Changing Global Cloudiness

Surface-Based and Satellite Cloud Observations
The existing long-term collection of cloud measurements made by surface observers provides an important baseline record of cloud phenomena. Although these historical observations provide a useful context in which to study clouds, they do not provide the data that scientists need to conduct a more thorough and detailed study. Specifically, scientists require observations to be taken more frequently (at least daily), over much wider areas (up to global scale), and at many more wavelengths of the electromagnetic spectrum than can be detected by the human eye, ranging into the infrared (or heat) region. In short, satellites can provide much more quantitative data than can be gathered by surface observations.

The International Satellite Cloud Climatology Project, begun in the early 1980s, is an effort to take advantage of the visible light and infrared information currently available from existing meteorological satellites by constructing a detailed database of average global cloud cover and cloud types (Figure 3). Although these observations are useful, their spatial resolution is limited to about 2.5 miles (four kilometers), and data are available only at two wavelengths-one in visible light and one in the infrared region of the spectrum. While these data allow estimations of cloud top pressure, cloud coverage, and cloud optical depth (a measure of the amount of sunlight that passes through), they still do not provide the detail scientists need to accurately model the roles clouds play in climate change.

Figure 3: Total fractional cloud cover annual averaged from 1983-1990, compiled using data from the International Satellite Cloud Climatology Project (ISCCP).

Studies show that observations with a spatial resolution of about 800 feet (250 meters) are needed to accurately resolve cloud distributions. With observations at multiple wavelengths of the electromagnetic spectrum, the size of cloud particles and more accurate estimates of cloud radiative effects can be obtained. In addition, observations taken at several viewing angles are necessary to better define the effects of clouds on climate.

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Changing Global Cloudiness
How do Clouds Form?
Clouds and Climate Change
Surface-Based and Satellite Cloud Observations
Terra and Cloud Observations