April is one of the wettest times of the year in Illinois and across the Midwest. On average, Illinois receives about 4.24 inches of rain during this month. That rain can be light, with very small raindrops, or heavy, with large raindrops. Let’s explore how raindrops form and their size, and speed. I’ll try to keep the conversation from being dry.
How does rain form?
Rain forms from the change of water vapor into liquid water or ice crystals. For this to happen, the incredibly small water molecules must first attach themselves to particles like dust, smoke, or bacteria. We see this first in the formation of clouds. Within clouds, especially taller or higher ones, there will be a combination of tiny water droplets and ice crystals. Without getting into tedious details, the ice crystals will pull water away from the tiny water drops, and they will grow larger.
Wait, you are saying rain starts out as ice crystals or snowflakes? Yes! Even in summer, rain can start as snow. It must fall about 1,000 feet in above-freezing temperatures to melt into rain. Due to turbulence inside the cloud, ice crystals and water droplets are banging into each other, which makes the crystals and water droplets grow. Once the droplets are .008 inches or larger, they are heavy enough to start falling. As they start falling, they are still banging into other droplets, and still growing.
What is the shape of a raindrop?
We often see illustrations of happy little raindrops falling in the shape of a teardrop. The real shape is more like what you might think a flying saucer looks like. Raindrops typically have a flat base due to them falling through the air and have a relatively round top. Bigger drops may have more of a convex bottom.
How big can raindrops get?
Raindrops cannot get excessively big because as they get bigger, they tend to break up. Raindrops rarely get larger than a quarter inch, although it may feel like it if you are in a heavy rain.
How fast do raindrops fall?
Fall speed is affected by the size of the drop and air turbulence. The fastest speed would be around 90 feet per second. Drizzle may fall at less than 6 feet per second.
If there is a strong updraft, raindrops can be pushed up in the clouds for thousands of feet.
Do raindrops affect downbursts and tornadoes?
Research has explored the size of raindrops in storms and their potential effects on tornado development. NASA has been examining the drop size distribution in storms, which is one of many factors that determines how big a storm will grow, how long it will last, and how much rain it will ultimately produce. Smaller drops, for instance, will tend to evaporate faster and cool the air more. This leads to a stronger flow of downward moving air that can cause damaging winds when they reach the ground. However, these same downdrafts can interfere with the upward flowing air that fuels the storm and cause the storm to weaken or dissipate.
Has raindrop size changed over time?
While the size of raindrops does not appear to have changed much, the intensity of rain has increased significantly in the past 40 years in the Midwest. Rainfall events of 2 inches or more become more common.