PRECIPITATION

What is evapotranspiration?

Evapotranspiration is the combination of water vapor being evaporated from the surface of the Earth (such as from lakes, rivers, or puddles) into the atmosphere, and transpiration, which is the movement of water from plants to the air.

How does the hydrologic cycle work?

The movement of water from the atmosphere to the land, rivers, oceans, and plants and then back into the atmosphere is known as the hydrologic cycle. We can pick an arbitrary point in the cycle to begin our examination. Water in the atmosphere forms clouds or fog and falls (precipitates) to the ground. Water then flows into the ground to nourish plants, or into streams that lead to rivers and then to oceans, or it can flow into the groundwater (underground sources of water). Over time, water sitting in puddles, rivers, lakes, and oceans is evaporated into the atmosphere. Water in plants is transpired into the atmosphere, too. The process of water moving into the atmosphere is collectively known as evapotranspiration.

What is latent heat?

The concept of latent heat was discovered by Scottish chemist Joseph Black (1728-1799), as well as, independently, by Swiss meteorologist, geologist, and physicist Jean André Deluc (1727-1817). When water condenses, cools, or freezes it loses energy and, thus, gives off heat. Water condensing into vapor releases an amount of energy equal to 600 calories per gram of water; when water freezes, it releases about 80 calories per gram. The heat released provides energy for storms to form.

What is a "white-out"?

An official definition for "white-out" does not exist. It is a colloquial term that can describe any condition during snowfall that severely restricts visibility. That may

How fast does rain fall?

The speed of rainfall varies with drop size, wind speed, and the size of the raindrops. A typical raindrop in still air falls about 7 miles (11 kilometers) per hour. Large raindrops can reach speeds of 16 to 20 miles (26 to 32 kilometers) per hour, while the tiniest drops are slower than a mile (1.6 kilometers) per hour as they drift to Earth. Thus, the larger drops can usually splash onto the ground about three minutes after being generated in a rain cloud that is about 5,000 feet (1,500 meters) above the surface.

mean a blizzard, or snow squall, etc. If you get some sunlight in the mix, that makes the situation even worse—it's like driving in fog with your headlights on high-beam. The light gets backscattered right into your eyes and you can't see.

What methods have been used to try to remove hazardous fog?

Helicopters have been used in the past to try and blow away fog; and in France people have used jet engines at airports to heat up the air. Both of these methods are very impractical, however.

Are there different categories of rainfall?

Yes. Rainfall is categorized into three types:

Convective rain happens when the Sun warms the air near the ground; as the air then rises, it cools in the higher altitudes and water droplets form, creating a rain shower.

Orographic rain is caused when air masses are elevated due to a geological feature such as a mountain. At the same time, land forms create a kind of squeegee effect on moisture as it runs into mountains. The result is the same as with convective rain, because as the air cools in the higher elevation, rain may result on the windward side of the hill or mountain.

Cyclonic rain is the result of interacting air masses, which collide and force warm air masses upwards. This type of rain formation often results in strong thunderstorms or hurricanes.

What is the shape of a raindrop?

Although a raindrop has been illustrated as being pear-shaped or tear-shaped, highspeed photographs reveal that a large raindrop has a spherical shape with a hole not quite through it (giving it a doughnut-like appearance). Water surface tension pulls the drop into this shape. As a drop larger than 0.08 inch (two millimeters) in diameter falls, it will become distorted. Air pressure flattens its bottom and its sides bulge. If it becomes larger than one-quarter inch (6.4 millimeters) across, it will keep spreading crosswise as it falls and will bulge more at its sides, while at the same time, its middle will thin into a bow-tie shape. Eventually in its path downward, it will divide into two smaller spherical drops.

 
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