Some of them look like big pillows or lines across the sky, while others may remind you of Winston Churchill's profile. But at any given moment, about two-thirds of the Earth's surface usually is covered by clouds, according to a NASA study based upon satellite observations.
For a long time, scientists have been trying to figure out exactly how that cloud cover might affect the climate change process - or how it might be altered by global warming.
Photos: Know Your Cloud Types
So that you can visualize just how cloudy our planet is, NASA scientists have created this very cool global cloud fraction map. It's based upon data collected between 2002 and 2015 by the Moderate Resolution Imaging Spectroradiometer, or MODIS, an instrument on NASA's 13-year-old Aqua satellite, which is studying the Earth's water cycle.
As you can see from the image, there are three bands where the skies are most likely to be overcast - a narrow strip near the Earth's equator and two wider swaths in the mid-latitudes, 60 degrees north and south of the equator. The equatorial cloud band is related to Hadley cells, which are low-attitude circulation patterns that cause warm, moist air to rise and cool in that area, which causes the water vapor to condense and form clouds. (That's also why the area has a lot of thunderstorms.)
NEWS: How Do Clouds Affect Earth's Climate?
The middle latitude bands, in contrast, where the edges of polar and mid-latitude Ferrel cells collide and push air upward, (For more info, here's a primer on cloud formation from the University Corporation for Atmospheric Research.
Clouds' effect on climate change - and vice-versa - long has been a matter of debate among climate scientists, as this National Science Foundation report details. Back in 1997, in fact, the Intergovernmental Panel on Climate Change described clouds as "the largest source of uncertainty" in climate change predictions.
Different types of clouds have different effects. Stratus clouds, for example, block sunlight from reaching the Earth, and actually tend to have a cooling effect, while wispy, feathery cirrus clouds tend to let sunlight through but trap reflected heat from the Earth's surface, magnifying warming.
Overall, the net effect of clouds has been cooling, and some scientists have held out hope that the cloud cover will respond to feedback and shift to mitigate much of the predicted rise in global temperatures.
But recent research, such as this 2009 study by University of Miami scientists and this 2014 study by University of Paris researchers, suggests the opposite.
In that scenario, climate change will alter the cloud cover, thinning it in a way that actually will exacerbate the warming effect.