As of today, August 4th 2009 it has been a record breaking cool summer across the Chicago area and the entire midwest. Many locations have reported their coolest Julys on record. So what is behind such an unusual pattern? Lets take a look...

 

First some weather 101 facts to better help you understand.

 

We have "weather" because our planet is round. This causes the surface to not heat up equally in all areas. Since earth is not racist it wants to try to balance out these temperature differences as best it can. This results in something called the jet stream. Warm and Cold air have different physical properties which cause the air to move. The jet stream separates these hot and cold air masses. It is an intense band of winds found in the upper areas of the atmosphere. These winds can easily reach speeds over 100mph.

 

Basically, air is colder to the north of the jet stream, and warmer south of it.

 

 

 

When the jet stream looks as shown above, we are in what is call zonal flow. The jet stream does not take on this pattern too often, usually in between major shifts. Typically what we see can be described as a hill on one side of the county and a valley on the other. In weather terms, the hill is called a ridge and the valley is called a trough. The warm and cool air follows these patterns just the same.

 

 

 

These troughs and ridges fluctuate the entire year, and are basically the driving force behind our weather patterns. The trough and ridge in the above image are fairly gradual. One could expect temperatures to be close to normal under such conditions, perhaps +/- 10 degrees depending on your location. However, there are many other factors that can affect temperatures as well so the position of the jet stream is not the ONLY culprit.

 

Sometimes these troughs and ridges can really take on a steep, dramatic nature. We call this a highly amplified jet stream pattern. That is exactly what has been happening for much of 2009, especially during the month of July. The pattern shown below illustrates what has been going on.

 

 

 

This is highly unusual for the summer months. Such patterns are most common during the spring and fall, which are transitional seasons. Usually such a pattern lasts a couple days, maybe even a week. What is remarkable about 2009 is this pattern has been very persistent and has not budged much at all. Since the winds in the jet stream always flow from west to east, you can see that based on Chicagos location in relation to the jet stream the primary wind flow is from the northwest. Easily enough this is called a northwest flow pattern. Such a pattern delivers a constant flow of cool, dry canadian air.

 

The result has been a summer with temperatures much below what we would normally see around here. The same pattern is behind record setting heat in the west. As a whole, temperatures across the planet are now balanced, and earth is happy.

 

Typically during the summer, the jet stream likes to hang out near the US/Canada border. During the fall it begins to migrate south as we lose heating and the days get shorter. In the winter the jet stream likes to hang out near the gulf coast and get a sun tan while the rest of us bundle up. In the spring, as things begin to heat up and the days increase, the jet stream begins its trek back north.

 

 

 

 

 

 

The weather never follows these exact patterns each year though, and thus we have years like 2009 where all kinds of records were broken. While the west coast boiled with record smashing heat, the midwest experienced temperatures much below normal. There are no exacts in the world of the weather, just general trends which are dubbed "normal." Usually a normal pattern is what we see, but as mentioned, not every year is a normal year, and 2009 is a great example of how the weather can pull a fast one on us sometimes.

 

Now, as far as storm systems go, storm lovers such as myself have had a miserable year around these parts. There has been very little storm activity due to the pattern described above. That is a whole new weather discussion, but it can be directly tied to the above pattern. The workings of the weather are very complicated, and there are numerous factors that determine the weather we have. To give you a quick idea though, storm systems basically follow the jet stream.