Drizzle can be pesky. Light to moderate showers can be refreshing. Heavy rain can be concerning if too much falls in a particular area.

However, before that first raindrop falls from a cloud, it has to grow.  

In a cloud, there are countless cloud droplets of various sizes suspended in the air that are not large enough to fall from the cloud as rain.

Through a process called Collision and Coalescence (for warm clouds above freezing at all levels), large cloud drops fall within the cloud and collide with smaller cloud drops.  

They end up sticking together or coalescing to form even bigger cloud drops. Through this process, the cloud droplets eventually grow large enough to fall from the cloud as rain. 

If a cloud’s temperature is both warm and cold (i.e. has liquid water and ice crystals), the cloud droplets go through a more complex process for precipitation formation. This process is called the Bergeron (Ice-Crystal) Process.

Raindrops are not all created equal once they form. Drizzle contains raindrops that have a diameter of less than 0.5 millimeters. Drizzle is the lightest form of rain and may seem to float, but unlike fog, it does hit the ground. 

A typical raindrop is roughly 2 millimeters, but a large raindrop can grow close to 5 millimeters in diameter. A drop that big usually breaks up into smaller droplets.  

Size matters in terms of a cloud drop graduating into a raindrop, but it doesn’t matter in terms of rainfall intensity. So, big raindrops don’t mean heavy rainfall. Rainfall intensity is more-so based on how much rain falls within a certain time. 

According to the book, Meteorology Today, by C. Donald Ahrens, rainfall is classified as:

• Light if less than 0.10 inches of rain falls per hour
• Moderate if 0.11 to 0.30 inches of rain falls per hour
• Heavy if more than 0.30 inches of rain falls per hour
  

So, the next time it rains, remember that those drops had some growing to do before they could fall in your area.