Weather: So Why Does It Rain?

So Why Does It Rain?

Wait a minute! After all this, we still don't know why it rains? Talk about beating around the bush! I guess that's what you'd expect from someone who's been a college professor for three decades. But we're almost there.

In order for it to rain, the air must be saturated: The relative humidity must be 100 percent, or very close to it. How does that happen? By lowering the temperature or bringing in more water vapor. But if we just bring in more water vapor and nothing else happens, we're still just stuck with microscopic droplets. So let's focus on lowering the temperature. The temperature can be lowered by a colder wind, possibly coming from the north. But those north winds contain less water vapor, so the temperature may lower, but the humidity could remain the same.

The temperature could also fall if a column of air rises. As an air column rises, there is less atmosphere around it, which reduces the pressure outside the column. So the column expands as it goes up. That expansion will lower the temperature because the air molecules will not hit each other as frequently. Eventually the relative humidity will reach 100 percent, and the rising column loses no water vapor. Rising air seems to be the surest way of getting a cloud to form.

Also, as the air rises, the moisture is distributed over a wide region. There are plenty of droplets available for coalescence, and if the rising takes the cloud to great heights, ice will form—even during the summer, even in tropical latitudes. So rising air delivers the best environment for cloud droplets to grow into raindrops. The figure below shows the dynamics involved with natural rainmaking.

And that is the message of this section. Upward motions make rain happen. The faster the air goes up; the harder the rain comes down. Everything else that occurs within the atmosphere simply builds on this basic principle.

Those Ups and Downs

A typical rainy day might be generated by relatively gentle upward motion—a couple centimeters per second. That would normally occur over 12 or 24 hours, possibly longer. The intensity of the precipitation would be directly related to the magnitude of that rising motion. The sky would first be covered by cirrus clouds, which would then thicken, lower, and eventually lead to nimbostratus clouds.

Weather and associated rising currents.

Weather Type	Vertical Velocity
Light rain	1-4 cm/sec
Moderate-heavy rain	4-20 cm/sec
Thunderstorms	20+ cm/sec
Severe storms and tornados	10-50 m/sec

In contrast to this slow and gentle process, the upward motions can be violent in those tall, cumulonimbus clouds that bring thunderstorms and even tornadoes. The vertical velocity can be a thousand times greater—20 meters per second or 40 miles per hour—with updrafts approaching 100 miles per hour. Large hail takes shape in these clouds because the upward motion keeps the ice suspended for an extended time, and the ice has a chance to build up. Eventually, it has no choice but to fall.

Of course, if the air motions are downward, the sky is clear and the weather is dry. Spells of bright weather are directly related to that subsiding motion. Instead of condensation taking place, evaporation occurs. Ahead of a storm the air is rising, but after the storm passes the atmosphere is sinking, which is why skies clear soon after a storm center moves away. Subsidence is not without its weather headaches. If the atmosphere always sinks, desert conditions will prevail. The desert zones around the earth occur where the air is sinking. Also, serious air-pollution levels appear when subsidence is present. Air pollutants are unable to rise through the stable atmosphere created by the subsidence.

We can explain much about the weather just by knowing the nature of the atmosphere's vertical motion. Throughout the coming chapters, we take a look at the mechanisms that cause that upward and downward motion to occur. But if you close this without learning anything else, be assured that you now understand a lot about meteorology. You know why it rains.