‘Night Clouds’ give Clues to Weather, Climate on Earth

The silver-blue clouds that hover near the poles at night could give scientists clues to understand the weather and climate of the rest of the planet. After being first spotted in 1885, the nature of the phenomenon seemed to change throughout the 20th century, occurring at increasingly lower latitudes. As a result, scientists wondered if these clouds, known as noctilucent clouds, had shifted in the region they inhabit.

A NASA mission called Aeronomy of Ice in the Mesosphere, or AIM, was launched in 2007 to observe noctilucent clouds but currently only has a view of the clouds near the poles. However, according to a release from NASA, scientists have been able to gather information from several other missions and combine it with computer simulations to show the presence of these bright, shining clouds has increased between 40 and 50 degrees north latitude, a region covering the northern third of the United Sates and the lowest parts of Canada.

"Noctilucent clouds occur at altitudes of 50 miles above the surface - so high that they can reflect light from the sun back down to Earth," said James Russell, an atmospheric and planetary scientist at Hampton University in Hampton, Va. "AIM and other research has shown that in order for the clouds to form, three things are needed: very cold temperatures, water vapor, and meteoric dust. The meteoric dust provides sites that the water vapor can cling to until the cold temperatures cause water ice to form."

To study long-term changes in noctilucent clouds, Russell and his colleagues used historical temperature and water vapor records and a validated model to translate the data into information on the presence of the clouds. The model reflected an increase of noctilucent clouds from 2002 to 2011. This appears related to a decrease in temperature at the height where noctilucent clouds exist in the atmosphere.

Russell and his team will research further to determine if the frequency increase and temperature decrease could be due to a reduction in the sun's energy and heat, which naturally occurred as the solar output went from solar maximum in 2002 to solar minimum in 2009.

"As the sun goes to solar minimum, the solar heating of the atmosphere decreases, and a cooling trend would be expected," said Russell.

Russell served as the main author on the study, which was published online in the Journal of Geophysical Research: Atmospheres.