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".
The Cloudsat spacecraft carries an extremely sensitive radar.
"A tiny, tiny fraction of the water on our planet is in clouds and yet that tiny, tiny fraction is what provides the fresh water on which humans depend"
"The strength of the return from the radar is actually directly related to the amount of water that's in clouds. Effectively, it allows us to weigh the clouds," explained Dr Stephens.
"The time delay of the pulses means we can look at different levels and that gives us the profile of clouds."
Many sources
Calipso stands for Cloud-Aerosol Lidar and Infrared Pathfinder Satellite.
It uses lidar, which fires pulses of shorter wave energy - in the infrared and visible part of the spectrum - down into the atmosphere to obtain a different, but complementary, set of data from Cloudsat's.
In particular, Calipso is concerned with aerosols. These very fine particles are thrown up into the atmosphere by natural processes, such as volcanic eruptions, dust and sand storms, and even sea spray.
Human activities, also, produce aerosols: through burning of forests; and industrial and vehicle emissions.
Aerosols take very complex forms, and contain a range of chemical compounds. They can be solid or liquid, or even solid material inside a drop of liquid.
And their longevity is highly varied, with some aerosols lasting a few days and others hanging in the air for months.
"The bottom line here is that to measure aerosols is very difficult; you need a variety of instruments," explains Dr Charles Trepte, Calipso project scientist from the US space agency's Langley Research Center.
"Nasa and other agencies have been making measurements of aerosols from space for many years, but the problem is that they haven't been able to measure all the properties; and one thing they are missing is the vertical distribution of aerosols in thin clouds."
Different ways
Aerosols have a fundamental relationship with clouds by providing the nuclei on which cloud droplets can form.
Clouds that form in clean air are made up of droplets that tend to get bigger because they form on fewer nuclei; and these clouds tend to rain more, too.
Clouds that develop in dirty air form many more, but smaller, droplets. These clouds also look brighter.
By picking apart these details, Calipso will help scientists understand the direct and indirect effects of aerosols on climate.
"Directly, they can scatter sunlight back to space and have a cooling effect just by reducing the amount of solar radiation reaching the Earth system," said Dr Trepte.
"They can also absorb solar radiation and warm the atmosphere, possibly alter circulations, change the thermal stability of the atmosphere and perhaps inhibit convection.
"And then they have the indirect effects of changing cloud properties, allowing them to last longer, changing the amount of precipitation - either increase it or decrease it - and perhaps even dim or brighten clouds so that they have better or worse reflecting properties."
The Cloudsat and Calipso missions have a number of objectives:
"A tiny, tiny fraction of the water on our planet is in clouds and yet that tiny, tiny fraction is what provides the fresh water on which humans depend," Dr Stephens said.
"Clouds replenish our fresh water resources and yet we can't really tell you today how clouds will change under the pressures of global climate change."
Cloudsat and Calipso join a fleet of other satellites - known as the A-Train - which are aiming to give a rounded picture of Earth's atmospheric and water systems.
The spacecraft circle the planet on a path that takes them over broadly the same observation point in quick succession.
The platforms carry different instrumentation to address specific questions.
Jonathan.Amos-INTERNET@bbc.co.uk
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