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?

Weather Science

Rainspotting: how to tell the difference between the many forms of winter precipitation.

When you walk outside this upcoming week you might notice something particularly alarming about the weather. It’s cold, real cold. Not the brutal cold Upstate New Yorkers are accustomed to, but cold enough to bring frost and possibly, just possibly a little bit of snow. Although there is a slight chance of a flurry in the forecast, Rochesterians have a better chance of seeing different forms of precipitation such as sleet, freezing rain, or even graupel. These other forms may not bring the beauty fresh white snowflakes bring, but that doesn’t mean they don’t have a significant impact on the surrounding environment.

You might recall one of those depressing winter days where the sun doesn’t seem to exist and you wished you owned a parka. It also just so happens that it is raining and the temperature at the surface is below the freezing point. You might venture outside only to fall right on your bottom due to the ice rink outside the front door.

Yeah, we know.
Photo: Wayne Nalbandian

This precipitation type is known as freezing rain. Warm air just above the surface allows falling snow to melt and fall as rain. A very shallow-layer of temperatures below 32 degrees hugs the surface causing rain to freeze on contact with roads, trees, power lines, and other structures. Light accumulations may cause dangerous travel, while heavier amounts can cause widespread, lengthy power outages.

Now think back to a time where you were driving east on the Thruway, and suddenly falling white pellets ambush your car, restricting your view to a few feet in front of you. This type of winter precipitation is known as sleet or ice pellets.

It’s always much prettier in animated form.

Sleet forms in a somewhat similar environment to freezing rain. Falling snow travels through a warm layer aloft where it melts into rain. After exiting this warmer layer, the raindrops then refreeze into pellets of ice as they fall into a very cold layer of sub-freezing air just above the surface of the earth. The difference between freezing rain and sleet is that with sleet, the cold layer near the surface is quite a bit deeper, allowing the falling rain time to refreeze. Ice pellets essentially take the form of frozen raindrops and even have the ability to accumulate. An important feature to note about sleet is that it is often see-through, solid ice. This can cause problems on any roadway, as sleet tends to act like snow when it accumulates.

Graupel. Because sleet can get more complicated.
Photo: Todd O’Bannon

Sleet is often confused with another form of wintry precipitation that people are not too familiar with known as graupel. Unlike sleet graupel is normally cloudy or white, not transparent. Graupel often forms when water droplets are collected and freeze on a falling snowflake. Since graupel is similar to sleet, it also has the ability to accumulate and cause problems on the roadways.

When you’re traveling around the Rochester area this winter, be sure to look out for these forms of wintry precipitation, as they surely will make a visit to our region.