Weather Science

What’s in a raindrop? How wind and temperature affect precipitation.

It’s amazing what a couple of degrees can do to the precipitation we see. One degree Fahrenheit can be the difference between a two-foot snowstorm or a paralyzing flash flood. Type of precipitation determines how we react to such conditions, but the size and shape of precipitation particles is equally as important when acclimating to certain elements.

The transition of seasons brings a variety of precipitation types that many people don’t even know exist. Northeasterners are more than accustomed to snow while folks in the Pacific Northwest see their fair share of rain each year. Interestingly enough, the rate that these precipitation types and various others fall at along with their initial size determines the form the precipitation takes when it reaches the surface.

For example, it’s a common misunderstanding that raindrops take a teardrop shape when falling. At the inception of a raindrop, the small diameter of the drop will allow it to fall as a small spherical figure. As rain continues to fall, the individual drops will collide and integrate into larger drops. When the drops become larger in diameter the air beneath the drop will force it to become more horizontally situated or more oblate, like a jelly bean on it’s side. Therefore, the heavier the raindrop, the more oblate it will become.

Horizontal winds can actually have more of an impact on particle size and shape than wind from under the drop. Strong winds in the horizontal will break up larger drops into the smaller spherical drops. This relationship can also be seen in snowflakes. Since snowflakes are considerably lighter than raindrops, they are able to break more easily and take the form of small flakes.

You might recall the difference in particle sizes between lake effect snow and nor’easters. Generally, lake effect storms entail high winds, which breaks flakes into smaller flakes. However, if we are on the outskirts of a large nor’easter flakes will tend to be larger with less wind.

Wind is a major factor in precipitation shape and size but temperature is the most important property when it comes to characteristics of precipitation. A slight warming of temperature (near the freezing mark) when snow is falling can create larger snowflakes since the ice crystals that make up snow can melt and become stickier, aggregating particles into larger flakes. An interesting thing to note is that around 28F, flakes can become triangular symmetrical, a rarity among flakes since generally flakes are irregular, one never resembling the next.
Particle geometries are one of the most interesting precipitation phenomena continuously being studied. New information about particle size and shape may give researchers a better understanding of what precipitation types form in certain conditions.