Category: Weather Science

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Weather Science

Our tepid Yuletide, by the numbers.

It’s easy, whenever there’s a seemingly radically different year or month’s worth of weather to start talking Climate Change. The truth is: climate and weather are very different things.

Neil DeGrasse Tyson had an interesting illustration of this in his Nova series. The whole episode is below for you to enjoy. But climate is like a man walking a dog, which is weather. The dog can bound from one side of it’s master to the other, sniffing back and forth, or higher and lower for the purposes of our discussion. But the master defines the range in which the dog can move.

https://www.youtube.com/watch?v=ubTJXF5MwMc

Similarly, we can experience great shifts of temperature, year over year, without them affecting the overall climate. Even more importantly from the perspective of planet-wide climate: one region’s weather does not a climate make.

Still, this past December was extraordinary. It’s one thing to have a “Brown Christmas,” of which every Rochesterian is acquainted. It is quite another to experience 70 degree temps for Christmas!

So, here’s a quick summary of the muddy Christmas of 2015, so you can really tell your grandkids. Someday..

[posts post__in=”17335,17337,17338,17336″ loop=”datagrams” post_type=”datagram”]

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Weather Science

El Niño? La Niña? No. Get ready for “La Nada.”

Picture: <a href="http://www.jpl.nasa.gov/m/news/index.cfm?release=2013-272">NASA-JPL</a>

Fall is undeniably in the air: the days are getting shorter, leaves are beginning to turn. And today, we’re jetting across the 70’s, skipping right over the 80’s, en route to a 90-degree day. What?

Maybe this unsettled weather we’ve been experiencing has an overarching explanation. Weather scientists at NASA are studying a phenomenon in the Pacific Ocean which is not like El Niño or its opposite, La Niña. They call this extended period of normalized water temperatures “La Nada“:

“Without an El Niño or La Niña signal present, other, less predictable, climatic factors will govern fall, winter and spring weather conditions,” said climatologist Bill Patzert of NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “Long-range forecasts are most successful during El Niño and La Niña episodes. The ‘in between’ ocean state, La Nada, is the dominant condition, and is frustrating for long-range forecasters. It’s like driving without a decent road map — it makes forecasting difficult.”

While most of us think of Niño/Niña dynamic in terms of weather patterns, they actually refer to water temperature patterns in the Pacific Ocean. Since above average water temperatures mean more evaporation, extended periods of high temps result in changes to our weather system first identified in the media as El Niño. When temperatures are below average, the opposite effect holds true. This system is referred to as La Niña.

But periods which are neither above or below average mean the weather they effect could swing wildly either way. Climate scientists note that some of the wettest and some of the driest conditions have occurred during this hilariously named La Nada.

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Weather Science

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

Photo: <a href="http://www.flickr.com/photos/aj-enthusiast-photography/8629682281/sizes/l/in/photostream/">AJ Enthusiast</a> @ Flickr.com

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.

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Weather Science

Wicked windy weather: the jet stream, “jet streaks,” and severe weather.

Last week we discussed the role jet streams have on temperature. Northward movement of the jet allows for warmer air from the south to penetrate into the higher latitudes. Conversely, as the jet moves south toward the equator, chilly air from our Canadian friends permeates southward into the heart of the U.S. These drastic temperature variations can happen quickly and become quite an annoyance. However, the jet is associated with much more than temperature differences, as any area in the path of a strong jet stream can be subject to severe weather and significant precipitation.

Pressure systems and the Jet

Regardless if you’re a weather novice or expert, most people have heard the terminology “low and high pressure systems”. Discussing the development and formation of pressure systems is a looooong conversation for another day, but there is an obvious correlation between these pressure systems and the mid-latitude jet.

As cold air pushes southward, the jet is thus forced in the same direction and a trough in the upper atmosphere (5–8 miles above the surface) digs southward. In the opposite direction, warm air forces the jet northward, resulting in a bump in the jet also known as a ridge.

Large surface low-pressure systems form immediately to the east of an upper-level jet in the trough. Most lows have fronts attached known as warm and cold fronts, and these fronts give us much of our severe and rainy weather. Often, the most intense weather is associated with cold fronts as cold air violently lifts warmer air upwards, triggering precipitation. The greater the temperature difference, the stronger the cold front which is then able to produce more lifting. The stronger the jet is aloft, the greater the temperature difference at the surface, which can result in more precipitation.

