Summer 2009… Cool!

Most people I talk to are taking our cool summer in stride. I suppose temperatures in the 70s and lower 80s are much more fun to be outdoors in than summers where we soar into the 90s with high humidity. However… there are a few of you die-hard summer fans who want to know why it’s been so cool.

For the answer, we need to look at the air flow in the upper atmosphere. When we look at weather maps of the middle and upper atmosphere, one of the things we look for are ”ridges” and ”troughs”. Ridges are where the jet stream flows northward, sometimes all the way into Canada before it crests then dives southward toward the Gulf of Mexico, forming a corresponding trough. Ridges and troughs both have their own weather associated with them. For example. The air under an upper level ridge sinks. As a result, there is very little cloud cover. The air flow is usually from the south. In addition… as the air sinks, it heats up. If an upper level ridges sets up shop over the Upper Midwest during the summer, our weather would be mainly dry, sunny, and quite warm. The stronger the ridge, the warmer and drier we get. We saw this in late June when temperatures soared into the middle and upper 90s in La Crosse. An upper ridge was sitting over the upper Midwest that week.

An upper level trough is the opposite. The air underneath it has a tendancy to rise. This forms clouds and sometimes light precipitation. In addition, the air flow is usually from the west and northwest. This, combined with persistent cloud cover leads to cooler temperatures.

So what drives these ridges and troughs? You have to look at the whole globe to answer that question. A trough over the western Pacific may cause a corresponding ridge over the eastern Pacific , which in turn, causes a corresponding ridge over the western United States and on and on and on… Think dominoes and you’ll get the picture.

Keep in mind this is a very simplified view of weather in the upper levels. There are many small disturbances that track along the jet stream that can flatten a ridge, or create a trough, but you should at least get some idea of how the upper flow can give us a peek into what to expect at the surface. And how weather in another part of the world can impact our weather here in La Crosse. More importantly, I hope it helps answer the question of why we’ve been so cool this summer.

Talk at you soon!

Cory