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.

Weather Science

The two forces that make #Sandy quite unlike Irene

Batten down the hatches; Mother Nature is about to unleash her wrath on the eastern seaboard Monday afternoon. Just in case you’ve been living under a rock, meteorologists have predicted for many days that Hurricane Sandy was going to be be “the storm”. You know, that storm people will reference back to and compare every other storm to twenty years down the road. She will be strong, she will be relentless and she will be sure to leave her mark on the Rochester region.

Without a doubt Upstate New Yorkers have faced their fair share of powerful tropical cyclones, Hurricanes Bob, Isabel and most recently Irene to name a few. These hurricanes packed a powerful punch in terms of precipitation, closing down the Thruway and paralyzing small communities due to localized flooding. However, these three storms were not able to provide the wind Sandy is projected to produce.

This is because Sandy will eventually transition from a tropical cyclone into an extra-tropical cyclone. As Sandy moves onshore, she will become integrated into a deep trough and upper-level jet stream associated with the cold air present over the northeast. When she merges with the deep trough, her central pressure will drop, winds will strengthen and she will spread out over a huge area. Tropical-storm force winds will extend over 500 miles northward of the center of Sandy – an unprecedented size! As a result, Sandy will produce sustained winds over 40 mph and gusts exceeding 55mph in the Rochester area.

Hurricane Sandy is projected to make landfall somewhere between southern Jersey and Delaware around midnight Tuesday and dump anywhere from ten to twelve inches of rain in the area. Although Rochester won’t see that much precipitation, parts of Western and Central New York could see over three inches of rain from Sandy, more than enough to cause localized flooding.

That amount of precipitation has the ability to dampen the ground so that with the strong winds, trees could be uprooted. Given the counter-clockwise flow around Sandy, winds will be coming from the northeast, opposite the typical prevailing wind direction, which could put added stress on trees. A positive thing to note though is that most of the leaves have fallen off the trees, lightening their weight. Nevertheless, be alert if your house is situated near tall trees, especially if they have a weak root structure.

As if there is not enough to worry about, Rochesterians will have to pay special attention to Lake Ontario throughout the storm. Sandy’s winds will blow across the flat lake, causing over 20 feet waves, almost unprecedented heights for Ontario.

It’s fair to say that the hype Sandy has brought is nothing short of extraordinary. However we will have to wait to see if she puts her money where her mouth is. My guess is, Sandy is not fooling around and the 50 million people she is projected to impact should take the necessary precautions to stay out of harms way.

Weather Science

Those ghostly moon halos and how they can help predict the weather.

Growing up, I’m sure Rochesterians are familiar with a number of mythical sayings for fascinating weather occurrences. For colorful, brilliant skies, there’s the well-known “Red sky at night, sailor’s delight. Red sky in morning, sailor’s warning”. There is also the less known but equally peculiar cricket theory, which states in order to calculate Fahrenheit temperature, count a cricket’s chirps over fourteen seconds and add fourteen. Many have actually seen success when predicting weather changes through this concept. However, for those of you who have difficulty keeping up with a cricket’s rapid chirp rate, you may want to look to the moon to foresee a change of weather.

Photo: Dan Bush (via

Large rings around the moon known as halos are without a doubt one of the most captivating weather phenomena, but did you ever realize you could predict the weather by looking at one? Before we jump into how it’s possible to forecast by merely peering at the sky, we need to understand how halos form around the moon. A halo is a whitish ring that encircles but does not touch the moon. They are merely an optical sensation, formed by the splitting of light (refraction) caused by atmospheric ice crystals. Since light must shine through this thin layer of ice crystals, halos are thus generally associated with cirrostratus (wispy, sheet like) clouds that drift about 20,000 feet above the earth’s surface.

As with any optical illusion, halos are based entirely on the perspective of the individual who sees them. That’s why, like rainbows, these rings around the moon are personal. Everyone sees their own particular halo, made by their own particular ice crystals, which are different from the ice crystals portraying the halo to the person standing next to you.