Streaking in the Jet

Although a jet stream is defined as a thin current of rapidly moving air, flowing west to east in the upper part of the Earth’s atmosphere, there are sections within the jet that are faster than its surroundings. These sections are known as jet streaks and are usually located between the trough and ridge in a jet. Since jet streaks are faster than their surroundings, the air aloft diverges faster, which creates lower pressure at the surface and consequently enhances the amount of precipitation.

Think of a jet streak as a bottle of soda. The regular jet stream is a gently shaken bottle of soda and when opened the soda might fizz to the top or barely fizz over, removing only a little soda from the bottle (creating a weak low pressure in the atmosphere). On the other hand a jet streak is like a violently shaken bottle of soda, when it’s opened the soda explodes out of it (creating a strong low pressure in the atmosphere).

The evolution of the jet stream is one of if not the most important weather phenomena to understand. If stormy weather is coming your way, the amplified jet is probably to blame.

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Weather Science

It’s spring!! But, not so you’d really notice it.. Jet stream blues.

Photo: <a href="http://www.flickr.com/photos/18663463@N03/8415284912/sizes/l/">wizardofgurl</a> @ Flickr.com

Since April fools day is just around the corner, many Rochesterians might be thinking the local meteorologists are playing some joke by forecasting highs that will barely reach the 40 degree mark along with a high possibility of a mix of rain and snow for Monday, April 1st. Unfortunately, that seven-day forecast graphic couldn’t be more accurate.

Although spring technically began March 20th, there has been little to no evidence of “spring” along the eastern seaboard so far this year. Rochester has been averaging about 4°F below the March average of 43°F. This is a far cry from a year ago as the average temp over the course of the month soared to 57°F. Without a doubt it is human nature to want to blame something for this awful spring.

So what do I blame?  I blame the jet stream.

What is a jet stream?

A jet stream is a thin current of rapidly moving air, flowing west to east, that is usually several thousand miles long and located in the upper part of the Earth’s atmosphere (~6–7 miles above the Earth’s surface). There are two main jet streams in each hemisphere, a weaker one in the subtropics, often crossing the southern portion of the U.S. and a more active jet in the mid-latitudes near the Canada/U.S. border..

This jet in the mid-latitudes is very active because of the collision of arctic and tropical air masses. The rapid change of temperature between these air masses near the surface, also known as the temperature gradient, creates a stronger jet aloft. Temperature differences create pressure differences, which leads to wind. Consequently, the greater temperature differences at the surface, the stronger and more active the jet aloft.

Because a jet stream is contingent on temperature differences, jets are most active during the winter over the mid-latitudes. As the northern hemisphere mid-latitudes begin to warm up into the spring and summer the jet stream moves north.

Why so cold, Rochester?

But so far this year, the jet hasn’t budged one bit. Unpleasant cold air has continually made its way into the Northeast from Canada due to a persistent pattern of surface lows, keeping the jet located to our south. Since the bitter air from Canada has been constant, the jet has not yet been able to begin its seasonal shift northward.

There is always hope though as the ground continues to warm throughout the spring. Eventually, the jet will start to push its way northward. And once Rochester is on the south side of the jet, say hello to swim trunks and tank tops.

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Weather Science

The future of meteorology visits Hobart William-Smith College

Photo: <a href="http://coloradostateofmind.wordpress.com/2011/06/06/tornadoes-storm-chasing/">Herb Stein / CSWR</a>

It’s amazing to think about how far the field of meteorology has come in the past 50 years or so. Atmospheric science has evolved from a minor concern among the public to a media cash cow. A primary reason of the growing interest in meteorology directly stems from great technological advancements in the 20th and 21st centuries. It seems like everyday there is a cutting-edge tool that will help meteorologists in their quest for the perfect forecast. Recently, this revolutionary invention is a portable doppler radar, known as the Doppler on Wheels (DOW).

Implemented in the 1940’s, radar was initially used to detect enemy aircraft during World War II. These radar sent out microwave signals towards a desired target and listened for its reflection, allowing the U.S. Navy to successfully decipher the enemy and their whereabouts.

When military radar operators noticed strange features on the radar, they reasoned that the radar must have picked up precipitation. Not too long after this, the first radar primarily used for weather was developed, commencing the need for weather instruments.

Due to its accuracy in pinpointing the location and evolution of precipitation, radar has been one of the most important tools used in meteorology. About twenty years ago, all weather radar was updated to Doppler radar, a feature allowing for the detection of wind flow within regions of precipitation. This upgrade allowed meteorologists to identify areas of rotation in regions of precipitation, a telltale sign for tornadoes.

