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.

Weather Science

50% chance of BS? Where do weathermen come up with those predictions?

For all the boaters out there, ever wake up on a beautiful, warm August morning and say to yourself, “today’s a great day to go out on the boat”. You jump out of your bed to check the forecast and to your astonishment see there is an 80% chance of rain for the day. You look back out your window in bewilderment, pondering how forecasters could predict a “likely” chance of rain on such a gorgeous day. Although many may think forecasters are making these percentages up, there is actual science behind the probability of precipitation.

The chance of rain is actually referred to by meteorologists as Probability of Precipitation (POP). POP is defined as the probability of any particular point location within a forecast area receiving measurable precipitation in a given time period. Essentially, this means that POP is the percentage chance of a specific location receiving measurable precipitation for a specific time. Measurable precipitation is defined by the National Weather Service as 1/100 of an inch.

So how do forecasters come to a certain percentage of predicted precipitation? There is a fairly easy equation that forecasters abide by to find this. This equation is POP = C x A. “C” is the confidence that precipitation will occur somewhere in the forecast area and “A” is the percent of the area that will receive measurable precipitation. So, if there is full (100% or 1) confidence that there will be rain over 60% (.6) of the forecasted area, there is a 60% chance of rain. Strangely enough, forecasters are not magicians and are not always certain if there will be precipitation. Therefore, sometimes forecasters will only be 60% confident if there will be precipitation over 50% of the forecasted area. In this case forecasters will predict a 30% (.5 x .6 = .3 or 30%) chance of precipitation. Another way to look at POP is looking at days where weather conditions are similar to that specific day and deciphering how often precipitation will occur. For example, if an area has a 30% chance of precipitation that means that 3 out of 10 days where the weather is similar, there will be a measurable amount of precipitation somewhere in the area.

As one might expect, this method is hit or miss depending on location. Often times, people will take precautionary matters when precipitation prediction is fairly high, even though it may not be for their exact spot. For example, if forecasters are 100% certain measurable precipitation is coming but only for 50% of the forecasted area, a 50% forecast for precipitation will be issued. This can cause problems for people when planning activities outdoors, especially in the summer.

Understanding how forecasters predict precipitation is important for figuring out outdoor activities. After all, who likes to be left in the rain?