Tag Archives: meteorology

Geoscientist’s Toolkit: Radiosonde

Radiosonde launch.  Image credit: Laikolosse (CC-BY-NC).
Radiosonde launch. Image credit: Laikolosse (CC-BY-NC).

Last weekend, I had the opportunity to tour the National Weather Service‘s Weather Forecast Office in Chanhassen, MN. One of the highlights of the tour was the upper air sounding facility, which launches a radiosonde (weather balloon) twice daily (at 0000 UTC and 1200 UTC, which is 7:00 local time here in Minnesota during the summer).

Radiosondes provide crucial information about the state of the atmosphere by measuring pressure, temperature, relative humidity, wind speed, and wind direction. Wind speed and direction are determined by the movement of the balloon as measured by GPS. Observations are transmitted back to the launching station via radio, where they are merged with all the other observations around the world. The merged observations from the most recent soundings are fed into the numeric weather prediction models used by your local weather service office (and your local TV meteorologist) to create forecasts and evaluate severe weather threats.

Radiosonde instrument packages are fairly small—about the size of a thick book, like Anna Karenina, but mostly styrofoam. A wire protrudes from the instrument package which acts as the radio antenna and holds the temperature sensor away from the body of the radiosonde. The instruments are suspended on a line beneath a parachute, which is in turn held up by a large helium-filled balloon, perhaps 1.5 m in diameter (~5′). Upon release, the balloon will rise around 5-8 m/s (11-18~mph), and can reach a peak altitude of more than 38 km (23.5 miles), well into the stratosphere. At these altitudes, where pressure is less than .5% of the surface pressure, the balloon can expand to a diameter of 7.5 m (~25′).

Temperature and dewpoint profiles are often plotted as functions of pressure (closely related to altitude). In the lower part of the atmosphere, the troposphere, temperature gradually decreases as altitude increases. This is why mountains have a snowline. Beyond the troposphere lies the stratosphere. In the stratosphere, the temperature tends to increase with altitude, due to the absorption of UV light by ozone. Pictured below is a temperature profile from Chanhassen, MN, on a clear, warm day.

Temperature (right) and dewpoint (left) profile from Chanhassen, MN, on a clear, warm evening.  The x-axis is showing temperatures (°C), while the y-axis is showing pressure (in hPa; 1000 hPa is roughly surface pressure near sea level).  Image credit: University of Wyoming Department of Atmospheric Science.
Temperature (right-most dark trace) and dewpoint (left-most dark trace) profile from Chanhassen, MN, on a clear, warm evening. The x-axis is showing temperatures (°C), while the y-axis is showing pressure (in hPa; 1000 hPa is roughly surface pressure near sea level). For those curious about the purple and green lines, see here and here. Image credit: University of Wyoming Department of Atmospheric Science.

With a little training (e.g. this web course), a meteorologist can look at a temperature/dewpoint profile and identify whether it is sunny, cloudy, or if severe storms are likely.

Radiosondes are a great way to get detailed data about the state of the atmosphere. Within the US, there are roughly 90 balloon sounding stations, and there are many more stations around the globe. If I had more time and the funding to do it, I would love to send radiosondes up from Heard Island—it certainly is an under-sampled area of the globe.

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.