Satellite data suggests that March, 2011 was the coolest in more than a decade. The average worldwide temperature in March was .18 degrees below the 30-year average for the month. It was the coldest March since 1999. February was also cold with temperatures running .03 degrees below the long-term average. Satellites began measuring temperature in 1978. The instruments measure the temperature of the atmosphere from the surface up to an altitude of about five miles above sea level. Satellites allow meteorologists to get accurate temperature readings for almost all regions of the Earth. This includes remote desert, ocean and rain forest areas where reliable climate data are not otherwise available. The cooling was largely driven by La Nina, which is a cooling of the equatorial Pacific Ocean.