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Your search for "water vapor" gave back 31 results.
Show me the Water
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This short video, derived from the Science on a Sphere film "Water Falls", explores how Earth's freshwater resources are allocated and used.
Photo of a drop of water rippling in a pond
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Part 1 of a 4 part webquest that teaches the basics of precipitation science. Prepares students for the GPM Anime Contest.
The Water Cycle - Animation
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This animation shows the entire process of the water cycle throughout the course of a day.
Screenshot from the water cycle animation
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This flash animation takes viewers through four aspects of the water cycle: rain, water storage, vapor, and clouds.
Hydrologic Cycle diagram
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The water, or hydrologic, cycle describes the pilgrimage of water as water molecules make their way from the Earth’s surface to the atmosphere and back again, in some cases to below the surface.
Understanding Earth: Whats Up With Precipitation?
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An educational brochure which explores the importance of fresh water to life on Earth and how NASA is monitoring and studying precipitation using satellites like GPM.

What causes thunderstorms?

Thunderstorms form when an air mass becomes unstable (when air in the lowest layers is very warm and humid, or air in the upper layers is unusually cold, or if both occur). Rising near-surface air in an unstable air mass expands and cools, making it warmer than its environment, which causes it to rise even farther. If enough water vapor is present, some of this vapor condenses into a cloud, releasing heat, which makes the air parcel even warmer, forcing it to rise yet again.  Water vapor fuels the storm.

Earth System thumbnail
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This unit will introduce you to many of the complex issues surrounding the Earth as a system and will help you to look at Earth in a new way—as a living system.

What are clouds made of? Are they more likely to form in polluted air or in pristine air?

Depending on their type, clouds can consist of dry air mixed with liquid water drops, ice particles, or both. Low, shallow clouds are mostly made of water droplets of various sizes. Thin, upper level clouds (cirrus) are made of tiny ice particles. Deep thunderstorm clouds which can reach up to 20 km in height contain both liquid and ice in the form of cloud and raindrops, cloud ice, snow, graupel and hail.

It is important to understand that even a cloud that looks impenetrably dark is almost entirely made of dry air. Water vapor and precipitation each make up a maximum of just a couple of percent of the mass of a cloud, except in a few very intense storms.

How do these precipitation particles form? First, tiny cloud droplets are born when the water vapor in the air is cooled and starts to condense around tiny "condensation nuclei" (particles so small they are invisible to the naked eye). The presence of these aerosols is crucial: without them, in absolutely clean air, condensation would not start until the relative humidity has reached several hundred percent (this suggests that the "saturation" level of 100% humidity is poorly defined; in fact, the atmosphere always contains more than enough nuclei of all sorts for condensation to start as soon as the dew point temperature is reached). The more particles there are in the atmosphere, the easier cloud droplets will be formed and the smaller they will be (since more particles will be competing for the same amount of water, so each one of them will attract less). This is why clouds over land have more droplets of smaller sizes than clouds over oceans where the air is generally much cleaner.

The process of ice formation similarly requires the presence of nuclei. However, there are much fewer particles which make suitable ice nuclei. This is why freezing often does not start until the temperature of the air reaches -15° C (if there are no ice nuclei at all, freezing will not occur before the temperature drops to -40° C). Hence, clouds with temperatures below 0° C can still consist of water droplets called "supercooled" water. These drops freeze immediately upon contact with any surface. When they fall to the ground as freezing rain, they can form a thin layer of sleet on roadways, an almost invisible and very dangerous hazard for drivers.

How does a hurricane form?

As rising water vapor condenses and latent heat is released, surrounding air is warmed and made less dense, causing the air to rise.  The thunderstorms that make up the hurricane’s core are strengthened by this process. As air rises within the storms, pressure at the surface decreases and moister, tropical air is drawn to the center of the circulation, providing even more water vapor to fuel the hurricane.  A hurricane has sustained wind gusts of at least 74 mph.

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