Space Weather

Its spring in Rochester, but fall on Saturn’s moon, Titan

Here in Rochester, it is easy to lament the long transition from winter into summer. Not only long, but inconstant and subject to depressing changes that leave us shivering in our spring clothing. Still, as seasonal changes go, one can hardly compare our Earth’s cycle to that of the Saturnian moon Titan, where the change takes seven years.

Scientists studying Titan have observed an ice cloud that hovers over Titan’s northern hemisphere for some time. But as the seasons change on Titan and the wind patterns begin to shift, that ice cloud has shifted southward in a process similar to Earth’s own Hadley cell. This seasonal change on Earth is closely associated with our jet streams:

 At first blush, the southern ice cloud seems to be building rapidly. The northern ice cloud, on the other hand, was present when Cassini first arrived and has been slowly fading the entire time the spacecraft has been observing it.

So far, the identity of the ice in these clouds has eluded scientists, though they have ruled out simple chemicals, such as methane, ethane and hydrogen cyanide, which are typically associated with Titan. One possibility is that “species X,” as some team members call the ice, could be a mixture of organic compounds.

So there you have it: Rochester weather may be cold and damp when we least like it, but at least we have a 365 day cycle. And that’s not nothin’…

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.

Weather Science

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

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.