Across the U.S., there are 155 Doppler radar that meteorologists use on a daily basis.  However, there will always be one major downfall with stationary radar: they can only “see” so far away. Essentially that means the further away an object is, the less accurate the radar is.

That’s why atmospheric scientist Joshua Wurman created a fleet of three trucks known as the Doppler on Wheels. The concept behind the DOW is the closer it is to the weather phenomenon, the better data the radar will receive. This allows the DOW to be a pioneer in severe weather research.  Over the past 15 years, the DOW has collected data within a mile of a numerous tornadoes and within the eye wall of multiple land falling hurricanes.

The DOW even measured the fastest wind speed ever recorded on earth, a 318 mph wind gust from a tornado outside of Oklahoma City in 1999.

Although the DOW operators pride themselves on being trailblazers for tornado and hurricane research, the DOW has also recently studied other weather phenomena like lake effect snow. In fact, this February the DOW made a two-week trip to work with students at Hobart & William Smith Colleges to study how lake effect bands behave. During this visit, students were given the opportunity to operate the DOW and decipher the movement and precipitation type of lake effect off Lake Ontario. This is the same work done by real meteorologists in the field.

Despite the fact that the DOW has departed HWS, students will have the chance to work with it once more, as it will make an extended two-month visit next December when all three DOW’s will travel to upstate New York to further study lake effect precipitation.

 

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Space Weather Weather Science

There is no life on Earth’s evil twin.

Photo: <a href="http://nssdc.gsfc.nasa.gov/image/planetary/venus/mgn_eistla_regio.jpg">NASA.gov</a>

The conflict between good and evil is a concept that stems from an ancient myth thousands of years ago. However, long before this idea was conceptualized, the battle between good and evil existed on a planetary scale.

In our galaxy (the Milky Way), all eight planets have unique size, characteristics and appearance. However, even though these eight planets have such distinct features, there are two planets known as twins. With luscious vegetation and more than half of the planet covered with water, Earth is the ideal planet for all living organisms. On the other hand, the planet that is strikingly similar to Earth in size, mass and composition, Venus, has temperatures upwards of 1000°F and an atmosphere 100 times thicker than Earth’s. Therefore, it’s no surprise that the planet known as the “Morning Star” is commonly referred to as Earth’s evil twin.

The Amazon in Space?

Venus’ composition and weather is a fairly new understanding though, as many scientists actually believed Earth’s evil twin possibly could have similar weather and surface features to our planet. This idea stemmed from the fact that Venus is essentially covered in clouds. Since clouds on Earth are composed of water vapor, researchers believed that there must be some sort of tropical “paradise” like lush rainforests or jungles encompassing Venus. However, this ideology came to an abrupt end when scientists learned of the hellish-hot temperatures on Venus.   To accompany these temperatures, the clouds on Venus are composed of drops of sulfuric acid.

Composition of Venus’s Bizarre Clouds

Scientists believe there are a couple of ways these sulfuric acid clouds formed. One is that these clouds were actually formed by early volcanic activity that released sulfur into the atmosphere and trapped it in the clouds. The sulfur was able to melt in the atmosphere since the melting point of sulfur is 386K and the surface temperature on Venus is about 750K. The other way is through photo dissociation (breakup) of carbon dioxide into carbon monoxide and atomic oxygen. Since atomic oxygen is highly reactive, when it reacts with sulfur dioxide, it results in sulfur trioxide, which can combine with water vapor to create sulfuric acid.

Although these clouds have a much different composition than water vapor clouds on Earth, the sulfuric acid clouds surrounding Venus do precipitate. Sulfuric rain falls from the atmosphere of Venus, however does not reach the surface due to the extreme heat that evaporates the rain and forms clouds again. This sulfuric rain is much different from acidic rain on Earth since Earth’s acid rain is water with small amounts of sulfuric and nitric acid and Venus’s acidic rain is composed entirely of sulfuric acid.

Due to its extremely close proximity to the sun and interesting atmospheric features, Venus’s weather has been long debated among scientists. But since we now know Earth has an evil sibling, it’s fair to say we lucked out.

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VIDEO Weather Science

Why fog is unique in the Rochester area

Photo: <a href="http://www.hws.edu/news/twip/?twip=THIS%20WEEK%20IN%20PHOTOS%20-%20January%2019%20-%20January%2025">Hobart William-Smith</a>

Ever walk to your car on a cool but humid late summer morning and realize that you can’t even see the end of your driveway? Dense, heavy fog smothers everything around you making it even difficult to make out your feet. You might stand in amazement for a couple of seconds, pondering how you will make it to work. Well, as many Rochesterians know fog isn’t just reserved for the summer; in fact different varieties of dense heavy fog can make travel difficult at any time throughout the year.

Before we delve into the numerous types of fog, understanding the basics of this phenomenon is a necessity. Fog is essentially a collection of liquid water droplets or ice crystals suspended in the air just above Earth’s surface. When the air temperature cools to equal the dew point temperature, the air becomes saturated condensing into droplets and creating fog. This is the same process as cloud formation, thus it is fair to say that fog is essentially clouds at the surface. However, there are some differences between clouds and fog, mainly in the ways they are formed. In the upper atmosphere, the air is cooled as it rises, forming a cloud. At the surface the air is cooled in a multitude of ways, creating the many types of fog.

Anyone who lives on or near any of Upstate New York’s lakes knows that throughout the winter, steam sometimes appears to come off the lake. During the early morning, very cool air will tend to move over a warmer, moist body of water. When the cool air mixes with the warm moist air directly over the water, the moist air cools until it becomes saturated and fog forms. The following is a video of steam fog over Lake Ontario during the winter of 2005. Fog like this is common over Lake Ontario and many of the Finger Lakes throughout the winter.

[youtube]http://www.youtube.com/watch?v=r8FPWRj_TKo[/youtube]

As late summer approaches and fall is imminent, another type of fog called radiation fog is pretty common in Western NY. Radiation fog forms at night under clear skies with calm winds when heat absorbed by the earth’s surface during the day is released into the atmosphere. As the earth’s surface continues to cool, the air will then become saturated and dense fog will form if enough moisture is present. You might see this fog in the early morning before the sun heats the surface.

Sometimes water droplets that compose fog are supercooled, or in a liquid form at temperatures below freeing.  This fog is termed “freezing fog”.  These water droplets remain in the liquid state until they come into contact with a surface upon which they freeze. As a result, any object the freezing fog comes into contact with will become coated with ice.

Rochesterians should always be on the lookout for different types of fog as upstate NY is a unique region where this phenomenon is prevalent.

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Technology Weather Science

NASA’s RapidScat-ISS is a DIY dream

Photo: <a href="http://www.jpl.nasa.gov/news/news.php?release=2013-037&cid=release_2013-037">NASA/JPL</a>

Republicans looking for “wasteful government spending” should look elsewhere than the team at NASA/JPL. When pressed to solve a problem, the engineers are perfectly ready, willing and able to put together old parts to make something new.

Case in point, NASA’s ISS-RapidScat system. Planned to be installed into the International Space Station in 2014, RapidScat is a scattetometer that microwaves to study the scattering patterns of wind. This new tool, aimed at studying oceanic wind currents and their effect on high energy storms like hurricanes, is being cobbled together from parts of another scatterometer, the QuickScat satellite that stopped working in 2009:

ISS-RapidScat will have measurement accuracy similar to QuikScat’s and will survey all regions of Earth accessible from the space station’s orbit. The instrument will be launched to the space station aboard a SpaceX Dragon cargo spacecraft. It will be installed on the end of the station’s Columbus laboratory as an autonomous payload requiring no interaction by station crew members. It is expected to operate aboard the station for two years.

ISS-RapidScat will take advantage of the space station’s unique characteristics to advance understanding of Earth’s winds. Current scatterometer orbits pass the same point on Earth at approximately the same time every day. Since the space station’s orbit intersects the orbits of each of these satellites about once every hour, ISS-RapidScat can serve as a calibration standard and help scientists stitch together the data from multiple sources into a long-term record.

The original QuickScat satellite stopped working in 2009, the new ISS launch is expected to last for two years, and next-generation equipment is being looked into for the next step.

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Weather Science

Into the deep freeze: what large-scale climatic changes mean for Rochester weather

Graphic: <a href="http://www.pnas.org/content/107/52/22442/F1.expansion.html">PNAS</a>

Mid-January has arrived which means a couple of things in the Northeast. College students reluctantly return to the grind of classes, football season is coming to an end and the winter cold has arrived in full force. Yes indeed, the bone chilling cold returned last week from a long hiatus and is back with a vengeance.

Recently, (although not so much this week), Arctic air from Canada caused frigid temperatures in locations such as International Falls, Minnesota (-35?F) while cities such as Boston and New York barely made it out of single digits. Although it may appear that subzero temperatures can seemingly come out of the blue, forecasters have the ability to predict the severity of the winter cold far before winter arrives.

Large-scale climatic teleconnections have an important influence on the weather pattern for a specific region. These teleconnections such as the North Atlantic Oscillation (NAO) and the Pacific North American pattern (PNA) relate large-scale weather patterns across a large distance, consequently having a direct impact on the weather we have experienced this winter.

The PNA is one of the most recognized, influential teleconnection patterns in the Northern Hemisphere. The positive phase of the PNA oscillation tends to be associated with warming over the Pacific and the negative phase tends to be associated with cooling over the Pacific. This warming/cooling is directly correlated to the temperature anomaly in the United States. During a positive PNA there is a ridge in the jet stream over the western U.S. with warm air infiltrating from Mexico resulting in above average temperatures. During this stage there consequently is a trough in the jet stream over the Eastern U.S. with cold air coming down from Canada resulting in cooler than normal temperatures. During a negative PNA, the temperature anomaly is directly opposite to the positive stage with cooler temperatures out west and warmer temperatures in the east. Recently, with the cold air that funneled into the northeastern U.S., the PNA shifted to the positive phase.

Another influential oscillation, the NAO is simply a “blocking” pattern that affects the location and intensity of cold air. The positive phase of the NAO tends to bring above normal temperatures along with relatively wet conditions over the Eastern seaboard. These conditions are associated with a fairly strong upper-level jet stream. During the negative phase, the upper-level jet stream weakens, allowing cold air to filter down along the east coast of the U.S. Additionally; the negative NAO tends to bring conditions drier and cooler than normal. The recent cold blast pushed the NAO into negative territory for only the second time since the beginning of winter.

Distinct changes in temperature and precipitation throughout the winter always correlate to the large-scale meteorological patterns. Understanding how these large-scale teleconnections behave during the winter is extremely important for any meteorologist when making a forecast.

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Weather Science

Hypemageddon? Frankenhype? The Weather Channel to begin naming “noteworthy” storms.

Photo: <a href="theweatherchannel.com">The Weather Channel</a>

Who could ever forget powerful storms such as hurricanes Andrew, Katrina or Sandy? Not only did these storms have a devastating impact but the hype surrounding these storms was unprecedented. By personifying these storms with human names and qualities such as “powerful”, “mean” or “brutal”, meteorologists attempt to better warn people about a dangerous situation that could occur in their area. That is why beginning this year; The Weather Channel (TWC) will name “noteworthy” winter storms.

The intention is good; The Weather Channel wants to better prepare Americans across the country when a threatening winter storm is headed their way. But is this really the best way to accomplish this goal?

According to their website, the goal of naming winter storms is “to better communicate the threat and the timing of the significant impacts that accompany these events”. TWC meteorologist Tom Niziol believes that along with a heightened sense of awareness that naming a storm brings, it will be easier for the general audience to track a weather system progress. The Weather Channel also says that when winter storms are named, much like hurricanes, they will tend to take on a personality of their own, adding to the heightened awareness among the public. Niziol also claims that in today’s social media world, a name makes it much easier to reference in communication.

The Weather Channel’s proposed storm names. Um. No hype here.

However this naming could easily confuse people rather than helping them prepare. One thing to note is that there is no exact criterion for naming these storms. What I mean by that is that for a hurricane, a certain wind speed must be met to reach certain categories of a hurricane. For example, a tropical cyclone is determined a category one hurricane when sustained wind speeds are 74-95 mph; category two is 96-110 mph sustained winds and so on. TWC does not have these qualifications for their winter storms; in fact they are determining a “noteworthy” winter storm solely on impacts.

According to The Weather Channel “The process for naming a winter storm will reflect a more complete assessment of several variables that combine to produce disruptive impacts including snowfall, ice, wind and temperature. In addition, the time of day (rush hour vs. overnight) and the day of the week (weekday school and work travel vs. weekends) will be taken into consideration…”

Essentially, a storm that drops 20 inches of snow in the boonies of North Dakota may not get a name, but a storm that drops 2 inches of snow in Atlanta gets a hashtag (those that tweet know what I’m talking about) and a special news report all because it has a name.

And what about lake effect snow? If Rochester or Buffalo is blanketed by 30 inches in a two-day span, does that event get a name? I guess only time will tell, but one thing is for certain, those names are pretty lame.

The future of weather journalism? Cheap click-throughs and Twitter hash tags?
Categories
Weather Science

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

Photo: <a href="http://awesomnessproductions.webs.com/apps/photos/photo?photoid=71198809">Awesomenessproductions.com</a>

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?