People often wonder if these halos are consistent or if they vary at all. Different crystal habits, orientations, and zenith angles can produce different halos, the most common being the 22° halo. This halo gets its name by forming a circle 22° away from the moon. Because ice crystals are randomly oriented in space, there are a variety of directions light rays can enter and exit the crystals. However, light rays that form the 22° halos enter one side and exit out the opposite, the most common occurrence, making this the most common halo.

The other known halo is the 46°, located 46° away from the moon. This halo is much less common because ice crystals do not enter one side and exit the other; they exit out of the bottom or top of the ice crystal.

Photo: Colin Chatfield (via

Regardless of the type of halo, many have claimed this optical phenomenon can be a telltale sign of precipitation in the near future. People have stuck to the old saying of “ring around the moon means rain soon”. Although this may be seen as a tad far-fetched, there is truth to this saying, since high cirrostratus clouds often come before a storm. So in addition to listening to the weatherman for tomorrows forecast, check the moon before bed, you never know what you may find.

Weather Science

So, what does the lake effect, exactly? Its not just a winter weather phenomenon:

Envision this setting: mid-day lunch-break on a brisk, bright December afternoon in Rochester. It’s one of those days where you think the blinding sun would warm the air slightly, however the air remains bitter. Now fast-forward an hour, you take a quick glance out your window to find blizzard conditions. This rapid change of conditions that Rochesterians are far too familiar with is lake-effect snow.

Although we likely won’t experience lake-effect snow for another month or so, that doesn’t mean upstate New York won’t be impacted by lake effect precipitation. Lake effect isn’t all about snow, as lake effect rain can occur in September and October.

Generally, cool air temperatures traveling over a much warmer body of water causes lake effect precipitation. Strong, chilly winds blow across a lake, picking up moisture from the water. The weather nerds call that latent heat flux. As the warmer air near the surface rises, it begins to cool and as a result is able to hold less moisture. This drop in temperature and overload of moisture causes the vapor in the air to undergo condensation (to liquid water) or deposition (to ice) forming clouds. When the water droplets or ice crystals grow to a large enough size precipitation falls from the clouds onto the downwind shores.

Upstate New York cities like Rochester are situated in the perfect position for lake effect precipitation. Throughout the year, there is a prevailing wind from the west or northwest over Lakes Ontario and Erie. Since Rochester is downwind, this region is continually pounded with lake effect precipitation from late fall, when the temperatures begin to cool, until March, which is when air temperatures come in line with the temperature of the lakes again.

One might wonder what determines if Rochester receives lake effect rain or lake effect snow. This solely depends on the air temperature. Temperatures greater than the freezing mark during a lake effect event will produce rain, or perhaps sleet; accordingly air temperatures below 32° Fahrenheit will produce snow. As a weather nerd in Upstate New York, lake effect precipitation is one of the most thrilling weather phenomena. Since lake effect precipitation is so localized, predicting where it will hit is almost an art. For example, Boonville, NY averages approximately 220 inches of snow annually, while Utica, NY averages just over 100 inches a year – a difference of 100 inches of snow in just 40 miles that mostly results from lake effect.

These drastic differences in annual snowfall occur near regions of high intensity lake-effect snowfall known as snowbelts. These are regions directly south and east of the body of water, essentially the kill zone for lake effect snow. When passing through these snowbelts traveling on I90, one will often experience snow bands with visibilities at times reduced to near zero. Take some advice from the weather wonk and always expect the unexpected when driving during lake-effect snow season.

Weather Science

Wild weather on the Great Lakes: 160 tornado-like downspouts this season!

Until I took my first college meteorology class, I always thought the large tornado-like things known as waterspouts were merely a myth. I was fascinated with the raw power and terrifying beauty of tornadoes, but never could grasp the concept of tornadoes over water. Little did I know, not only do waterspouts exist, but there have been an unprecedented number of them spotted over the Great Lakes this year.

