Clouds play an important role in regulating the flow of radiation at the top of the atmosphere and at the surface. They are also integral to the atmospheric hydrological cycle via their integral influence on the balance between radiative and latent heating. The response of cloud cover to increasing greenhouse gases currently represents the largest uncertainty in model predictions of climate sensitivity (see Chapter 8). Surface observations made at weather stations and onboard ships, dating back over a century, provide the longest available records of cloud cover changes. Surface observers report the all-sky conditions, which include the sides as well as bottoms of clouds, but are unable to report upper-level clouds that may be obscured from the observer’s view. Although limited by potential inhomogeneities in observation times and methodology, the surface-observed cloud changes are often associated with physically consistent changes in correlative data, strengthening their credibility. Since the mid-1990s, especially in the USA and Canada, human observations at the surface have been widely replaced with automated ceilometer measurements, which measure only directly overhead low clouds rather than all-sky conditions. In contrast, satellites generally only observe the uppermost level of clouds and have difficulty detecting optically thin clouds. While satellite measurements do provide much better spatial and temporal sampling than can be obtained from the surface, their record is much shorter in length. These disparities in how cloud cover is observed contribute to the lack of consistency between surface- and satellite-measured changes in cloudiness. Condensation trails (‘contrails’) from aircraft exhaust may expand to form cirrus clouds and these and cosmic ray relations to clouds are addressed in Chapter 2.