The Earth's land surfaces, life, oceans, and atmosphere endlessly recycle the planet's water. Liquid water on the surface is transformed into water vapor by wind and sunlight, and it is also released by plants during photosynthesis. Water vapor is what makes the air feel humid. It condenses into water droplets or freezes into ice crystals, making clouds, rain, and snow. Air temperature and water vapor play the biggest role in forming clouds.
The cloud maps show what fraction of an area was cloudy on average for the month. The measurements were collected by the MODIS sensors on NASA's Terra and Aqua satellites. Colors range from blue (no clouds) to white (totally cloudy).
The water vapor maps show the total amount of water vapor in the column of air between the surface and the top of the atmosphere on average for the month. The observations were made by the MODIS sensor on NASA's Aqua satellite. Places where the air was dry are white, while places where water vapor was abundant are blue.
The comparison of global cloudiness and water vapor shows that year-round, the tropics are both the cloudiest and the most humid zone. This pattern makes sense because tropical latitudes receive the Sun's most direct rays for more of the year than higher latitudes, and the steady sunlight and warmth fuel evaporation from the ocean. On land, tropical forests release water vapor during photosynthesis. Because of this steady heating and evaporation, air over the tropics is usually warm, moist, and buoyant. It rises high into the atmosphere, where it cools, and clouds form.
Another interesting relationship is that while the places with the most water vapor in any month are always among the cloudiest, it is not always true that the cloudiest places are among the most humid locations. The tropics are both very humid and very cloudy, but in many months, the Southern Ocean is among the cloudiest places on the planet, even though the amount of water vapor is relatively low. This pattern occurs because cloud formation depends on both water vapor and air temperatures. The colder the air, the more readily any water vapor in the air will condense into clouds.
View, download, or analyze more of these data from NASA Earth Observations (NEO):