in the picture at the top of this post.

In the southern hemisphere, there is much less land to disrupt the smooth flow of air than there is in the northern hemisphere. Consequently, the winds tend to be stronger more consistent. At Atlas Cove, the average wind speeds are around 26-33.5 km/h (16-21 mph), with gusts recorded up to ~180 km/h (110 mph). That means we can expect damage similar to that found from a borderline EF1/EF2 tornado.***

In short, the climate of Heard Island is near-freezing, wet, and strongly influenced and moderated by the water around it. Be prepared for cold, snow, rain, and wind. There will be some sunshine in there too, but it won’t be the norm.

* The National Weather Service computes this over the 30-year period of 1981-2010.
** Variable would have been a more appropriate word, because devious has deviated from the sense which would be useful here.
*** Tornado categories use the Enhanced Fujita scale (0-5, 5 is strongest), and are estimates of wind speed based on damage. Obviously we’d rather use a direct measurement and have our camp sturdy enough to not have damage to use for estimating wind speed. More on the inner workings of the (enhanced) Fujita scale can be found at the NOAA Storm Prediction Center.