Weather Science

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

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.


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.

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.