Precipitation
Water falls from the sky in many forms. What causes it? In a word, the answer is lifting. The idea is that air can hold a certain amount of water depending on the temperature and pressure. Water is "dissolved" as a gas in air (mostly nitrogen, N2 and oxygen, O2) much as a liquid solute is dissolved in a liquid solvent. Hot air can hold more water as a gas than cold air. As air cools, the solubility of gaseous water in it decreases.
The graph below shows the maximum partial pressure (in atmospheres) of water in air for air between 0˚C and 100˚C.
Maximum partial pressure of water in air
as a function of temperature
You can see from the graph that warm air can hold more water than cold air. So if we take a mass of warm air with a certain amount of water dissolved in it, and cool it down, at some point the water will condense and drop out of solution – it will precipitate.
In general, with just a couple of special exceptions, air temperatures are cooler as we go higher into the atmosphere. That's why it's always so much cooler high in the mountains. That relative coolness means if a mass of water-laden air is lifted to a higher altitude, its temperature will generally drop, causing it to be able to hold less water, so some of it may condense into clouds, which are water droplets or ice crystals suspended (not dissolved) in air, and some may even form droplets heavy enough that they fall to the ground as rain, snow or some other form of precipitation.
Any form of lifting of an air mass can lead to precipitation. In this section we'll include the big four of these. They are
- Frontal lifting
- Orographic lifting
- Cyclonic (or convergent) lifting
- Thunderstorm lifting
Differential heating by the sun, which occurs because of differences in latitude, the angle with which the sun passes through the atmosphere, whether sunlight strikes continents or oceans, and a lot of other kinds of differences, leads to warmer air masses and cooler ones being formed over time. As these move around the planet by the prevailing winds (west-to-east in the northern hemisphere and east-to-west in the southern), the rising of warm air masses and the sinking of cold ones, and other more local forces like terrain, they sometimes collide. We call the leading edges of these air masses warm fronts and cold fronts
If a warm air mass collides with a cooler air mass, the lower density of the warm mass will cause it to ride up over the colder mass, thus lifting a mass of air that can hold more water and causing it to cool.
Many storms that produce rain or snow are caused by frontal lifting. When a warm front hits a cold front, the lifting can be dramatic, causing heavy precipiation events. A cold front can move in "behind" a warm front, lifting it in the same way, its denser air sliding in beneath the less-dense warm air.
Top: A faster-moving warm front overtakes a slower-moving cold front. Center: Warmer air rises over the colder air, cooling in the process. Bottom: Water vapor in the warm air mass condenses and precipitates in the form of rain or snow.
When warm and cool air masses meet, the warm mass rises up over the colder mass, cooling the warm air, potentially producing precipitation.
Orographic lifting
Sometimes in mountaineering, we say that "mountains make their own weather." That's partly true: The potential for weather was there; it was the mountain that made it happen.
Consider the diagram. If a moisture-laden mass of air is moving across the land and it encounters a mountain, the slopes of the mountain can force the mass upward (the only place it can go if it's moving horizontally) into the cooler air above. This is called orographic lifting. Orographic means "relating to the shape of mountains."
Orographic lifting is the reason why mountains tend to receive more rain and snow than flat lands. These regions are very important for our fresh water supply. They're storehouses of fresh water in the form of snow. They charge aquifers (underground reservoirs of fresh water) and if there is enough snow accumulation, glaciers – massive storehouses of fresh water in the form of snow compressed into ice – can form, grow and slowly flow down the mountain.
Top: A warm air mass moves through warm air until it hits a geographic obstacle, like a mountain range. Center: winds force the warm air up the range into the cooler air. Top: When the warm mass is cool enough, the water vapor it carries nucleates and condenses into precipitation – snow if the air is cold enough.
Cyclonic lifting
Because of the spinning of Earth on its axis, any linear motion on a large scale, such as the prevailing west → east winds of the northern hemisphere, are more complicated than they seem.