As we approach mid-October fall seems to be in full swing. Temperatures will continue to drop and soon enough Rochester will experience its first frost of the season. Summertime severe weather appears to have come to an end. However as recently as last week, waterspouts were spotted over lakes Michigan, Erie, and Ontario. Doesn’t it seem a bit strange for these small-scale tornadoes to be present in late September and early October? In order to answer this, it’s necessary to take a closer look into what a waterspout actually is.

According to the National Weather Service waterspouts are similar to tornadoes over water and are broken into two categories: fair weather waterspouts and tornadic waterspouts.

Tornadic waterspouts are merely tornadoes that form over water or move over water and have the same characteristics of regular tornadoes. They are commonly associated with severe thunderstorms, high winds, large hail, and frequent dangerous lightning. Since these waterspouts are associated with severe thunderstorms they are less common during autumn over the Great Lakes.

The waterspouts Rochesterians and Upstate New Yorkers most generally see throughout fall are fair-weather waterspouts. These waterspouts are usually less dangerous than a tornadic waterspout since fair weather waterspouts are not associated with severe thunderstorms. Typically, fair weather waterspouts dissipate rapidly when they make landfall.

These waterspouts occur most frequently during the months of August, September, and October, when the waters of the Great Lakes are near their warmest levels of the year. Fair weather waterspout formation typically occurs when cold air moves across the Great Lakes and results in large temperature differences between the warm water and the overriding cold air.

According to Wade Szilagyi, head of Canada’s International Centre for Waterspout Research, there have been over 160 waterspouts recorded over the Great Lakes this season, an unprecedented amount. Why such an increase? Well, this unusually warm summer warmed the Great Lakes to above normal temperatures. The recent periodic cool outbreaks over the lakes results in a larger than normal temperature variations with height, enabling a more-unstable environment, and more frequent waterspouts.

Another major reason for the increased frequency is you, the public. Since almost every cell-phone is equipped with a camera these days, it’s so easy to capture a waterspout via photo or video and upload it to a social media website.

Waterspouts are one of the more fascinating aspects of weather especially during this time of the year. So next time you are near the shore of Lake Ontario be sure to grab your camera, as predicting waterspouts is never easy.

Weather Science

Rochester’s mild drought didn’t affect the fall foliage! How temperature and rainfall affect leaf colors.

Before you venture out apple picking this fall, be sure to have your camera on standby as the vibrant upstate New York foliage is predicted to be as spectacular as ever this year. Many Rochesterians were able to experience the beginning of the foliage season this weekend, especially apple pickers who flocked to festivals all over the area such as the 32nd annual Hilton Apple Fest held at Hilton Apple Fest Farm Market.
It has been a disappointing season for apple pickers this autumn due to the significant decrease in production. Although many apple farms have been at a loss of words with the notable decline in apples this year, people all over the region will be able to enjoy the brilliant colors of autumn as peak hits this next week.

Some speculated that the dry conditions we have seen in Upstate New York will have a negative impact on the foliage this fall, however as you may have already noticed, that is far from the truth.

Map of the height of this summer’s drought, via the University of Nebraska’s Drought Watch

Over the past four months or so, upstate New York has been entrenched in a moderate drought, which has led to an increase in food prices and distraught flowers. Along with these impacts, many New Yorkers expressed their concern over the lack of vibrant colors they would see due to the drought. However, Donald Leopold, chairman of environmental and forest biology at SUNY College of Environmental Science and Forestry, explains that drought is not always bad for foliage: “Mild droughts are generally good for fall leaf color. It can enhance production of certain pigments in leaves that produce color”.

An example of the “scorching” that occurs in leaves when the environment is too hot and dry. Via

It’s common sense that trees need the necessary water and sunlight to survive: this beautiful concept is known as photosynthesis. But too much rain or sunlight throughout the spring and summer could have a negative impact on the health of the tree and more importantly, the leaves.
According to meteorologist David Epstein, an abundance of precipitation can encourage disease on the leaves, creating dull or bland colors. While not enough precipitation causes the leaves to dry up and fall early, not allowing for colorful leaves. Luckily, our moderate drought was not severe enough to have a negative affect on the foliage.

