I had absolutely no intention of writing anything today, but my commute changed everything. I have always been fascinated by rain. For much of my career at NASA, I served as Deputy Project Scientist for the Global Precipitation Measurement (GPM) Mission. As I drove Georgia's 316 with the wiper blades feverishly trying to remove water from my windshield, 4 odd facts about rain came to mind that I wanted to share.
Raindrops are shaped more like hamburger buns. As a rain drop falls, it becomes less spherical in shape and becomes more flattened on the bottom like a hamburger bun. This is primarily related to their fall speed. According to the NASA educational website associated with the precipitation program:
Air flow on the bottom of the water drop is greater than the airflow at the top. At the top, small air circulation disturbances create less air pressure. The surface tension at the top allows the raindrop to remain more spherical while the bottom gets more flattened out.
Over time, large raindrops may break-up into smaller drops. Check out this video for more discussion.
Yep, rain does have a smell. Petrichor is the word that describes the scent of rain. If we want to be technically correct, however, it describes an oil released from the Earth. The word is apparently derived from the Greek words "petra" (stones of the Earth) and "ichor." Ichor is from Greek mythology and is related to the blood of the gods. The basic chemistry of the process is related to decomposed organic material fused with soil, rock, and minerals in an interesting chemical brew. Livescience describes the process more eloquently than I can:
Some plants secrete oils during dry periods, and when it rains, these oils are released into the air. The second reaction that creates petrichor occurs when chemicals produced by soil-dwelling bacteria known as actinomycetes are released. These aromatic compounds combine to create the pleasant petrichor scent when rain hits the ground.
Even though it is raining where you are, it probably started as snow. In many places, the Bergeron or "cold rain" process is responsible for the formation of rain (even on a hot summer day). Clouds are often a mixture of ice crystals and super-cooled water drops, and some interesting things happen because of that fact. I like the discussion on the College of Dupage Nexlab website:
The air reaches saturation and some of the resulting droplets will come in contact with freezing nuclei (assuming they have reached the activation temperature ). We will now have a combination of ice crystals and supercooled water droplets. From the perspective of the supercooled droplets, the air is in equilibrium at saturation, but from the perspective of the ice crystals, the air is supersaturated. Therefore, water vapor will sublimate on the ice crystals. Since the amount of water vapor in the air has decreased, and from the perspective of the supercooled water droplet, the air is subsaturated, the supercooled water will evaporate until the air once again reaches saturation. The process then continues.
Eventually, the ice crystals (snow) will fall. The ice crystals may also grow through accretion of more supercooled liquid or aggregation (clumping) of other crystals. The type of precipitation that we ultimately see at the surface depends on the temperature of the atmosphere from the base of the cloud to the ground (see the figure below).
There is also process by which rain grows through "warm rain processes" as well, but this is more common in clouds that remain completely above the freezing point. This condition is likely found in the tropics and in some parts of the mid-latitudes but is less important than the Bergeron process. The main growth mechanisms in warm rain processes is "Collision and Coalescence". You have probably noticed an example of this when drops collide on your car windshield and then grow larger as they trickle downward.
Rain may worsen Asthma. Intuitively, you might think that rain would lessen suffering from asthma. However, research led by my colleague Dr. Andrew Grunstein along with colleagues at the University of Georgia and Emory University have added to the body of literature that thunderstorms can trigger asthma response. I was also a part of one of the studies. The hypothesis are still not conclusive, but the thinking is that winds associated with thunderstorms lift pollen and mold spores. The rainfall bursts the particles into smaller aero-allergens that can get into the lungs. The thunderstorm downdrafts then spread the particles. This article in the Washington Post is a nice summary of the history of this fascinating work.
Ok, back out into the rain and off to the University of Georgia to teach about precipitation.
