Terra Satellite Data Confirm Unusually Warm, Dry U.S. Winter

Press Release From: Goddard Space Flight Center
Posted: Tuesday, April 9, 2002

New maps of land surface temperature and snow cover produced by NASA's Terra satellite show this year's winter was warmer than last year's, and the snow line stayed farther north than normal. The observations confirm earlier National Oceanic and Atmospheric Administration reports that the United States was unusually warm and dry this past winter.

For the last two years, a new sensor aboard Terra has been collecting the most detailed global measurements ever made of our world's land surface temperatures and snow cover. The Moderate-resolution Imaging Spectroradiometer (MODIS) is already giving scientists new insights into our changing planet.

MODIS data reveal that average winter temperatures in clear-sky conditions during the 2001-02 winter in the contiguous United States were more than 3 C (5.4 F) warmer in the daytime and more than 2 C (3.6 F) warmer at night than the winter of 2000-01.

Daytime temperatures in December 2001 were 4.6 C (8.3 F) warmer than December 2000. January and February 2002 were also warmer than in 2001, and unseasonable warmth extended back into the fall, with November temperatures almost 6 C (10.8 F) warmer in 2001 than 2000. Some of the biggest temperature differences occurred in the northern Great Plains, which were much warmer than last year, and northern Utah, which was colder.

Zhengming Wan, a remote-sensing scientist at University of California, Santa Barbara, developed the new technique for using MODIS data to determine the surface temperature of the Earth. According to Wan, "The land surface temperature maps from MODIS provide independent evidence of previous reports that this past winter was warmer than normal and confirm our ability to observe from space a characteristic of the Earth that is important for studying global change."

His team's initial evaluation of the MODIS land surface temperature observations found that most of MODIS' space-based temperature measurements agreed with comparable ground-based measurements to within 1 C (1.8 F). Results will be published in a special issue of the journal Remote Sensing of Environment later this year.

Unlike conventional observations of surface temperature that are actually measurements of air temperature collected by thermometers 2 meters (6.6 feet) above the ground, MODIS measures precisely the thermal radiation emitted from the planet's surface -- whether that surface is bare ground, lakes, treetops, or rooftops. This additional detail means farmers could know the temperature of the air around their crops and the temperature of the crops themselves, which helps farmers better estimate productivity and water requirements.

MODIS measures the temperature of nearly every square kilometer (0.4 square miles) of the Earth's surface roughly twice a day. This regular coverage enables MODIS to observe snow cover, as well. Monthly MODIS maps showing snow-covered areas from November 2001 to February 2002 reveal that along with high temperatures, snow was late to arrive and early to recede in many parts of the United States.

November's snowline remained well north of its average location near the border of the United States and Canada, and large areas of the U.S. Rocky Mountains saw little persistent snow cover that month. In February, at least 16 states from the Rocky Mountains eastward showed little to none of their expected snow cover.

Many parts of the U.S. depend on snowmelt for recharging public water supplies. MODIS' observations of the extent of snow cover allow scientists to more accurately estimate water availability in the spring and summer months.

Launched December 18, 1999, NASA's Terra satellite is the flagship of the Earth Observing System series of satellites, part of NASA's Earth Science Enterprise, a long-term research program dedicated to understanding how human-induced and natural changes affect our global environment. Terra MODIS observations are expected to continue through at least 2004.

More information, including images and animations, can be found at:

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