All over the planet the sun helps nature recycle water in the water cycle, also known as the hydrologic cycle. Water keeps us and the Earth system healthy. All living things are connected to the planet’s water. That means we shouldn’t waste it, and everyone must be responsible for making sure that our water quality is good. A scientist who studies how water moves around the planet is a hydrologist.
Condensation happens when air cannot hold any more water vapor. To make condensation, just keep filling the air with water vapor by evaporating more and more of it. When we shower, many of us make condensation clouds. Another way to make condensation is to cool the air. This can happen when wind pushes air up the side of a mountain, where it cools. We find condensation on the outside of a glass of ice water. Clouds form when water vapor condenses in air, or when water vapor changes to ice crystals in air.
There are two special kinds of condensation where no cloud is formed. They both happen after a clear night, with calm wind. Sometimes we wake up to find small beads of water on grass, or cars. That’s dew. When the temperature is below freezing you might see ice crystals instead, especially on windows. That’s frost. You can tell that it didn’t fall from the sky because the sky is clear. That type of condensation is called deposition- when water vapor changes directly to ice crystals. Dew and frost happen when the temperature cools down and the air can’t hold any more water vapor. The dewpoint temperature is a way to measure how much moisture is floating in air. When the temperature is cooled to the dew point temperature, dew can form.
Precipitation is any form of water that falls from clouds. It may be liquid or solid, and that includes rain, snow, sleet, and hail. Since precipitation falls, you can call it a meteor or hydrometeor. That’s where the word meteorology comes from- the study of precipitation. Too much rain or snow causes problems for people, plants, and animals. Too little rain or snow also causes problems because we would not be able to store enough water to drink, bathe, cook, wash, and stay comfortable. We also would be in trouble without water to fight fires.
Rain is liquid water that falls from clouds. If clouds are small and spread apart, you might get just a little rain that starts and stops. Rain that falls from large, dark clouds can be light or very heavy, and it could last for many hours or even for several days. Rain is a critical way to get fresh water for farms, animals, grass, and trees. Records of rainfall tell us that in the rainforests of Hawaii more than ten feet of rain falls each year, while in the deserts of Arizona, no more than 6 inches of rain falls in most years. Usually rain is fresh, clean water, but when it falls through polluted air it can carry pollution down to the ground as acid rain.
Cold weather creates snow instead of rain, in many regions of the world. We also find snow at the tops of tall mountains where the air stays very cold. Snow is tiny crystals of ice that stick together in the clouds. The crystals grow until they are heavy enough to fall to the ground. These ice crystals are just like the ones you find on things inside your freezer. Snowflakes are light but when they fall and add up into piles and mounds, they become very heavy. It takes a lot of energy and money to clear snow from streets to make travel safer. Snow that melts will soak into the ground or runoff and flow into rivers and lakes where it is stored. Snow may also change directly back to a gas, without melting first. That’s called sublimation.
Sleet is a mixture of rain and snow. It is often snow that halfway melts before reaching the ground, or it can be little pellets of rain or half-melted snow that freeze in the air, and often bounce when they hit the ground or a hard surface.
Hail is balls or chunks of ice that falls from clouds. A lot of hail comes from tall thunderstorms, even on a hot day, because the temperature in the tops of the thunderstorms can be far below freezing. Hail can be as tiny as a pebble, or the size of a marble. If a storm is strong, hail can be the size of a golf ball. The largest hail from severe thunderstorms is the size of a grapefruit. When hail falls, it tells you that the ice formed many miles above the ground where the temperature is far below freezing. Big hail from thunderstorms is dangerous, especially to animals. Some hail falls from clouds that are just cold, not thunderstorm clouds. That hail is usually not very big.
Runoff describes what happens to precipitation after it hits the ground. It simply will run off, downhill. Of course, if it is snow, sleet, or hail, it has to melt first, but a lot of the water that falls from the sky flows in streams and rivers until it ends up in a lake or ocean. When water runoff is extreme, it carries pieces of soil and rock and moves them downhill. That’s called erosion. Runoff from heavy precipitation can be destructive. It can fill streams with sediment to make it hard for fish to breath. Heavy runoff can also carry pollution great distances. It can muddy the water, so plants have a hard time getting sunlight to remain healthy. Runoff may weaken tree roots. It can be heavy in big cities, and in fields where there is little grass or plant life to help slow the water and soak it up. Precipitation that does not runoff slowly soaks into the soil. This is called percolation. As the water sinks into the ground, the soil and plants help to filter it and clean it. Water that percolates deep into the ground is then called groundwater.
Floods happen when too much water rises in one spot. Flash flooding is the fastest type of rising water, common when heavy rain falls over the same spot and can't drain fast enough. The floods may come from intense thunderstorms sitting in place, or from several hours of heavy rain. Water always runs downhill and rises in the low areas within a watershed. Floods could be standing water or moving water. Swiftly moving water, especially when it is carrying debris, has the energy to eat away hillsides and road underpasses to undermine the soil. In the worst cases, a road may be washed away beneath the floodwaters. Flash flooding takes lives because people underestimate the power of moving water, even when the water seems to be shallow. A vehicle crossing water that's not very deep can easily lose traction and be swept off a roadway into deeper water. A person walking through fast-moving water that's less than a foot deep can easily lose footing and be knocked down and carried away.
River flooding can happen suddenly, or it can be a much slower process. Some river flooding is seasonal, when the warmth of spring triggers the melting of a snowpack built up over the winter. In extreme cases, rivers increase in width, depth and in speed. They spill over their banks and flood low-lying land in floodplains for days or weeks. Rivers with melting ice also may rise further if the ice creates dams. With population growth and expansion, many homes and businesses are developed closer to, and inside of, floodplains. A floodplain is the natural basin for river overflow. New development also can redirect runoff, making water rise in places where it never flooded in the past.
A Flood Watch means flooding is possible and you should be prepared to take action. A Flood Warning means flooding is happening and you must take safety steps immediately.