Tag: snow

The Curious Case of Lake Superior's Shrinking Cloud Street Droplets


Parallel lines of cumulus clouds often appear when frigid, dry winds rush over comparatively warm bodies of water. NASA satellites have observed the striking cloud formations – which atmospheric scientists call “cloud streets” — over the Hudson Bay, Greenland Sea, Bering Sea, and the Amery Ice Shelf a number of times in the past.

Recently, University of Wisconsin scientist Steve Ackerman was combing through data from NASA’s MODIS instrument as part of an effort to catalog and classify different cloud types. Something about the street clouds in this image of Lake Superior (above) struck him as peculiar. We caught up with him during a poster session at an American Geophysical Union meeting in San Francisco to find out more.

WoE: What are we looking at here?

Ackerman: These are cloud streets. They’re really quite interesting clouds. They occur when you get cold air blowing over warm water. You get them frequently over the Great Lakes and off the East Coast as well.

WoE: What was it about this particular cloud street set that you found notable?

Ackerman: We actually looked at a series of these, and what we found was that the clouds start small, grow in altitude, get thicker optically, and then do something quite strange and unexpected.

WoE: Strange and unexpected? Please explain…

Ackerman: Yes, often what happens is that the size of the cloud droplets grow as we’d expect at first, but then partway across the lake the size of the particles starts to decrease.

WoE: And that’s surprising?

Ackerman: Yes, we have no idea why they’d do that. They should be getting progressively bigger as they move across the lake and pick up moisture.

WoE: About how big are these cloud droplets, and how do they change over time?

Ackerman: They start off at about 5 microns. (For reference, human hair is about 100 microns.) They grow up to about 20 microns, and then they drop down to 10 microns.

WoE: How long does that process take?

Ackerman: About four hours.

WoE: Why do think it’s happening? 

Ackerman: We’re really not sure. Perhaps dry air is coming in from above.

WoE: Is this the only time you’ve observed this phenomenon?

Ackerman: It’s pretty rare. We found it in the MODIS imagery in the five years that we looked about 15 times.

WoE: What makes a peculiar phenomenon like this worth studying?

Ackerman: The next step is to work with cloud modelers and to see if they’re modeling things well enough to explain what’s going on. If the models can’t recreate unusual events like these cloud streets, we know they’re not getting things right. We need models to get the global climate right, and also the weather prediction right. 

The top image comes from the Moderate Resolution Imaging Spectroradiometer (MODIS).  The other two images are courtesy of Steve Ackerman.

–Adam Voiland, NASA’s Earth Science News Team

Snow Views

What on Earth was that? It may have looked like an amoeba, but it’s actually a microscopic view of a wind-blown snowflake as viewed by a scanning electron microscope. Scientists in the Electron Microscopy Unit at the Beltsville Agricultural Research Center, which is just up the road from NASA’s Goddard Space Flight Center in Maryland, captured the image. Scientists at Goddard typically study snow from above using satellite instruments that fly high above the surface. Despite their differences, both perspectives offer views of bewildering beauty.  See more microscopic snow crystal imagery here. 



Satellite imagery courtesy of the NASA Earth Observatory.  Snowflake imagery courtesy of the USDA.


–Adam Voiland, NASA’s Earth Science News Team

Snowpocalypse Revisited


Though the summer heat and humidity makes it seem like a lifetime ago, the record-breaking snows in the eastern U.S. last winter are not something we will soon forget. Several feet of powder fell on most of the Mid-Atlantic region during February 2010, and this week a study from Columbia University’s Lamont-Doherty Earth Observatory gives us new insight into what caused the freaky weather.

A rare combination of weather — not climate — patterns seems to be the culprit. El Niño produced abnormally wet conditions in the southeastern U.S.; a negative North Atlantic Oscillation pushed frigid Arctic air down from the North. This collision of moisture with abnormally cold air led to more than six feet of snow over the region between December 2009 and February 2010.

The visualization above, derived from the Goddard Earth Observing System Model Version 5 (GEOS-5) and created by NASA Goddard’s Scientific Visualization Studio, shows the first wave of the February snowstorms hitting the East Coast about four seconds into the animation. The second wave forms off the west coast of Mexico’s Yucatan peninsula — about twelve seconds in — and then pummels the East Coast.

— Michelle Williams, NASA’s Goddard Space Flight Center