As for temperatures, colder temperatures, especially at night, breaks down chlorophyll in the leaves, exposing the red and orange pigments we are so accustomed to during autumn. On the other hand, warmer temperatures don’t allow for this breakdown in chlorophyll, creating delay in pigments and accordingly delay in foliage change.

Regions such as the Midwest have not been as lucky as we have this fall. A persistent severe drought has not only killed the economy of the Midwest but foliage is almost nonexistent this autumn, thanks to the drought. Rochesterians should cherish this falls’ colors, as you never know what next fall brings.

Weather Science

Get more rain than your neighbors? Microclimates could be to blame.

Understanding your local weather is a necessity for any setting. Whether you live in hot, arid conditions such as the southwestern U.S., or in a four-season weather pattern like the northeastern U.S., you must learn to live with the weather or more specifically, your climate. Climate is prevailing general weather conditions over a long period of time, for a specific region. Basic climates of the U.S. and even the world are common knowledge, but did you know that there are climates within climates?

These miniature climates within large-scale climates are known as “micro-climates”. According to the Cornell University Department of Horticulture, a microclimate is defined as “The climate of a small area that is different from the area around it. It may be warmer or colder, wetter or drier, or more or less prone to frosts”. These peculiar climates may range from a few square feet to a couple of square miles.

Imagine an ideal garden, receiving the perfect amount of shade, water and sunlight. This type of environment stays relatively cool and moist whereas a nearby environment receiving mainly sunlight will have a much warmer, drier microclimate. On the other hand, doesn’t it always seem hotter in a big city?

Walking through downtown Rochester during the summer, the heat can be stifling. This is because urban environments are filled with objects that have a low specific heat capacity such as asphalt, bricks, and concrete. These objects absorb the sun’s radiation, warm rapidly, then give off that heat, creating much warmer temperatures. As a result, cities are often urban heat islands, featuring temperatures 2–5°F warmer than the surrounding areas.

Illustration of microclimates that exist on Hawaii. Photo courtesy Hilo Living.

Other factors such as wind direction and elevation contribute to the formation of microclimates. The Big Island of Hawaii has ten different microclimates in an area roughly the size of Connecticut. The prevailing easterly trade winds and precise placement of volcanoes, result in climates that can vary from hot desert to tropical rainforest in a matter of fifty miles.

Bodies of water can also cause a microclimate, especially in our region. Unlike land, water has a high specific heat capacity, enabling more solar radiation absorption.. Thus, temperatures will increase and decrease at a much slower rate near water, creating a more temperate environment. It’s for this reason that Vineyards are quite common in the Finger Lakes region. The neighboring lakes release their stored heat in the fall, allowing for temperatures to stay relatively mild, preventing an early frost. These microclimates are a growing economic boon to the central New York economy.

Geography and large-scale weather patterns dictate the general climate of the U.S., but did you ever realize climates can vary in your backyard?

Rochester Weather Science

Bundle up! How dew point affects temperature as Rochester chills for fall.

As seasons change and days become shorter, Rochesterians begin to brace themselves for another long winter. Opening the front door on an early fall morning, you may expect familiar summer air only to be rudely welcomed by a blanket of frost.

Long before winter makes her presence felt on the northeast, the transition into fall not only brings the beauty of radiant deciduous trees but also the cool, crisp air of autumn.  Did you ever wonder why during this period, daytime temperatures may still climb into the 80s but come nightfall, temperatures dip well into the 40s? Or in a matter of minutes following sunset, temperatures will drop ten or almost twenty degrees?