Because Earth is a spinning sphere (it's very nearly spherical), larger storms in the northern hemisphere tend to spin in a counterclockwise direction, opposite their counterparts in the southern hemisphere. This spinning is due to the coriolis effect. You might have heard that water in sink drains in the southern hemisphere spins in the opposite direction as it does in the north. While the idea is sound, the coriolis effect is too weak to work on water at that small scale. But giant storm fronts moving across the surface of a planet are indeed subject to this effect. The resulting cyclonic flow tends to lift air in the center of spiraling wind.
The diagram shows that as cyclonic winds circulate, they push air into the center of the spiral. Because the air can't expand downward into the ground, it moves upwward, pushing low-level air into colder regions above. If the lower-level air is moisture laden, precipitation is likely to occur in the center of the cyclonic flow.
Cyclonic flow, by the way, is a way to determine whether a large storm system is on its way in or out. Take a look at that spiral diagram. A storm moving from west to east in the northern hemisphere will begin with winds from the south and southeast. On its way out, the winds experienced on the ground will shift to north and northwest. Try it the next time a multi-day storm front passes your area.
Thunderstorm lifting
Thunderstorms generally build, occur and diminish over a cycle of about 12 hours in most locations. They begin with heating of moist land areas by the sun, usually on summer days when solar heating is most intense. Water that evaporates from bodies of water, wetlands, farmland, forests, and so on, is heated by the sun and rises into the atmosphere, where it eventually condenses into clouds as water droplets or ice crystals not yet heavy enough to fall, but dense enough to scatter light and be seen as white, fluffy clouds.
As the day goes on and solar gain continues on the ground, more rapid heating of moist, low air causes clouds to build and combine into large thunderheads, towering clouds that can reach the stratosphere. These clouds can build so high that the winds in upper layers of the atmosphere can begin to shear off the tops, forming a recognizeable anvil shape. They are dense enough that they become dark near the bottom as they block sunlight.
Air currents in thunder clouds can be complicated. Intense heating of low-level air can cause high-velocity updrafts (upward winds), and the downward forces of droplets or ice crystals now big enough to precipitate can create a competing downward flow of air. In these conditions, friction between droplets or ice crystals can cause stripping of electrons away from molecules, resulting in a charge difference between areas of the cloud (usually vertically separated), that can cause lightning, sudden discharges of electricity that ease or neutralize the charge imbalance in the cloud.
At some point, updraft winds and the masses of the precipatated droplets or crystals inside the cloud become unbalanced and the cloud begins to precipitate. Whether that precipitation is in the form of rain or ice depends on conditions in the cloud. Hailstones can form when temperatures in the cloud are so low that droplets freeze. These are often transported by updraft winds back to the upper reaches of the cloud, where more freezing water attaches. This cycle can occur many times, building the average size of the hailstones as it repeats. Golf-ball sized or larger hailstones have been recorded. A large hailstone, preserved frozen and cut in half, will look like an onion, composed of layer-upon-layer of deposited ice.
As a thunder cloud "rains out", the resulting downdraft can push wind out in advance of the moving cloud, producing tremendous thunderstorm winds in front of the rain.
One of the most interesting encounters of a human with a thunder storm is the experience of Lt. Colonel William Rankin, who in 1920 had to parachute from his damaged plane. He ejected and fell through a thunder cloud. The fall took roughly 40 minutes as he fell and was lifted again over an over inside the cloud. You can read more about his story here, and he wrote a book about it called "The Man Who Rode Thunder" (ISBN 9780135482711).
Because of the recognizeable pattern of building to the storm, thunder storms are in one sense easy to anticipate. They are, however, unpredictable in their final stages. They can produce tremendous amounts of rain, lightning, very strong winds and even tornados. A good rule of thumb for being in the outdoors, especially at high altitudes, is to be well out of harm's way well before thunderstorms build to the point that they're dangerous. This means getting off of high ridges early in the day.
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