Cartoon cloud reflects heat rays from bottom back toward Earth's surface.

Or do clouds make Earth warmer?

Here is a riddle—a serious one, not a joke:

As the ocean warms up, more water evaporates into the air. So does more water vapor then mean more warming? And does more warming mean more water vapor? And ‘round and ‘round we go?

Drawing shows how clouds have a greenhouse effect at night. Clouds trap some of the heat coming from Earth's surface, preventing it from escaping into space.

At night, clouds trap some of the heat from Earth's surface. Thus, it does not escape back into space.

Or, since more water vapor means more clouds, will the fluffy white clouds reflect enough sunlight back into space to make up for the warming?

Drawing shows arrows that represent sun's energy reflecting off tops of clouds.

During the day, clouds reflect the Sun's energy back to space, before it has a chance to heat Earth's surface.

This cloud riddle has scientists scratching their heads and trying to figure it out. NASA is helping with satellites like Aqua and CloudSat, which study the Earth's water cycle and clouds in 3-D.

Image taken by a satellite of a hurricane. Below is a cross-section of the storm clouds, colored to show how much water is contained in the clouds at different heights.

The top image is a hurricane, as seen by a satellite. Below is a cross-section of the storm clouds. This colorful image was made with data from the CloudSat satellite. It shows with different colors how much water is contained in the clouds at different heights.