The first thing a visitor to the Center for Clouds, Chemistry, and Climate notices is that there’s not a single picture of a cloud in sight. No photos of brooding thunderclouds, no cloud-category posters. The center (nicknamed C4) occupies a low wooden building on a hillside overlooking the ocean in La Jolla, California--a prime cloud-viewing site. Yet in the director’s office, Veerabhadran Ramanathan sits with his back to the window as decks of stratus clouds sweep in from the Pacific. The visual statement, he explains cheerfully, is unintended but telling. We’re not ready to deal with real clouds yet, he says. They’re just so complicated and ugly.
It’s not that a scientist like Ramanathan can’t appreciate the beautiful infinity of shapes and shades clouds can assume. But these qualities make clouds almost impossible to reduce to hard numbers.
Until recently there was little reason to do so. The only numbers that seemed relevant to clouds were the few minutes or hours they might last before raining themselves out or just dissipating away. Given their ephemeral existence, clouds were regarded more as passive indicators of short-term weather than driving forces of long-term climate. Only now are researchers beginning to appreciate the degree to which clouds determine how much sunlight Earth accepts or rejects and how much heat it yields back to space. That’s what is motivating research groups like C4 (a consortium of university, government, and industrial researchers in the United States and Europe) to study clouds with satellites, sensors, and spy planes. They are trying to uncover the radiative effects of clouds--that is, what clouds do to the heat and light that try to get through them.
Although at any moment they can cover as much as three-quarters of the Earth, clouds remain the most mysterious factor in projecting climate. They both cool and heat the planet, and we don’t know which side of their nature will prevail as the climate itself changes. Moreover, Ramanathan and his colleagues have realized that the power of clouds doesn’t just lie hidden in the future: they have a far bigger hand than we once imagined in guiding the present.
Clouds are the product of moisture-laden air that has been pushed upward by the heat absorbed from the sun-warmed Earth, forced aloft by mountain slopes, or plowed up by a wedge of colder, denser air. Since the atmosphere cools with altitude and since cool air can hold less water vapor, some of the vapor condenses into droplets or ice crystals, growing around seeds of dust or salt. This liquid and solid water is the cloud- stuff that scatters light and makes clouds visible. Yet clouds remain insubstantial--droplets or ice make up as little as a millionth of their volume.
