#cloud streets

A Cloud and Ice Factory : Image of the Day

For most of each winter, the Sea of Okhotsk is covered in ice. This pair of images shows the sea before and after this year’s transformation. The Sea of Okhotsk is what oceanographers call a marginal sea: a region of a larger ocean basin that is partly enclosed by islands and peninsulas hugging a continental coast. With the Kamchatka Peninsula, the Kuril Islands, and Sakhalin Island partly sheltering the sea from the Pacific Ocean, and with prevailing, frigid northwesterly winds blowing out from Siberia, the sea is a winter ice factory and a year-round cloud factory.

The first natural-color image was acquired by the Visible Infrared Imaging Radiometer Suite (VIIRS) instrument on the Suomi NPP satellite on November 25, 2017. Snow and clouds cover much of the coastal land, and some sea ice appears toward the lower left. The dominant feature of the image is the pattern of cloud streets—parallel lines and arcs of cumulus clouds over the water.

Cloud streets typically form when cold air blows over warmer waters, picking up heat and moisture. As this air becomes warmer and wetter, it starts to rise in columns until its hit a warmer air layer, which makes the rising thermals roll over and loop back on themselves, creating parallel cylinders of rotating air. On the upper edge of these cylinders, clouds form. On the falling side (descending air), skies stay clear. Thanks to strong, cool land breezes and warmer Pacific waters, the Sea of Okhotsk is covered by clouds (with or without streets) for much of the year.

The image also shows some sea ice forming close to the northwest coast; click on the large image download and zoom in. You will also note that the cloud streets start to form at some distance from the shore. This is because the northwesterly winds need some time and distance to pick up enough heat and moisture to start rising thermals. Well offshore, another wind pattern puts a kink in the cloud streets.

A Chilly End to 2017 for the Northeast : Natural Hazards

As 2017 drew toward a close, bitterly cold Arctic air spilled into the eastern United States for several days. Blasts of cool air set up a white Christmas for many Americans and were poised to make New Year’s Eve celebrations the coldest in recent memory in many areas.

The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite captured this image of the Northeast’s frozen landscape on December 28, 2017. Brisk northwesterly winds created rows of “cloud streets” as cold air blew over Lake Ontario and the Atlantic Ocean. A layer of snow covered much of New England and upstate New York.

Cloud streets are long parallel bands of cumulus clouds that form when cold air blows over warmer waters and a warmer air layer (temperature inversion) rests over the top of both. The comparatively warm water gives up heat and moisture to the cold air above, and columns of heated air called thermals naturally rise through the atmosphere. The temperature inversion acts like a lid. When the rising thermals hit it, they roll over and loop back on themselves, creating parallel cylinders of rotating air. As this happens, the moisture cools and condenses into flat-bottomed, fluffy-topped cumulus clouds that line up parallel to the direction of the prevailing winds.

While the cold streak has not broken all-time cold records, it is breaking records for individual days. On the day the image was acquired, weather observers on Mount Washington in New Hampshire recorded a daily record low of -34 degrees Fahrenheit (-36° Celsius). Cities that have seen records fall during this cold spell include Baltimore, Boston, Flint, New York, Montreal, and Toronto.

When the ball drops in New York City on New Year’s Eve, forecasters expect air temperatures of 10°F (-12°C) and wind chills of -5°F ( -15°C). The last time it was so cold was in 1962; 1917 holds the record for the coldest ball drop, a year when the air temperature was just 1°F (-17°C), noted Jason Samenow of the Capital Weather Gang.

Cloud Streets Over the Yellow Sea : Image of the Day

On November 18, 2017, the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite acquired this natural-color image of cloud streets—long parallel bands of cumulus clouds—over the Yellow Sea.

Cloud streets typically form when cold air blows over warmer water. To transport the heat away from the sea surface, columns of heated air called thermals naturally rise through the atmosphere. The air masses rise until they hit a warmer air layer (temperature inversion), which acts like a lid. The rising thermals then roll over and loop back on themselves, creating parallel cylinders of rotating air. On the upper edge of these cylinders of air, clouds form. Along the downward side (descending air), skies are clear.

According to Seok-Min Hong, an atmospheric scientist at Kongju National University in South Korea, the cold season is the right time of year to see the phenomenon unfold over the Yellow Sea. Cold, dry air from Siberia is warmed and moistened as it moves over the sea surface. But the transformation does not happen right away; it takes the air mass some time over the sea surface to pick up its the heat and moisture, which is why the region close to shore is cloud-free.

As the air moves further over the sea and cloud streets develop, the tops of the clouds reach higher into the atmosphere. The cloud streets continue to mature and change to open-cell and then to closed-cell clouds. The cells of closed-cell clouds display opaque cumulus clouds at their centers. Conversely, the cells of open-cell clouds are ringed with clouds and their centers are cloud-free.