Clouds
Clouds are formed from water vapor condensation on cloud condensation nuclei (CCN) or deposition on ice nuclei (IN). In the field, clouds can be monitored by ground-based (e.g. radars and ceilometers) and satellite-based (e.g. CloudSat) remote-sensing instruments over a long term. They can also be surveyed by air-borne instruments during field campaigns. The global cloud radiative effects on current and future climate are evaluated by numerical models. The best estimate on the present-day net cloud radiative forcing is -20 W m^-2. Currently, clouds are not well-represented in climate models, either due to the coarse resolution of models or due to poor understanding in key cloud processes. In Chapter 7 of IPCC AR5, the following aspects of cloud studies are assessed: