Clouds consist of liquid water droplets and / or ice crystals. Depending on their vertical extent, growth history and water phase clouds obscure the sun partly or completely. They can cause more or less precipitation in various forms. Clouds play a fundamental role for the short- and long-wave radiation budget of the Earth's surface and the Earth as a whole. Different cloud layers occur which, depending on the cloud, may have different typical distributions and concentrations of cloud droplets and/or ice crystals.
Imagining satellite senors like the Advanced Very High Resolution Radiometer (AVHRR), the Moderate Resolution Imaging Spectroradiometer (MODIS), or the classical geostationary weather satellites: Meteosat, are suited to monitor total cloud cover - provided that the contrast between cloud and Earth's surface is large enough. This applies to both, the optical (only useful under daylight conditions) and the infrared spectral range: clouds exhibiting an albedo and/or surface temperature similar to the Earth's surface cannot be detected.
Profiling sensors scan the vertical structure of the clouds by coupling to changes in cloud composition (cloud particle phase), particle size distribution, and vertical cloud water distribution. Such sensors operate in the infrared spectral range such as the Atmospheric Infrared Sounder (AIRS) and the Infrared Atmospheric Sounding Interferometer (IASI). Actively profiling sensors like the CALIPSO Lidar and the CloudSat radar have allowed quite some progress in recent years with regard to resolving the vertical cloud structure in more detail.
The GEWEX Radiation Panel has been working on an assessment of currently available possibilities to remotely sense clouds; a preliminary report of this panel which illustrates current possibilities and limitations is given here.
Aerosols impact cloud and precipitation formation processes, the radiation budget of the clouds, the atmosphere and the Earths' surface. Appropriate knowledge about aerosol type and concentration is required by remote sensing methods needing atmospheric corrections. Monitoring of aerosol parameters also allows us to identify and better quantify the impact biomass burning and air pollution.
For model evaluation it is often better to, instead of looking at the clouds themselves, consider their effects on the radiation. One sensor specialized for this is the Clouds and Earth's Radiant Energy System (CERES).
Another data source for radiation observations is the World Data Centre for Remote Sensing of the Atmosphere. Information and data sets at high temporal resolution about solar irradiation (also in the UV part of the electromagnetic spectrum), e.g. from MSG SEVIRI, can be obtained from there, among others - for instance via SOLEMI.
The sunshine duration is measured in hours (h) and the intensity of radiation in Watt per square meter (W/m²).
In the context with long-term measurements of the sunshine duration we recommend to take a look at the following paper:
Matuszko, D., A comparison of sunshine duration records from the Campbell-Stokes sunshine recorder and CSD3 sunshine duration sensor, Theor. Appl. Climatol., 119: 404-406, 2015.
Information and data sets about solar irradiation (and sunshine duration) as measured at ground-based observations stations can be obtained from the World Radiation Data Centre (WRDC). Here one can find a list of ground-based observation stations world-wide where- among other parameters - radiation and sunshine duration is measured and from where one can download the data.
In addition to radiative fluxes (short- and longwave) heat fluxes play a particularly important role in the Earth's Climate System. The sensible heat flux and the latent heat flux are the two most important heat fluxes for the interaction between the atmosphere and the Earth's surface for both, water and land. The first one is basically a function of the temperature gradient between the surface and the near-surface atmosphere. The second one is basically a function of the humidity gradient between the surface and the near-surface atmosphere.
Data of such heat fluxes are provided, e.g.: