Author: Benjamin Ayres

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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.

Categories
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.

Categories
Science

Reversal of roles: Vegetation actually impacts our weather

Photo: <a href="http://www.flickr.com/photos/tikarasha/8147440645/sizes/l/">tikarasha</a> @ Flickr.com

It’s okay to go outside now spring is finally here! Although it may still feel like January, it’s only a matter of time before snow banks and L.L. Bean Boots are replaced with chirping birds and flip-flops. Deciduous trees will soon become covered with leaves and springtime flowers like daffodils will make up flowerbeds across the northeast. But such change in scenery can surprisingly bring a change in weather, specifically an overabundance of precipitation resulting in flooding and even severe weather throughout the spring.

Each day during the winter, we pass trees that sit stripped of their leaves and plants that have little to no life. We never really think about what effect the lack of agriculture in the winter has on our weather. It’s common sense that an abundance of leaves can form a canopy or that all living vegetation consists of water, but why are these factors important in terms of how we will be affected in a weather sense?

The onset of spring means more daylight, which consequently means warmer temperatures. As the ground warms and habitual springtime rain falls, the mild spring atmosphere is able to evaporate more water from the surface and cause vegetation to transpire more water into the atmosphere. This means that anything that relies on water to flourish such as plants, crops, trees or even soil loses water to the atmosphere due to the mixture of evaporation and transpiration, this process is known as evapotranspiration.

The more vegetated a region is, regardless if it’s a thick wooded area or a vast corn field, the more evapotranspiration will take place. Since this water is evapotranspirated into the atmosphere it takes on the form of water vapor or moisture. Severe weather enthusiasts know that during the spring, the more moisture that is present in the atmosphere, the greater possibility for the development and maturation severe weather or heavy precipitation. Therefore, it can be inferred that more evapotranspiration can equate to severe weather or local flooding.

Locally, Upstate New York has a variety of vegetation that allows for large amounts of evapotranspiration. For example, the world-renowned Finger Lakes wine region is an ideal area for high amounts of moisture in the atmosphere. Since wine grapes feed off water from the soil and use a canopy of leaves and plants to protect themselves from incoming solar radiation, there is plenty of water for evapotranspiration of moisture into the atmosphere. And with hundred of wineries in the Finger Lakes region, there is no shortage of water in the grape vines, soil and canopy, therefore no shortage of evapotranspiration into moisture.

As the days continue to grow longer, we will not only benefit from the warm weather but also from the springtime scenery. But be aware, as vegetation continues to grow in the spring, so does the possibility of a severe thunderstorm or even a tornado.

Categories
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.

 

Categories
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.

Categories
Space Weather

Why Earth’s rotation makes Jupiter’s Red Spot impossible here.

Photo: <a href="http://www.nasa.gov/images/content/297677main_hst_img_20081218_HI.jpg">NASA.gov</a>

Did you actually think Sandy, Andrew even Katrina were bad? These hurricanes that caused widespread destruction to the U.S. are mere child’s play compared to some storms outside the Earth’s atmosphere. If you were sitting at home on your couch and your local meteorologist began to rant about the latest storm that will bring 400 mph winds and temperatures plunging below -250°F, you would surely think he is lying. Now imagine those conditions lasting for at least 400 years and counting. Well, although this sounds like the makings of a science-fiction movie, this storm does exist as the Great Red Spot on Jupiter.

All major storms on Earth usually have a large center of low atmospheric pressure with cyclonic motion (counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere). These cyclones dominate the Earth’s weather patterns and can cause significant destruction at the same time. However, interestingly, the Great Red spot in Jupiter’s Southern Hemisphere is actually associated with anti-cyclonic (high pressure) flow.

Although there are countless differences between the Great Red Spot on Jupiter and Earth storms, there are also some surprisingly striking similarities. All storms on Earth circulate due to Earth’s rotation.  This rotation deflects the direction of a moving object – a force known as the Coriolis. This deflection allows cyclones to rotate, giving them the ability to strengthen into powerful storms. Since all planets in our solar system rotate, the Coriolis effect is also present, ranging in strength due to size and rotational frequency of the planet. Since Jupiter is the biggest planet in the solar system and makes a complete rotation in only~10 hours, the Coriolis force has an exceptionally strong effect on the planet. This fast rotation is directly related to the strengthening of a storm and wind speeds resulting in a Great Red Spot that has winds up to 400 mph.

The strongest surface wind gust ever recorded on Earth was 253 mph during Cyclone Olivia in the late 90s. Winds at this strength have the ability to demolish almost all buildings in their path. By adding another 150 mph of sustained winds on top of this gust, there most likely would be no evidence that a structure ever existed. But before we put a category 20 hurricane on Earth, it is even possible for conditions like these to be on our planet?

Since, Jupiter rotates two and a half times faster than Earth, causing the stronger Coriolis force and winds, a storm like the Great Red Spot could not exist on Earth.  Thus, you don’t need to worry about a 400 mph storm busting down your door.  Just don’t let the people at the Weather Channel know how strong storms can get on Jupiter or they may have to come up with a whole new list of “storm” names.

Categories
Space Weather

How Coronal Mass Ejections affect life here on Earth

Illustration: <a href="http://www.universetoday.com/96286/incoming-cme-on-its-way-toward-earth/">Universe Today</a>

As meteorologists continue their fight against Mother Nature in hopes to produce the ”perfect” forecast, they may encounter some unusual problems outside of our atmosphere. Although outer space does not have an immediate or direct impact on the weather on Earth, space phenomena do have the ability to influence or disrupt the way meteorologists or the general public goes about their daily lives.

According to Earthsky blog writer, Christopher Crockett, Coronal Mass Ejections or CME’s are essentially “sun burps with the power of 20 million nuclear bombs”. Although these burps or hiccups are not totally understood, astronomers believe they are caused by twists or “kinks” in the Sun’s magnetic field, much like a phone cord or toy slinky. “These kinks snap the magnetic field and can potentially drive vast amounts of plasma into space” (Crockett). When the plasma is ejected into space it travels at a million miles per hour, that speed could get you from Boston to London in less than 30 minutes!

Since these explosions are angled at all different directions, they don’t always reach the Earth. However when they do come into contact with the Earth, a geomagnetic storm occurs. This means that the Earth’s magnetic field is temporarily altered as the day side of the magnetic field is compressed and the night side is stretched out. When this happens, the aurora lights can drift towards the mid-latitudes and display a magnificent natural light show.

The effects of CME’s are not always positive though; they can cause widespread power outages and sometimes even become deadly. Cosmic rays, which are very-high energy particles, can infiltrate the earth’s atmosphere and expose people to these deadly levels of radiation. This risk is elevated for those further away from the Earth’s surface such as astronauts or people in planes. For example, during a solar storm in 1989, astronauts aboard the Mir space station received their yearly radiation dose in just a few hours.

Lastly, the flurry of magnetic activity and induced electric currents can disrupt radio transmissions and cause damage to satellites and electrical transmission line facilities. This can severely disrupt power grids and communication networks, leaving millions of people without power.

Just like the weather on Earth, there is nothing we can do to prevent CME’s other than forecast and prepare for these events. In fact, NASA is predicting that we could see a very large CME this year, however they are urging people not to freak out and go on with their daily lives.

Categories
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.

Categories
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.

Categories
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?

Categories
Weather Science

Inversions: how the cool air just above your head could create a death cloud.

Photo: <a href="http://homecomingbook.wordpress.com/">Homecoming Book</a>

Being from an old mill city just outside of Boston, I am quite familiar with the effects pollutants have on the surrounding environment. The coughing and wheezing that is associated with poor breathing conditions is a common situation for many in the Rochester area. It is often true that the pollution we suffer from is man-made, but what you may not know is that a certain weather phenomenon known as an inversion can exacerbate these conditions.

Photo:

An inversion is a deviation from the normal change of temperature with height. In a normal environment, temperatures decrease with height until the stratosphere (about 12 km above the surface). During an inversion there is an increase in temperature with height near the surface caused by a layer of cool, stable air that often sets-up during the early morning hours. This stable air acts as a blocking mechanism or cap on the upward movement of air from near the surface. As a result, any pollution that is emitted within the inversion layer becomes trapped near the surface. You may have seen smoke from a factory spread out horizontally from a smokestack (see photo). This behavior is a telltale sign of an inversion., When pollution is trapped like this for a long period of time, it can have serious health effects..

Rochester in particular is known for having one of the worst air qualities in the entire country. Vehicles and power plant smokestacks are mainly to blame for the extreme levels of sulfur dioxide and other fine particles that can cause health problems. Although recent studies have shown that the effort to improve the Rochester air quality is on the right path, pollutants in a dense area remain detrimental to one’s well being.

Temperature inversions are not random and actually occur more frequently in certain areas or seasons. Surprisingly enough, Rochester oftentimes experiences inversions due to its location.

Inversions can take place in a marine environment. During the spring and early summer when the cold bodies of water are still recovering from the winter, air directly over the body of water will be cool compared to the land. During the day, this shallow layer of cool air can sometimes move onshore with a lake breeze, setting up an inversion near the lakeshore.

Inversions can also occur with frequency throughout the winter months. Much of our frigid Arctic air (assuming we get any this winter) is often very shallow in nature. Directly above the in put of Arctic air, somewhat warmer air can reside. This temperature set-up can once again result in an inversion. As we progress into the winter months it is fair to say inversions will be occurring on a frequent basis in the Rochester area. Even though Rochester air quality is better than it once was, be on the lookout if you happen to suffer from respiratory conditions.