It drizzles, it pours, it pelts. Rain falls on the just and the unjust, winners and losers alike. In some ways, rain is the planet's great equalizer, yet rain falls anything but equally around the globe. You could spend a hundred years in the middle of the Sahara and collect less rain than an Hawaiian villager might see on a single wet day.
The life of a raindrop
Rain is only one feature on the vast, endless loop scientists call the hydrologic cycle. A good place to hop aboard this cycle is over the ocean. Each day, trillions of gallons of water escape from the sea surface to join the atmosphere as water vapour. This takes energy. As each molecule of H20 evaporates, it absorbs a tiny bit of heat. The warmer the water, the more evaporation occurs. Lakes and other bodies of water also add moisture to the air, but more than 80 percent of the planet's water vapour comes from the sea.
It takes a little over a week for the average water-vapour molecule to fall back to Earth as rain. During this short spell of flight, it may get swept thousands of miles. In fact, much of the rain that falls on continents is comprised of moisture from far-off oceans. But before it can fall as rain, our wayfaring molecule has to join thousands of others to make a raindrop. This happens inside clouds, where the relative humidity is at, or just over, 100 percent. This air is saturated – it's been cooled (by lifting, usually) to the point where some of its water vapour must condense onto the nearest particle of salt, soot or other material. The end product is a cloud droplet, but one that's initially too small to fall as rain by itself.
There are two main avenues for raindrop production. The simpler of the two is the warm rain process. It predominates when most of a cloud is positioned lower than the freezing level, as is often the case in the tropics. A growing cumulus cloud might have one or two hundred droplets scattered through an area the size of your fingertip. Each of these droplets is approximately 0.001cm/0.0004in wide, less than half the width of a strand of human hair. As these droplets fall at varying speeds, based on their sizes, and bump into each other, some of them grow larger than others. The bigger they get, the faster they fall, and along the way, they coalesce with smaller droplets they collide with. This allows them to fall even faster and to absorb still more of their peers. In as little as half an hour, a raindrop is born, perhaps a hundred times wider than the many thousands of cloud droplets it incorporated.
When ice is present in a cloud, a different process – cold rain – takes the lead. Ice crystals are able to form only on a few special kinds of nuclei (primarily dust), so they're far outnumbered by water droplets. However, the crystals grow more quickly than the droplets, since water vapour tends to be drawn more to ice than to liquid water when in the presence of both. Some of these fast-growing crystals are soon big enough to qualify as snowflakes. Through mechanisms that aren't yet fully understood, the crystals throw off tiny shards that attract water vapour of their own. The result is snow, forming far above ground, that turns into rain on its descent through warmer air.
How fast and how much?
Only a fraction of the moisture in a typical thunderstorm becomes rain. Even the raindrops that exit a storm may not survive their trek to earth. When a rainshower forms above extremely dry air, the rain sometimes evaporates before it reaches the earth. The streaky curtain that hangs from the cloud base is called virga. It's a common sight across arid and semi-arid regions, especially in the summer.
Shallow, low-slung clouds – the kind you might see hugging a coastline or banked against a mountain range – don't have the vertical extent necessary to produce heavy rain. These make drizzle, the misty droplets that may be only a tenth of the diameter of a bona fide raindrop. Drizzle-producing clouds can be less than 300m/ 1000ft thick. The sun might be shining on top of a coastal mountain even as people on the beach are hauling out their raingear.
When air is being forced strongly upward – whether by winds blowing up-slope, the dynamic force of an approaching upper-level low, the buoyancy of a hot and humid air mass, or all of the above – a much deeper cloud may form Thunderstorms can stretch more than 16km/10 miles from top to bottom, and the temperatures inside may range from 15°C/ 59°F at cloud base to -60°C/-76°F within the icy cirrus doud at its crown. With this much of a cooldown for air parcels that rise to the top, a lot of moisture gets wrung out, and the droplets and ice crystals that are left behind have more room to collide and coalesce on their way down. All this leads to the potential for seriously heavy rain. Especially when thunderstorms are embedded in a tropical cyclone, the amounts they can dump are astonishing.
Even a less imposing garden-variety shower can rain with surprising gusto. Anyone who's visited the wet tropics knows how a seemingly innocuous batch of cloud can produce torrents in a few minutes, then nonchalantly move on What's helping these modest showers to pour is the sheer amount of water inside them. A nice, juicy tropical cumulus might have 50gm/2oz of water in a space the size of your living room. That doesn't sound too impressive until you realize just how large even a tiny cloud is. A cumulus that's only about 1.6km high and wide (1 mile in each direction) could contain over 3.8 million litres/1 million gallons of water. It doesn't take much time for the droplets in this dense a cloud to reach raindrop size.
Rain shadows and seasons
If all the rain and snow that fell throughout the course of a year were melted and spread evenly over the planet, it would form a layer roughly 1000mm/39in deep. Among the regions that average close to this global norm are western Europe and the prairies of the US and Canada. However, many places get substantially more, or much less, than this. The most dramatic rainfall contrasts within small areas are tied to mountainsides. The prevailing global wind patterns tend to dampen the west sides of mid-latitude mountains and the eastern sides of tropical peaks. These are some of the most dependably rainy places on earth, although a serious drought can alter things enough to dry out even the most sodden slopes. On the opposite sides of these mountains, you'll often find rain shadows. These regions get far less precipitation than their neighbours just over the ridge. Even a shift of a few miles or kilometres can cut average rainfall by half or more. Across the Big Island of Hawaii, rainfall drops from a yearly average of around 5100mm/200in at Kahuna Falls (which faces the trade winds), to less than 250mm/10in on the downwind Kohala Coast.
Most places tend to get more rain at one time of year than another. Usually the highest amounts are in the summer, if only because the air tends to be wanner and/or more humid. Mediterranean-style climates – like those of Italy or coastal California – are the exception that proves the rule. Here, persistent high pressure keeps the land drier in the summer, while winter storms bring the bulk of the rain. Monsoons – including the proto-typical ones across southern and eastern Asia – bring the most spectacular seasonal shifts in rainfall. After a bone-dry winter and a brutally hot spring, Lidia becomes a land of greenery and relief when the summer monsoon hits. A few months later, the circulations shift and the heavy rains move south toward Indonesia and northern Australia, only to return the following summer.
Drought can strike almost anywhere, but there are some parts of the globe where it simply doesn't rain much at all. These tend to be clustered around latitudes 30°N and 30°S, where the overall global circulation tends to produce sinking air and the most extensive deserts on earth.
All in all, it seems to be raining a bit more, and a bit harder, than it used to. Data spanning the entire Earth's surface is hard to come by, but the best estimates indicate that average annual global precipitation over land rose by perhaps one percent in the twentieth century. In many locations, the moisture that does fall is now focused in more intense bursts as opposed to longer, steadier rains. The most likely culprit is a global increase in water vapour, as suggested by data from balloon-borne instruments and satellites over the past several decades. Warmer oceans tend to evaporate more of their water, so the apparent water-vapour increase is one of the many hints that global warming is making its presence felt. On the other hand, warmer temperatures also help rainfall to evaporate more quickly, so the global risk of drought is not necessarily going down.