Look, up in the sky! It’s a Bunny, it’s a heart, it’s a UFO! No, it’s Global Cloud Climatology! (Photo Credit: International Satellite Cloud Climatology Project)
On average, the global cloud amount is approximately 65-72% (counting cirrus). - International Satellite Cloud Climatology Project
There’s more to cloud climatology than picking out what animal it most closely resembles. We can define a cloud as a visible region consisting of suspended particles of ice, liquid, or a combination of both. To form a cloud, we must increase the relative humidity such that it is greater than 100%. This can be done through radiative cooling or increasing the mixing ratio (i.e. the amount of the water vapor). There are 10 main cloud types formed from cirrus, stratus, and cumulus type clouds. Cloud climatology consists of defining clouds in terms of cloud amount, heights, cloud top temperature, optical depth, distribution, radiance, and lifecycle. Not all clouds precipitate either. (Precipitation is also intimately linked with global cloud climatology. Precipitation is typically associated with areas of rising air and low atmospheric pressure. The atmosphere itself is dramatically affected by latent heating associated with precipitation, an ultimate driver in overall precipitation patterns. Temperatures changes within the atmosphere also play a large role in the development and dissipation of clouds. Aerosols can act as cloud condensation nuclei, accelerating the rain process or inhibiting it).
The average cloud amount varies seasonally and with time of day, driven by solar radiation. Over land, cumulus tend to form in the early morning with dissipation in the early afternoon due to mixing. Cumulonimbus tend to take over in the late afternoon and evening hours in the continental summer, due to more organized forcing. The tropics have the highest total low cloud amount, while the total cloud amount is highest in the Northern Hemisphere winter associated with the midlatitude storm track. It is thought that high clouds could have an impact on warming and low clouds a net cooling effect, but these processes aren’t fully understood or use indirect observational evidence of low clouds. And yet, the Cloud-Radiative Forcing (CRF) experiment found that a climate system without clouds would have a net warming effect.
Ground-based cloud observations have been largely subjective in determining cloud type and somewhat sparse over the ocean. Detecting clouds through satellite data often involves techniques, such as cloud masking to identify the presence of clouds through comparison of the brightness temperature of pixels (i.e. cold pixels will be clouds). Over polar regions, the cooler brightness temperatures of snow and ice-covered surfaces may possibly be mistaken for clouds. Although 3% uncertainty exists with cloud amount in climatology, ground-based stations and satellite comparison should be able to minimize potential sources of error in cloud climatology and help to better understand the role clouds play in a changing climate.
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©2019 Meteorologist Sharon Sullivan