This is a direct result of a decrease of daily dew point temperatures. The dew point temperature is the temperature at which the air needs to cool to reach saturation. What does this mean? Essentially, dew point temperature is the measure of water vapor or moisture in the air. To grasp the idea of dew point temperature, think back two months to the unbearably muggy days of July. At some point everyone has said, “It’s way too hot outside”. Hot temperatures are exacerbated by exceptionally high dew point temperatures (approximately 65° and greater) not allowing perspiration or sweat to evaporate efficiently, which creates an oppressively hot, sticky feeling.

This week, Rochesterians have begun to experience “fall” like weather, especially in the mornings. Sure colder temperatures are associated with this feeling.  However, fall brings a certain crisp, dry air, allowing more evaporation of perspiration, and thus comfortable conditions.

Since the dew point temperature is the temperature at which air needs to cool to reach saturation, it is important to note that temperature cannot be lower than dew point temperature. Thus, one can use this fact to help predict the low temperature for a given day.  When the sun sets and earth cools due to lack of incoming solar radiation (heat from sun), the temperature will usually cool to around the dew point temperature. This is why lows in the summer often stay in the mid 60s or even 70s while lows in early autumn will drop into the 40s or below.

Dew point temperature and air temperature can be equal to one another though, which results in 100% relative humidity causing the air to condense and form early morning dew. As fall progresses, temperatures continue to drop, along with the dew point temperature, eventually falling below the freezing mark (32°F).  When the relative humidity is 100% and the temperature is below freezing, water in the air deposits on the ground as frost.

When frost coats the ground for the first time in autumn, it’s a sure thing that beach season has come to an unfortunate end.

Weather Science

In the path: how Southern storms influence Rochester’s weather

Could it already be that time of year again? The end of August marks the return to school, retirement of beach towels and of course, the heart of the Atlantic hurricane season. That’s right, although the Atlantic hurricane season technically started on June 1st, early September is the activity peak. This season has been no different as Hurricane Isaac ripped into the Gulf of Mexico and made its presence felt from the Gulf of Mexico north into western and central New York.

For weather nerds like myself, the hurricane season brings anticipation comparable to Christmas, as any day during the humid summer months could mark the formation of a major hurricane.

As the summer has roared along, we have seen spectacular weather in the northeastern U.S. Strange huh? I guess meteorologists can breathe a big sigh of relief. We haven’t heard the public complain too much about “wrong forecasts” this summer, tough to mess up “Hot n’ Hazy”. But since we are in the brunt of hurricane season, there’s no telling when one of these storms will come our way.

National Weather Service graphic shows the path of Hurricane Irene, cite:

For instance, last August Hurricane Irene made its landfall on the Outer Banks of the Carolinas and proceeded to dump 15 inches of rain throughout parts of New England, causing flash flooding and fatalities. Wait, I thought hurricanes weren’t supposed to happen in the northeast, at least that’s what the weatherman said.. In the case of Irene, the storm followed the natural east to west movement in the tropics due to the easterly trade winds as it strengthened over warm ocean waters. However, as Irene progressed northward into our neck of the woods, the prevailing winds shift from easterly to predominately westerly, allowing for direct aim on the northeast.

National Weather Service graphic shows the path of Hurricane Isaac. Cite:

Hurricane Isaac followed this pattern into the Gulf, moving westward and making its long-awaited landfall just west of New Orleans. The Louisiana and Mississippi coasts took the brunt of Isaac due to their location in the northeastern quadrant of the hurricane. In any hurricane, it is common for extreme flooding in the northeastern quadrant of the storm due to its counter-clockwise rotation, pushing water on shore, resulting in serious flooding and storm surge. Isaac was also a slow mover, remaining nearly stationary for almost 3 days, exacerbating the flooding.

As Isaac progressed northward, weakening over land due to lack of moisture, (hurricanes need warm water to survive – it’s their fuel!) the mid-latitude westerly winds pushed moisture from Isaac to the northeast providing our region with crucial rain that helped alleviate our recent arid conditions. Let’s all face it, clear skies and 80 degrees everyday is pretty freakin sweet, but a little rain here and there is necessary. Besides, pretty sure the weatherman hit this one on the head, pretty sweet right?