Some of the gaping holes that exist in our understanding of the Earth's atmosphere will be answered by two new satellites launched on Friday.
The Cloudsat and Calipso missions will study how clouds and aerosols (fine particles) form, evolve and affect our climate, the weather and air quality.
Scientists say knowledge gaps in such areas severely hamper their ability to forecast future climate change.
Different types of cloud, for example, can help cool or warm the planet.
"We will be making the key observations that address this problem," said Dr Graeme Stephens, the Cloudsat principal investigator from Colorado State University, US.
The US space agency (Nasa) satellites were launched from the Vandenberg Air Force Base in California at 1002 GMT, after a week of delays due to technical problems and unfavourable wind conditions.
They have been put in a 705km (438 miles) circular, sun-synchronous polar orbit, where they will fly in formation just 15 seconds apart.The spacecraft are part of an Earth-observation constellation Nasa calls the "A-Train".